Geology of the Provinces of Canterbury and Westland, New Zealand : a report comprising the results of official explorations
Hitherto I have not made any mention of Banks' Peninsula, beyond alluding to the small zone of palæozoic sedimentary rocks situated at the head of Lyttelton Harbour, probably belonging to the Waihao formation, of which a portion is considerably altered, and of another zone in the same locality consisting of quartziferous porphyries, pitchstones, rhyolites, and tufas, partly covering the former rocks. This chapter will be devoted to a description of the geological features of that remarkable volcanic zone as a whole, tracing its origin from the first eruption of quartziferous porphyries and the deposition of tufas and agglomerates in connection with them, to the extinction of the volcanic foci by which it has been built up.
When standing on the Canterbury plains the most striking feature in the landscape is Banks' Peninsula, rising so remarkably above the sea horizon, that its regular form at once attracts our attention. First we observe a series of mountains, of which the summits are all nearly of the same altitude, which, as it appears to us, as far as our eye can follow their outlines, form nearly a circle, from which a great number of ridges slope with a nearly uniform gradient towards south, west, and north. Above them, in the centre, stands conspicuously a higher truncated mountain with precipitous escarpments, assuming, according to the position of the traveller, a different aspect. The rim of the lower mountains in front rises to an average height of 1600 feet, whilst the central system attains an altitude of 3050 feet. On reaching Banks' Peninsula from the sea, we find that several deep indentations, forming splendid harbours, enter far into the outer rim of the moun-page 325tains, passing for a considerable distance along the higher central range. Similar indentations are also found to exist towards the Canterbury plains, but they have either been already filled by alluvial deposits forming fertile valleys, such as the Kaituna valley, or they appear in the form of a lake (Lake Forsyth). In examining the nature of the rocks of which the system under consideration is composed, we find that, with the exception of a small zone at the head of Lyttelton Harbour, the whole is composed of volcanic rocks; that the deep indentations are ancient crater walls, so-called calderas, into which a channel with precipitous walls, the barranco, leads; and that they consist of a series of lava-streams, with agglomerates consisting of scoriæ, lapilli, ashes, and tufas interstratified with them. These beds have all a qua-qua versal dip, that is to say, they all incline outwards from the centre of the cavity. The higher mountains in the centre consist also of volcanic rocks of a similar composition, which appear either horizontal or, when the direction of the lava-streams composing them can be ascertained, are found to flow into the calderas previously formed, from which we can at once conclude that they are of younger origin. Finally, we find mostly in or near the centre of these deep cavities, or calderas, either a small island or a peninsula stretching so far into these harbours. They consist also of volcanic rocks, having been preserved above the last centre of eruption. This last sign of vulcanicity is on a smaller scale than the previous ones. The whole of Banks' Peninsula, measuring along its longest axis from north-west to south-east, has a length of 31 miles, with a greatest breadth of 20 miles, and if we do not take the numerous indentations into account, it has a circumference of 88 miles, which corresponds closely with that of the base of Mount Etna.
In the Geological Map attached to this report, I have marked with circles, more or less perfect, according to the preservation of the lips or rims, all the principal centres of eruption which I have traced during my surveys, and I have no doubt that the remnants of others, which have escaped my observation, and are for the greater part concealed under younger lava-streams, will be found during future examinations. On the line between Lyttelton Harbour and the head of Akaroa Harbour, the highest portion of the Peninsula is found where the small craters of Mount Herbert and Mount Sinclair are situated. Having thus given an outline of the general features of the volcanic system under consideration, I shall now proceed to offer a short history of its origin, which will at the same time serve as an explanation to the general sections added.page 326
The oldest rocks in Banks Peninsula form a small zone of palæozoic sedimentary strata, possessing a slightly altered structure, many of them forming beds of chert, others, peculiar light-coloured brecciated schists; however, sandstones and dark clay-slates are also represented. This zone has a north and south direction, and reaches to the southern watershed of McQueen's Pass, which leads from the head of Lyttelton Harbour to Lake Ellesmere. Near this Pass, slates appear as high as 600 feet above the sea-level. On the western slopes of Castle Hill, the south-western continuation of Mount Herbert, 2900 feet high, which rises so conspicuously above Lyttelton Harbour, they reach an altitude of nearly 1000 feet, where they are overlaid by the older lavas, forming the Lytteiton Harbour caldera. Thus a sub-marine hill stood here in the young mesozoic sea, of which portions of the summit and the slopes were gradually covered by agglomerates and brecciated beds. These beds were formed during and after the eruption of quartziferous porphyries, of which here and there portions of the coulées have been preserved. Some of these quartziferous porphyries resemble in every respect those from the Malvern Hills and Mount Somers- They are also accompanied by pitchstones, porphyritic from the presence of numerous well formed crystals o£ sanidine or glassy felspar, and occasionally of garnets. Other portions of the quartziferous porphyries, as for instance, the whole coulée of which Manson's Peninsula is formed, have a rougher, more trachytic matrix. They are full of grains and small crystals of white greyish or smoky quartz. The brecciated beds have a hard felsitic matrix, and the angular fragments of rocks enclosed in them belong to a variety of eruptive rocks of many colours, and of different texture, often forming a rock of striking character. They appear conspicuously on the summit of Grebbie's Pass, having been washed into cliffs of picturesque forms, and covering the palæozoic sedimentary beds from one side of the Pass to the other. On the southern side of this Pass, about 200 feet above the sea-level, occur two beds of shales with stems and roots of carbonized plants, but too indistinct for recognition. They are associated with coarse sands. Each of them is about 50 feet thick, separated by about 130 feet of loose conglomerate, the whole standing at a very steep angle, dipping 76 degrees to the southsouth-west. They are situated at some distance from any locality where the brecciated porphyry agglomerate upon which they appear to rest, crops out. Thus they will be of more recent age than the former; however, no clear section is exposed anywhere, from which this point could be settled quite satisfactorily. After the page 327formation of the brecciated agglomerates, new eruptions of acidic rocks took place, now in the form of rhyolites, the highly liquid matter reaching the surface through broad channels, of which one has been preserved as a large dyke, forming a beautiful section on the northern side of Gebbie's Pass, not far from the summit. The dyke is here about 100 feet thick, half of which is formed by the central portion, consisting of a whitish rhyolite with a fine laminated structure, breaking in prismatic blocks; the rest on both sides, where in contact with the agglomerates, has cooled more rapidly, and has assumed the character of an obsidian. This obsidian is greenish or brownish black, very brittle, and imperfect crystals of sanidine are enclosed in it. This dyke can be traced for a considerable distance upwards. Where overflowing and covering the agglomerates it forms the highest peak on the western side of Grebbie's Pass, well visible from Lyttelton Harbour. The rock here is divided into small pentagonal columns, with a vertical arrangement; lower down the Pass, the same coulée has a tabular structure. There is no evidence from which we can conclude when these beds were raised above the sea, but there is no doubt that this was accomplished in connection with volcanic disturbances close to them, beginning probably towards the latter part of the tertiary period, when the foundation of the oldest crater in Banks' Peninsula was laid. It is, however, clear that before and during that time, these quartziferous porphyries and agglomeratic beds underwent considerable denudation and disintegration, because we find at their base deposits of thick bedded sandstones, consisting almost entirely of grains and crystals of quartz, not very strongly adhering together, an imperfection detracting considerably from the value of this otherwise beautiful freestone. That these quartzose sands were deposited over a considerable area can be concluded from their occurrence in different localities, at considerable distances from each other, the principal ones being situated at Ohinitahi (near the head of Lyttelton Harbour), at Little Quail Island (between Quail Island and the mainland), and some way inland in Charteris Bay. Little Quail Island has been preserved under peculiarly favourable circumstances, as close by numerous volcanic eruptions have taken place, of which the latest formed the greater and highest portion of Quail Island.
Owing to the fact that agglomerates, consisting of volcanic boulders and pebbles, occur about 1,500 feet above the sea-level on the flanks of Castle Hill, I came to the conclusion, during my first examination of Banks' Peninsula, that the volcanic rocks of which it has been built up page 328were mostly of submarine origin; but further and more detailed search has since proved, that although there are a few beds, which might be of marine, at least all the older portion is of subærial origin. The boulders and pebbles in question have either been rounded by attrition during their ejection, or they have been rolled in brooks descending along the slopes of high volcanic ridges, which were mostly destroyed before or during the formation of the Mount Herbert system. Moreover, no marine shells or exuviæ of other marine animals have ever been discovered in the tufaceous deposits interstratified with the lavastreams; whilst the existence of timber which has assumed the character of an altered coal, and is obtained in these deposits near Akaroa, points, from the manner in which it occurs, to a land, origin, which, moreover, is indicated by numerous beds of laterite, representing, doubtless, ancient soils.
The oldest crater, of which the principal boundaries can be traced at the present time, is the Lyttelton Harbour caldera, having a general diameter of about two miles, the centre of which is situated a little to the south of Quail Island. The general structure of this crater, even before the Christchurch and Lyttelton Railway tunnel was entirely pierced through, could easily be made out by studying the numerous sections exposed in many directions, and by ascending the steep escarpments of the caldera wall, where a succession of streams of stony or scoriaceous lava, interstratified with beds of agglomerates, ashes, tufas, and laterites can be traced to the very summit. Still clearer sections are open to our inspection if we follow the barranco or entrance into the harbour, forming sometimes vertical cliffs of considerable altitude, and where the whole series of beds can easily be followed. However, the most interesting and complete insight was obtained in the railway tunnel passing through the caldera wall, and of which, as the work gradually advanced, I prepared a careful section. The main results of this survey, together with a description of some portions of the tunnel, of which sections in chromo-lithography have been added, will be offered at the end of this chapter. The succession and dip of the lava-streams and the intervening beds can also be made out by following the slopes of the ridges between the deep valleys washed out on the outer side of the crater wall, where it will be found that the lava-streams forming the lip of the crater have generally a slighter inclination than those lower down, the dip of the upper ones being only nine degrees in the average. In the tunnel the dip is greater, an inclination of twenty degrees not page 329being uncommon. It is evident that the building up of such a huge system during numerous eruptions, often of great magnitude, could not be accomplished without a great destruction of portions of the beds previously formed, taking place, the point of eruption in the crater shifting continuously about the centre. If, at the same time, we examine the lava-streams and the interstratified agglomerate and ash beds along the water's edge, we have to come to the conclusion that all the eruptions by which the caldera wall was formed from summit to bottom, occurred under the same physical conditions.
For forming a true conception of the manner in which the crater wall of Lyttelton Harbour was raised, I cannot do better than refer the reader to the observations which have been made by excellent and competent geologists of the changes which occurred in Mount Vesuvius and Mount Etna in recent times, during violent eruptions. It has been made evident that these eruptions, principally at the beginning, could not have occurred without great convulsions taking place in the earth's crust, so that earthquakes of considerable vehemence must have preceeded them. After the crater was once formed, by the ejection of lapilli, scoriæ and ashes, over which streams of stony lava had been cooling, so as to preserve them from destruction, it existed either in the form of a large cauldron filled with liquid lava, resembling some of the volcanoes in the Sandwich Islands, or after partial solidification it formed a large rocky plain with a number of smaller vents over which ashcones were built up, or with numerous fissures from which vapours and gases issued such as, before the great eruption of 1822, the crater of Vesuvius appeared, as described by Sir Charles Lyell in his Classical Principles of Geology. During that eruption, the whole of this rocky plain was blown out, and an immense abyss formed, which was partly filled up by portions of the walls, more than 800 feet of which had been carried away by the explosion, so that the altitude of the mountain was reduced from 4200 to 3400 feet. Similar occurences have without doubt repeatedly taken place during the building up of the Lyttelton caldera wall. Examining into its formation and beginning our observations in the harbour, we find that many lava-streams have been preserved which have cooled in their ascent; others lie horizontal for a short distance, and are then seen to descend, conforming to the gradient of the underlying lava-streams, or agglomerate beds. In many instances we have also clear evidence, that considerable destruction of the beds previously formed had taken place before new streams flowed over the lip of the crater, or before beds of ashes, scoriæ and page 330lapilli, were deposited anew. The tunnel section in this respect is also very instructive. Thus, in course of time, the great crater wall was formed, rising to an altitude of nearly two thousand feet, and having a diameter of more than five miles at its crest. It is clear that close to the vent, from which scoriæ and ashes were thrown out in large quantities, the greatest thickness of the agglomerate beds ought to be formed, and this, in fact, is the case, as the largest beds, having sometimes a thickness of several hundred feet, are situated within the inner side of the caldera wall. The lava-streams here between these agglomerates are irregular in their direction, and mostly of small dimensions. The more we advance towards the outer slopes of the caldera wall, the less frequent become these agglomeratic or tufaceous layers, whilst the lava-streams, which towards the centre have the greatest bulk, and are very stony and compact, become now gradually more and more numerous, but of smaller size and more porphyritic or scoriaceous, according to the laws by which the flow, dimensions and cooling of the lava-streams are regulated. It is, moreover, evident that many of them, owing to want of material, scarcely reach half way down the slopes of the caldera wall, that others rapidly thin out, and that many which for some distance, after flowing over the lip of the crater, had been of large dimensions and stony, become, long before its outer edge is reached, thin and scoriaceous, so that here streams of five feet in thickness are not uncommon. Although the tunnel does not offer us the necessary data to judge of the breadth of the lava-streams, we have for that purpose ample evidence in Grodley Heads, the sea wall near Sumner, and many other localities. There are streams which are 500 feet broad, others only 30 to 40, but all without exception are somewhat scoriaceous on the bottom, where the lava flowing over cold ground cooled more rapidly. In many instances this is well exhibited by the existence of a small bed of laterite, a brick-red coloured rock, sometimes only a few inches thick, which doubtless was a layer of soil on the decomposed upper portion of a lava-stream, or agglomerate bed exposed for a considerable time to atmospheric action before the new eruption took place. The lava in the larger streams and in its central portion, principally very stony and of a blackish colour, gradually becomes, as we approach the surface, more porphyritic, with a more open texture, and assumes pinkish or lilac tints, till it changes into scoriæ. The decomposition or alteration is here often so great that it is impossible to trace the top of the line of contact between the surface of the stream and the bottom of the overlying bed, both forming a layer of coarse page 331agglomerate. In other instances the rough, uneven scoriaceous surface of the lava-streams has been well preserved, the hollow spaces being filled up by ashes and ejecta, in which case they resemble many of the recent lava-streams which I examined in Mount Vesuvius and Mount Etna shortly after they had issued from the crater.
From the cliffs outside Lyttelton Harbour and at Sumner a very fair idea can be gained how the whole assemblage of beds was formed, the lava-streams and agglomerate beds in turn filling up pre-existing hollows, and thus forming and equalising the surface. In other sections it is observable that when the lava-streams were flowing at a higher angle than usual they shrunk considerably, whilst where hollows were in their course, which they usually filled out, they became very thick. The lava of which the caldera wall under consideration has been built up, consists of basic rocks, changing from a dolerite to a fine-grained basalt. Some of the lava-streams, however, as previously pointed out, show also a remarkable difference in the structure of the rock of which they are composed, the central portion being a compact basalt with a few crystals of augite, basaltic hornblende, and labradorite, whilst the upper portion consists of a lighter coloured porphyritic dolerite, sometimes so replete with good-sized crystals of labradorite that the greater portion of the rock is formed of that mineral. Again, most of the lava-streams on the inner slopes, or on the rim of the caldera wall, are of a compact basaltic nature, and consist, lower down and more distant from the centre of eruption (becoming at the same time much thinner), of a greyish doleritic rock full of crystals of labradorite, augite,* basaltic hornblende, and rubellan. Even in the most compact streams olivine is seldom present.
There is a great difference in the texture and character of the rocks forming the innumerable lava-streams, according to the conditions under which they have cooled and consolidated, so that, judging from hand specimens only, a great variety of rocks can be made out. However, as it is possible to follow in many lava-streams the gradual change from a compact black basalt into an almost earthy or scoriaceous vesicular lava, passing through all the varieties of a porphyritic or crystalline granular compound, which could be claimed as anamesites or dolerites, I have thought it best to include the whole series under page 332the general term, "basalt." Of secondary minerals, we find, either lining or filling cavities, sphærosiderite, carbonate of lime, (in the form of calcareous spar and aragonite, of which the latter is the younger), chalcedony, hyalite, opal, jasper, natrolite, mesotype, iron pyrites, and several others.
The internal structure of the lava-streams is generally massive, but being divided by more or less numerous joints into polyhedric blocks some of them have a tendency to spheroidal structure, each polyhedric piece exfoliating with concentric layers with a hard kernel in the centre; such a structure is principally observable when the rock is partially decomposed.
Dr. Hector, at my request, had a number of specimens obtained by me during the tunnel survey, analysed at the Colonial laboratory at Wellington, the main results of which, embodied in three tables, will be found attached to this chapter. I have added to Dr. Hector's nomenclature, based without doubt upon the amount of silica contained in each specimen, the terms by which I designate them in accordance with the occurrence, character, and nature of each. I have also added the analysis of a remarkable trachyte, forming a lava-stream of considerable size, and having an average thickness of eighty feet, which is interstratified between two others of a basic character. This peculiar stream occurs between Lyttelton and the pass to Sumner. It is the only trachyte lava known to me as having flowed from any of the different centres of eruption of Banks Peninsula, all the other acidic rocks, as I shall show in the sequel, having been ejected into fissures of more recent date. This lavastream consists of a white vesicular trachyte rich in quartz, resembling closely some of the domites of the Auvergne, from which, however, it is distinguished by its larger amount of silica, although it approaches it again in its considerable percentage of potash. A vertical dyke, about eight feet thick, of a peculiar flaky, silky trachyte passes through this lava-stream, narrowing, however, in its upper portion. Although this acidic lava is rather soft and friable in small pieces, it has nevertheless resisted the disintegrating agencies at work far better than the hard basaltic lavas and agglomerates in its neighbourhood. The analyses of Nos. 10 and 191a, the specimens for which were taken from two small basaltic lava-streams, agree very well with the analyses of basaltic rocks from extinct European volcanoes, made by Rammelsberg, Engelbach, Streng, and others. The rock No. 180, page 333of which there are several streams towards the centre of the tunnel, some of considerable size, and which has as high a percentage of silica as 65.57, presents some peeuliar features not found in any other rocks in the Peninsula. It has a brownish black colour, with a somewhat waxy lustre, and resembles a palagonite tufa, from which it is, however, distinguished by its high amount of silica, which would place it with the trachytic rocks; small crystals of felspar with a high vitreous lustre are disseminated through it. The small (not uncommon) cavities or fissures are either filled or lined with sphærosiderite, and some of the joints are covered with a fine coating of the same mineral.
Returning to the orifice or orifices from which the material for the formation of the caldera wall was ejected, and to which also the numerous dykes, mostly having a vertical position, intersecting it, can be traced, it appears that the principal focus of eruption was situated a little to the south-west of Quail Island, as the greatest portion of the dykes radiate from here, and the eastern and southern sides of Quail Island, and the shores near Charteris Bay, are formed of tufaceous agglomeratic and brecciated beds, in which a number of angular blocks of rock are enclosed, having all a very bleached appearance. Many of these blocks are trachytic or porphyritic, others are porcelain jaspers and chalcedonies, and the whole has the peculiar altered look of rocks which have for a long time undergone the action of gases, vapour, and heat at the mouth of a volcano. The whole is so intersected with dykes, between which in many instances the bed rock has been washed away, that they look like remnants of the spokes of a gigantic wheel, of which the centre was situated at the spot close to Quail Island, as previously pointed out.
Another centre of eruption is close to Manson's Peninsula, which consist of quartziferous porphyry. A great number of dykes can be traced crossing that Peninsula, having the peculiarity that they generally stand well out in relief on its western or outer side, whilst on the eastern or inner side they have been washed out; these dykes are of various thicknesses, ranging from 3 to 25 feet, and have mostly a vertical position, forming protuberances all along the ridge. For the greatest part they radiate from a point in the centre of the shallow bay, situated east of that Peninsula. However, near its northern point the system of dykes becomes more complicated. First, two dykes five and six feet broad, cross each other, of which, the one pointing page 334towards Quail Island is the younger. Close to the end of the Peninsula, the crossing point of three dykes of considerable size is reached, the oldest pointing to the centre of the bay, the second towards Raupaki, and the third or youngest again towards Quail Island. At the extremity of the same Peninsula there are such a number of dykes intersecting each other in all directions, that it is impossible to trace their relations in detail, without devoting considerable time to it. However, it is evident that not far from this spot all the principal centres of eruption seem to have been situated It is here also that some of the dykes divide into several branches" and others anastomose repeatedly. The most striking fact in connection with the system of dykes of the Lyttelton caldera and to which I have devoted considerable attention is their size longitudinal extent, and constancy in direction. From the researches of numerous observers, it has been proved that all the dykes of Mount Vesuvius and Mount Etna do not extend much beyond the centres of eruption, so that they advance only a short distance, and rapidly thinning out, soon disappear, a fact which my own observations along the crater walls of both mountains have amply confirmed However, I have no doubt that other volcanoes similar in construction to Banks' Peninsula, and differing as considerably from these two European volcanic mountains, will be found to possess their systems of dykes developed in the same manner. During a number of years, it has been well ascertained by me that the dykes radiating from the several centres of eruption situated not far from each other, continue in many instances without notable interruption from the former mouth of the crater to the outer slopes of the caldera, where they disappear below the sea, or under the deposits now forming the Canterbury plains. Very often the principal dykes rise nearly 2000 feet above the sea level They are well visible from the harbour to the summit of the rim of the caldera wall, above which, in some instances, they stand prominently as a wall, often six or eight feet high. Where proper measurements of the same dyke can be obtained for a long distance, it has been found that generally, as it advances towards the outer circle, it diminishes in breadth; however, in other instances, this is not the case, as repeatedly I have found some which, after narrowing on their outward course considerably enlarge againbef ore reaching the foot of the caldera. Thus to give a few examples, the large dyke of trachyte, which is crossed in the railway tunnel, about 29 chains from the Heathcote end, is first seen west of the town of Lyttelton, near Naval Point, where it is nearly 40 feet thick. On the summit of the caldera wall, not far from the page 335top of the Bridlepath, it has narrowed to 23 feet 9 inches after which it gradually gains in proportion, so that in Thompson's quarry it has enlarged to 26 feet, a breadth which it still has in the tunnel. A mile, beyond the quarry the spur along which its course can be followed, runs out in the Heathcote valley, where it disappears below the Loess.
Two remarkable dykes, reaching the summit of Dyke Hill, about 2000 feet high, west-south-west of Castlehill, are very conspicuous. They both project boldly from the mountain, with a space of 35 feet between them. The eastern one is 18 feet, and the western 12 feet broad. Two similar dykes exist on the opposite side, and run up the caldera wall behind Raupaki. To mention a few others, there are some important dykes south of Dyer's Pass, which, after crossing Manson's Peninsula, are again met with at Ohinitahi (Governor's Bay), and of which several, after ascending to the very summit of the caldera, reach to the foot of the Peninsula near Cashmere, being extensively quarried in different localities along their course. These dykes, like many others which cross the caldera wall towards the Canterbury plains, mostly all radiate from a point lying in the centre of the Bay, formed by Manson's Peninsula on the one side, and Potts' Peninsula on the other, both of which consist of quartziferous porphyries and Between which this newer focus has been formed after the greatest portion of the caldera wall had already been built up. There is also the large dyke which crosses the Lyttelton-Sumner road at right angles, on the very summit of Evans' Pass, and which is repeatedly passed by the road winding in and out the different bays before reaching that Pass. It can be followed to Taylor's Mistake. Everywhere along the sea cliffs at and near the entrance of Lyttelton Harbour, numerous dykes, mostly all in a vertical position, can be seen pointing towards the centre of that harbour. A few, however, stand in a • slanting position, and others have a tortuous course. As one of the remarkable changes which some of the dykes have undergone since their formation, I may also mention one which is well exposed in the seacliffe at Ohinitahi, Governor's Bay; here, a dyke of domite, about nine feet broad, crosses in a nearly vertical position the so-called trachyte sandstone deposited on the slopes of the quartziferous porphyry. After its solidification, a new fissure, about three feet broad, has been formed parallel to its direction, and running along its centre which has been injected from below by domitic matter, but slightly different from the former however, instead of continuing to the top page 336of the cliff, about twelve feet above the sea-level, the new dyke is seen to turn from its vertical, to a nearly horizontal position, and to thin out considerably at the same time, disappearing altogether when it touches the side wall of the bed rock. The older dyke, above this change of direction, is considerably shattered and broken.
Before proceeding, it will perhaps be useful if I offer a few remarks on the causes which led to the formation of these remarkable dykes. I consider this the more important, as nowhere, as far as I am aware, do they exist in such great numbers, nor do they possess such a large longitudinal extent, as in the volcanic system under consideration. It appears to me that the immediate cause of the formation of a radiating system of dykes may be traced to the choked up vent or chimney of a volcano, the mouth of which, after an eruption of considerable dimensions, is thoroughly filled up, either by its sides falling in, by the cooling of ascending lava-streams, or by both causes combined. When, from abyssological origination, masses of steam and gases have collected below this vent, and new matter is ready to be erupted, an enormous effort of nature will be necessary to clear out the old, or form a new chimney, which cannot be accomplished without a series of violent earthquakes, succeeded by an enormous explosion, by which the mouth of the volcano is cleared out or newly formed, and of the magnitude of which we can scarcely form a conception. A. similar effect, on a gigantic scale, must have been produced repeatedly by the compressed masses of gases and steam during the formation of the Lyttelton caldera wall, when the upper portion of the closed-up volcano was not only removed, but vast, quantities of ashes, scoriae and lapilli were thrown out, together with lava-streams which flowed in various directions. Before, or during these eruptions, molten matter in a high state of fusion, generally rushed up in the fissures which had been formed at the time, radiating from the focus like the spokes of a wheel. An examination of these dyke rocks will show at a glance that most of them are quite different in composition and character from those of which the lava-streams have been formed. The latter, as already explained, with one notable exception, alluded to on page 332, all consist of true basic rocks basalts often assuming a doleritic texture, the dyke rocks being generally acidic, having either the composition of a trachyte or domite. We are able to judge of the more or less high state of fusion in which the molten matter ascended the open fissures from the effect produced on the walls on both sides. The trachytic matter forming the dykes, which are principally developed on the eastern side of the caldera wall, page 337has evidently been in such a condition that it could exercise a most powerful effect on both walls of the fissure, the rocks often, for several Inches, being changed to tachylite, a peculiar basic volcanic glass, quite distinct from obsidian. This change in the character of the rock is most observable when the dykes pass along tufaceous or agglomeratic beds. Here the reddish or light purple rocks have been altered to a black vitreous mass, containing small crystals of felspar. The domitic dykes, mostly confined to the western half of the caldera wall, seem not to have excercised such a great influence as the former, as in most instances the walls on both sides of the dykes are only slightly hardened. However, there is no constant rule; large dykes, as for instance, the huge domitie dyke at Governor's Bay, running for a considerable distance parallel to the coast, and forming such a conspicuous object along the picturesque beach road, lately constructed, has scarcely made any alteration on either side, whilst smaller dykes of the same rock, only a few feet in thickness, are sometimes accompanied by a welldefined selvage of tachylite. The same may be said of the basaltic dykes of which, however, by far the greatest part has caused no visible alteration along the walls on either side. The trachytic varieties, of which, most of the dykes on the eastern side of Lyttelton Harbour consist, are formed generally of a peculiarly lustrous and flaky rock, sometimes vesicular with small crystals of sanidine. This rock has a light greyish colour, and its small cavities are lined by sphærosiderite, On both sides of the dyke the rock is generally tabular—parallel to the direction of the flow, and is massive in the centre with polyhedric joints, of which the principal ones appear at right angles to the flow. There are also a few trachytic dykes, principally small ones, where the sides, for half an inch to one inch, consist of a rather brittle obsidian, doubtless the effect of rapid cooling. Some very thin thread-like dykes, about one to two inches thick, consist entirely of that peculiar form of acidic volcanic rock.
The chemical analyses of the same dyke, No. 17, of Carl Ritter von Hauer, and of 29a centre, and No. 29b side, made in the Colonial Laboratory are very instructive, because they show us that there is a great difference in the composition of the same dyke, if the specimens to be analysed are taken from different localities. The specimen analysed in Vienna in 1863 was obtained from near the summit of the hill, whilst the two others were taken from the tunnel shortly after this dyke had been crossed by the miners. It is interesting to observe, that although the appearance of the rock is in every respect page 338similar in both localities, the chemical constituents yary so very considerably. Thus, whilst the specimen from near the summit is very rich in lime and almost wanting in alkalis, those from the tunnel contain only the usual percentage of the former mineral, and the amount of soda is even in excess of that usually obtained in these acidic rocks. The other rock, of which mostly all the dykes towards the head of the harbour are composed, is of a domitic nature. It has a. whitish claystone matrix, with a number of small crystals of felspar (sanidine). The dykes themselves show a tabular arrangement on both sides, parallel to their direction, the centre being divided first by polyhedric joints, of which the horizontal ones are the most prominent. Each prismatic portion thus formed has generally a spheroidal concretionary structure, the central nucleus consisting of a very hard kernel. This latter arrangement is beautifully shown when the spray of the sea has had access to the dykes, through which process decomposition has been much accelerated. A number of gigantic rosettes have been formed, in which yellowish and pinkish tints bring out more vividly the remaining white colour of the rock, other portions being veined and stained by a dark yellow ferruginous colouration. Fine examples of these rosettes are to be seen in Governor's Bay, near Mr. Potts' residence, and close to Little Raupaki. Some dykes of a more compact nature consist of a greyish rock containing small crystals of sanidine, others have to be referred to the trachydolerites, of which the principal mass is dark ash grey, with numerous crystals of felspar, of which many are striated (oligoclose?), and in which smaller crystals or grains of hornblende are embedded. The dykes Nos. 44 and 55 passed in the tunnel, and of which an analysis has been made in the Colonial Laboratory, belong to this class.
There are also some dykes, often of great magnitude, consisting of a basaltic rock, of which that remarkable dyke at Sumner near Morton's Hotel, is a well-known instance. Here the face of the nearly vertical cliff, by which the spur terminates, is nearly 200 feet high, and consists of a series of scoriaceous lava-streams and agglomeratic beds, from their nature having offered much less resistance to the waves of the sea, than the dyke passing through them. This dyke has been left standing free for a distance of about 100 feet, and is six feet six inches broad, striking from N. 20° E to S. 20° W., reaching to the summit of the cliff; but higher up the spur it cannot any longer be traced, so that in all probability it does not reach to the summit of the caldera wall. Taken along its whole face page 339it stands vertical, although its course is a little tortuous and wavy. I add a faithful sketch of the locality on Plate, of sections No. 6, from the pencil of Mr. E. Dobson, C.E., from which it will be observed that this dyke forms a conspicuous object in the landscape. The first owner of the ground used it for a wall, and has broken a doorway through it. It consists of a hard tabular basalt with crystals of hornblende and labradorite, and sounds like a clinkstone when struck with the hammer. The tabular arrangement is parallel to the flow, with no cross divisions at right angles to it, except on both sides, where the rock has a polygonic structure (see sketch on the same plate), the horizontal divisions being four to six inches deep.
Another dyke of a similar character, reaching to the very summit of the caldera wall, is the one in which Mr. Fred. Thompson opened up a quarry many years ago, and which afterwards has been worked by Mr. Ellis. This dyke, having a direction of nearly west and east and pointing towards Little Quail Island, is about 18 feet thick, and like many others, is not quite vertical, having a dip of 85 deg. towards the north. It stands several feet above the ground, forming quite a rocky wall, and consists for the first 20 feet from its summit of a grey porphyritic rock full of crystals of labradorite and basaltic hornblende. It is a handsome building stone, being much liked in Christchurch, and amongst other buildings it has been used in the erection of the Bank of New Zealand, and for ornamental work in the Provincial Council Chamber. On the southern side, the dyke has a tabular structure for 6 feet 9 inches, and on the northern for 5 feet 6 inches, the centre consisting of a homogeneous mass, which can be quarried frequently in blocks of large size, indeed as large as 10 feet by 6 feet. In quarrying downwards, the rock gets gradually darker in the centre, and after a short distance assumes the character of a black dolerite, containing crystals of labradorite, augite, and hornblende, and is of such hardness that it is useless for building purposes; the sides however, still continue for some distance lower down to consist of the same greyish dolerite, of which higher up—to use a quarryman's expression—the heart had been formed (see section, plate 6). It is thus evident that the sides and upper portion, by cooling more rapidly and not being subjected to so great a pressure, could assume a more porphyritic texture than the centre of the dyke, resembling the basaltic rock of which the main mass of the larger lava-streams is formed.page 340
In studying the position of the dykes it becomes manifest that they have been formed at different times; however, the altitude of their uppermost portion does not indicate their age. I have no doubt that many of them, which scarcely reach above high-water mark, are not older than others of the same petrological nature, which reach to the very summit of the caldera wall. In the present state of our knowledge it is impossible to solve this interesting question in all its bearings, and I can therefore only suggest that dykes containing rocks of exactly the same lithological character have most probably been formed during the same eruption. It is also evident that a number of dykes were formed long before the whole of the caldera wall was built up, and that they were partly destroyed during one of the nest eruptions. One clear instance of the occurrence of such older dykes is to be found near Cliff's Cove in Lyttelton Harbour, where several trachydoleritic dykes were injected when the rest of the caldera wall was at least 1000 feet lower than at present. They pass through a basaltic lava-stream, which latter was afterwards partly destroyed along with them, the whole possessing now a nearly straight surface, upon which a large bed of agglomerate has been deposited. There are a great many instances of this kind. However, what is of the greatest interest in the history of the volcanic systems under consideration is the predominating acidic character of the dykes when compared with the basic lava-streams. In Vesuvius and Etna all the dykes are formed by the same kind of rock as the lava-streams are composed of, but they are generally more compact, having, as Lyell suggests, cooled and consolidated under greater pressure. It is evident that they owe their existence to the same subterranean efforts by which the lava-streams were ejected from the mouth of the crater, the fissures in which they were formed being evidently filled up from the same focus, and about the same time as the eruption of the lava-streams took place. But such a simple process cannot be admitted for the greater portion of the dykes of Banks' Peninsula, which must owe their existence to paroxysmal perturbations in the earth's crust, distinct from those during which the caldera walls were built up. It is evident that a great portion of the lava-streams and agglomeratic beds which once formed the crater of the volcanic system of Lyttelton Harbour, must have been blown away, or at least removed during one of those violent outbursts of subterranean forces necessary to clear the choked vent of the volcano similar to those by which in recent times the upper portions of active volcanoes have repeatedly been destroyed under the eyes of the trembling population in the neighbourhood.page 341
For an explanation, we might go back to Durocher's views, that all igneous rocks, even the most modern lavas, are derived from two distinct magmas which co-exist below the solid crust of the globe, each of them occupying a well-defined position. According to this distinguished French chemist, the uppermost portion is occupied by the acidic magma, which, besides being of lighter specific gravity, possesses a larger amount of silica and less iron oxyde than the other or basic magma. From the upper layer the granites, porphyries, and trachytes, according to his views, are derived, the zone of contact producing rocks of an intermediate character, such as trachydolerites. If this theory is correct, we have to admit that not only the dyke rocks were injected in rents formed during earthquakes, or immediately before volcanic eruptions had taken place from the opened chimney of the volcano, but that in each case the molten matter was furnished both from the upper and lower stratum of incandescent matter below the hard crust of the globe. There is, however, one great difficulty which crops up here, and which I wish to point out, and that is, the presence of dykes of basic rocks and of others of an intermediate character. If all the radiating fissures without exception had been filled up by acidic rocks, this would go far to prove the existence of such an upper acidic incandescent magma; in which case we should be forced to the conclusion that the chimney of the volcano reached lower down to the lower or basic layer. But it is difficult to understand how all the radiating fissures over an area of 12 miles in diameter could pass through the solid crust of the earth and through the fluid acidic magma, and how the lower basic rocks could be injected into them from below without disturbing the acidic magma, which certainly should have been forced up before. This difficulty might, however, be met by the suggestion that the radiating fissures in this instance did not reach so far down as the fluid acidic magma, and that the material for the formation of the dykes had been furnished from the crater itself, but it is scarcely conceivable that for a distance of six miles and for an altitude of several thousand feet the molten matter would have been forced in all directions from the central axis of eruption along these fissures often only a few feet wide. Mr. E. Mallet, in the "Transactions of the Royal Society (Phil. Trans. 1873,") has proposed another theory, namely, that the principal cause of vulcanicity is to be sought in the compressing and crushing action taking place beneath the crust of the earth, and by which such a great amount of heat is generated that a. fusion of rocks, often on a large scale, is easily produced. This theory page 342would so far explain very well the difference in the composition of the rocks varying according to the depth where the crushing action was actually taking place; thus, if the same action were to act upon granites, trachytes, and other acidic rocks, the result would be the production of trachytes, whilst if basic rocks were fused, basalts would ascend towards or to the surface. Here, however, another great difficulty presents itself in the fact that, although the number of volcanic eruptions during which the caldera walls were built up, must have been very great, no trachytic lava streams, with one single exception, have made their appearance, the whole series being of a basic, whilst most of the principal dykes are of an acidic nature. In such a case, the crushing of acidic rocks would have exclusively taken place when the dykes were being formed, and never when lava-streams issued from the crater's mouth, which is altogether improbable.
Although I have carefully read every work accessible to me in English, German, French, and Italian, treating on vulcanicity, I have not been able to find either any account of the existence of dykes in other volcanic regions converging so regularly to a few centres close to each other, or continuing over such a large area, (always keeping the general direction with which they set out,) as do those of the Lyttelton caldera, or again offering an explanation for the difference in the composition of the dyke rocks when compared with the lava streams or agglomeratic beds through which they pass. Mr. R. Mallet's excellent paper on the " Mechanism of Production of Volcanic Dykes," and of those of Mount Somma, in the " Quarterly Journal of the Greological Society of London, No. 128, Nov. 1876," in which an exhaustive account of the physical features of the dykes in the old caldera wall of Mount Vesuvius is given, unfortunately does not contain any physical theory to account for the mode by which fissures are produced, forming, when filled, volcanic dykes. If we take the heterogeneous nature of the material of which the caldera wall has been built up into account, it is astonishing that the dykes show such a remarkable regularity, always starting from a few points not far from each other, from which they radiate in all directions. It is still more remarkable to observe that all dykes which are cut by the Christchurch and Lyttelton railway tunnel have such a constant direction that they all, with one or two exceptions, appear to converge to one single axis behind Quail Island, a fact worthy of note, if we consider the distance, which is more than four miles, measured to the most distant dyke in that tunnel. The only dyke with which I am page 343acquainted, showing some remarkable irregularity, is the one in which the so-called Ellis Quarry is situated. This dyke, which strikes nearly east and west, goes out about 400 feet below the summit, where a saddle intersects the spur. Shortly above its lower termination it sends off a smaller branch in a south-west direction, also ceasing after a short course. Whilst the main dyke does not appear any more above the surface, the smaller south-western branch crops up again on the other side of the depression, now gradually changing its direction, so that, in its lower course, about 300 feet above the plains, it crosses the spur in a south-east and north-west direction. The whole system of dykes in the Lyttelton caldera wall is thus very different from the dykes of Mount Somma, of which, in his paper, Mr. R. M. Mallet gives us such a lucid and suggestive account, and of which many are fractured, displaced, and crushed, and have at the same time a wedge-shaped form. We can therefore safely assume that the fissures and dykes in the Lyttelton caldera were only formed after the latter had been so thoroughly consolidated that, after the formation of the fissures and their filling up by the principal dykes, no more changes of any importance took place in them; and that, moreover, the forces by which the walls of the volcano were starred from top to bottom, must have been far deeper seated and more effective than the agencies by which Mount Somma was rent.
Leaving for the present the Lyttelton caldera, the genetic history of which will also serve to explain the mode of formation of the other calderas of the volcanic system under consideration, I shall now proceed to treat of another volcanic focus, either contemporaneous with the former, or at least formed shortly afterwards. Of this second centre of eruption, which is so greatly destroyed or hidden by lavastreams and agglomeratic beds of younger age, ejected from other Volcanic foci, that only a portion of the western caldera can be made out, the Tent was situated somewhere in the valley of the Little River. All the rocks are similar in character to those of the Lyttelton caldera.
The system next in age, and distinguished both by its size and the splendid preservation of its nearly entire wall, is the Akaroa caldera. I should only have to repeat myself were I to give a description of the lava-streams, their mode of deposition, and of the beds of agglomerate, ashes, and tufas interstratified with them, by which this caldera wall has been built up, as it resembles closely the one at Lyttelton. The lava-streams consist also of a basaltic page 344rock, but sometimes a little different from that obtained in the former, it being more silky in appearance and containing more olivine. When porphyritic, the crystals of labradorite and augite are well formed; the scoriaceous lava is not so porphyritic, and has generally a reddish colour. Most of the ashbeds are also different, being of greater thickness before they alter to agglomerates, and having a peculiar chocolate purple colour. Only one other similar bed is found in the Lyttelton caldera, at Quail Island, the last centre of eruption. The tufaceous beds contain sometimes small portions of wood, so much altered as to assume a nearly anthracitic character. Iron pyrites (marcasite) has also been found in the same deposits. Most of the secondary minerals examined from the Lyttelton caldera have also been collected in Akaroa Harbour Another distinguishing feature in this harbour is the preservation of a portion of the sides of the old crater, which reaches to the very centre of the volcanic vent, from which the whole caldera wall around it has been built up. This portion forms a peninsula, owing doubtless its preservation to the existence—if I might thus express myself—of a core of a peculiarly hard lava, which has been found nowhere else in Banks' Peninsula, consisting of a very granitoid trachyte, containing crystals of quartz and sanidine, and forming at the southernmost point of the Akaroa Peninsula a hill several hundred feet high. Between this hill, which in former times was crowned by a strongly fortified Native pa, and the caldera wall, a succession of tufaceous and agglomerate beds, mostly of reddish-brown or purple tints, have been preserved, and, as it is perfectly evident, have repeatedly undergone great changes. They are intersected by numerous dykes belonging to two systems, of which one consists of domitic rocks, running mostly from north-west to south-east, the other of basaltic rocks having generally a south-west and north-east direction. They are generally not quite vertical, and have sometimes a slightly tortuous course. Beautiful sections of the innumerable lava-streams and the agglomeratic and other beds of similar origin of which this system has been built up can be obtained all around Akaroa Harbour and in the barranco leading into it. The walls of this caldera have an average altitude of 2500 feet, rising in Saddle mountain to 2750 feet above the sea-level. Below that mountain a portion of the older caldera wall of the Little River vent is hidden; the Devil's Peak, 2050 feet high, lying to the south-west of the former mountain, being doubtless a remnant of the latter system.page 345
After the formation of the Akaroa volcano, a long space of time must have intervened, so that great changes by denudation and disintegration could take place before new eruptions followed at different intervals. It is very probable that during that period of comparative repose the barraneo leading into the Lyttelton caldera was forming, and that the upper portion of the quartziferous porphyries, with the quartzose sandstones resting on their flanks, both forming a portion of the caldera wall where now Gebbie's and McQueen's Passes are situated, together with the lava-streams and agglomeratic beds on both declivities were partly removed.; of the latter some remnants are still to be found near Lake Ellesmere, where the road crosses Gebbie's Flat. After this period of quiescence new eruptions took place from two principal centres, of which the best preserved and highest is situated near the summit of Mount Herbert. It rises on the southern side of Lyttelton Harbour, while the other is found north of the Akaroa caldera, the highest remaining portion being designated Mount Sinclair (2800 feet). I am unable to say which of the two is the earliest, but I shall first speak of the Mount Herbert system, that being the most conspicuous. Several new vents were opened near the rim of the Lyttelton caldera, by which a volcano was built up by lavas-treams, ashes, and other ejecta, rising much above the remnants of the former system. Although much disintegrated, the remaining portions of this newer volcano still rise in Mount Herbert to 3050 feet, and in Castlehill to 2900 feet—the altitudes are taken from the Admiralty chart, with which my own calculations from barometrical observations closely agree. These remains of the newer volcanic cone consist of basic rocks, agreeing much more with those of the Akaroa than with those of the Lyttelton caldera. These basalts are fine-grained, with a peculiar silky lustre, and contain sometimes large crystals of labradorite and small needles of basaltic hornblende. They have often a tabular jointing. Others are lighter in colour, and resemble some of the South American andesites. The scoriaceous lavas have generally a brownish colour with a reddish tinge. Descending for a considerable distance from the summit of both mountains, we find that the lava-streams lie nearly horizontal, and are interstratified with beds of agglomerate, tufa, and laterite. The agglomerates are generally formed of more or less angular pieces of rocks ejected from the volcanic vent, but in the one instance alluded to previously, there occurs a bed at the base of the newer volcanic deposits, consisting of rounded boulders or pebbles, mostly of small size, the origin of which is not yet quite clear to me. These fragments might either have been page 346rounded by attrition during their ascent from the volcanic vent and before they had found a final resting place, or they might have been rolled in a water-course descending from near the summit of a volcanic cone, of which at present the remains are only to be found in Mount Sinclair, which in that case would be of greater age than the Mount Herbertsystem. The principallava-streams which issued from this volcano, and have been preserved, run in a northerly direction into Lyttelton caldera, and can be followed from the summit of Mount Herbert to the sea. They form that remarkable assemblage of streams bounded on one side by Rhodes' Bay and on the other by Charteris Bay, and it is in following them from Diamond Harbour that the summit of Mount Herbert—where the most extensive panoramic view of this part of New Zealand can be obtained—is reached with the least difficulty. The remnants of several craters on and near the summit of Mount Herbert, of which about one-half of the rim in each case has been preserved, can easily be seen; the principal ones are open towards Lyttelton Harbour and the Kaituna valley.
The other system, of which the remaining caldera wall rises in Mount Sinclair to 2800 feet, intersects both the Little Eiver and Akaroa calderas. Its barranco at the entrance to Pigeon Bay leads into the sea. It consists of rocks similar to those described as forming the Mount Herbert system, and its relations to the latter are rather obscure and complicated, as the lava-streams of both mis with each other, and owing to dense forest, generally clothing the slopes, no clear sections can be obtained to settle several important points in connection with them. It is an important fact, that the formation of dykes ceased after the older caldera walls of Banks' Peninsula had received their present form, and before the Mount Herbert and Mount Sinclairs ystems had been built up. All the lava-streams belonging to these two systems, even on their very summits, have never been fissured in any way, so as to prove that the volcanic energy by which the dykes were formed had already spent itself, or at least, if still existing, could not reach so high as to bring its effects under our observation.
To facilitate their task to those of my readers who have an opportunity of examining the remains of both volcanic systems now forming Lyttelton Harbour, I shall here offer the necessary data to distinguish between them. The older caldera wall beginning on the northern side of Gebbie's Pass rises soon to a considerable altitude, forming seven prominent peaks, as far as Dyer's Pass (957 feet). They are called page 347the Seven Brothers, of which the two highest (the Knobs) reach an altitude of 1880 feet. On the northern side of that pass Cass' Peak rises to 1660 feet, and from here to Mount Pleasant (1615 feet), of which the summit is situated behind Lyttelton, the caldera wall is much broken, one of the principal saddles being the Bridlepath (1080 feet) formerly much used by riders and travellers on foot between Lyttelton and Christchurch. Between Mount Pleasant and One-tree-hill (2310 feet) on the southern side of Lyttelton Harbour, the barranco or entrance passing through the caldera wall has been formed. Separated from the last-mentioned mountain by a saddle about 1200 feet high, appears Rhodes' Sugar Loaf (2005 feet), of which the western continuation, after half a mile, disappears under the slopes formed by the lava-streams of Mount Herbert. Then follows the newer Mount Herbert and Castlehill system, terminating in a precipitous slope on the western side of Kaituna Pass. From here to the southern side of Gebbie's Pass opening, the older caldera wall, rising again to a considerable altitude, continues without any other break, falling abruptly towards that latter pass.
We arrive now at the last stage of volcanic action, of which any clear signs have been preserved, and which again took place in the very centre of Lyttelton Harbour. This time the new eruption was doubtless of submarine origin. It appears that the country had sunk gradually, at least 300 feet below the sea-level, when from the present centre of Quail Island and in the neighbourhood, eruptions of basaltic rocks repeatedly took place, the lava-streams being generally separated by beds of conglomerate which were formed between the eruptions. The magnificent sections open to our inspection on Quail Island give us a good insight into the modus operandi, and I have added, on plate No. 6, two illustrations of them to those showing the more general characteristics of Banks' Peninsula. The oldest portion of that picturesque island, the vertical cliffs of which towards Lyttelton form such a conspicuous feature, is its eastern side, where the island narrows in the centre. Here a series of beds have been preserved, with all the appearance of having been deposited close to the mouth of a volcano. They consist of ashes, tufaceous beds, and breccia, enclosing blocks and fragments of rock of all possible shapes, but which have all undergone such considerable changes that their lithological character has been considerably altered. There are many fragments resembling porcelain jaspers, some are chalcedonic, others, are quartziferous porphyries, trachytes, and other eruptive or volcanic page 348rocks; but the whole is so bleached or altered in colour by gases and vapour, that the different constituents are unrecognisable, so that their original texture can only be guessed at. The whole is intersected by a perfect network of dykes, all of a trachytic nature, of which the greatest number point to two centres, both a little to the south-west of Quail Island, although a few also come from the west. Before reaching the north-eastern point the whole group disappears below a series of basic lava-streams, which have all issued from a shallow crater now forming the centre of the island. The rock by which this small volcanic system, and a few others of still smaller size situated in and near Charteris and Rhodes' Bays, have been built up, consists of a fine-grained basalt containing grains of magnetic iron ore, and grains and concretions of olivine. It has either a columnar or tabular structure, or breaks in large prismatic blocks. These basaltic lava-streams have been well exposed by denudation on the north-eastern face of Quail Island. The vertical cliff, about 200 feet high, consists here of four lava streams, of which the uppermost and most important one is about 45 feet thick. This lava stream has the peculiar feature of possessing only irregular prismatic jointings, where it issues from the vent with a fall of about 15 degrees. For the rest of its course, where it has only a slight inclination towards the horizon, it is divided into two clearly defined parts, of which the lower one lying directly upon a bed of rounded conglomerate is divided into a series of regular vertical columns having four to six faces, whilst the upper one has only the irregular prismatic joints, which the whole stream possesses close to the crater's mouth. The lower lava-streams are also jointed in the same manner, the whole series being separated by beds of conglomerate and ashes. The conglomerate consists mostly of waterworn blocks of rock, sometimes of large size. All the rocks of which Banks' Peninsula is built up have contributed, but trachytic and domitic and bleached or altered rocks are the most numerous, proving that a greater portion of the older crater wall must have been existing when these last eruptions took place. The newer basalts, of which the streams themselves are composed, are, however, also well represented, whilst the older basic rocks, of which the original caldera wall has been formed, are, if not entirely wanting, at least of very rare occurrence. Many of the boulders have the appearance of having been much decomposed before they were deposited in their present position. They are interstratified, covered or mixed up with beds of ashes and scoriæ, from which we can conclude that the lava-streams were preceded by a clearing out of the vent, during which the ashes and scoriæ were ejected. The presence page 349of a bed of chocolate-coloured tufa, resembling some of the so-called aqueous lava-streams (lava d'aqua of Italy), reposing directly upon the remnant of the older crater wall, shows that similar causes, as obtained in the ancient Kingdom of Naples, were also here in operation.
I examined repeatedly the conglomeratic and agglomeratic beds in question for fossils, but in vain, not the least sign of marine shells or other organic life being in existence. The vent of the last eruption lies just below the remnant of the ancient crater wall, exposed in the north-eastern face of Quail Island. The loose agglomerate beds, of which it consists, are mostly of a red colour, and, like the domite dykes by which they are crossed are much altered. The chocolate-coloured tufa bed on the western corner of the cliff is covered by a large conglomeratic deposit, similar in its nature to those which are interstratified on the eastern side with tbe basaltic lava-streams. It rises to the summit of the island, and is overlaid by a number of streams of basalt which form the whole western side of the island, with the exception of a very few spots where remnants of the older crater are exposed to our view. After the formation of Quail Island, and the few other small centres of basaltic discharges already pointed out, no other volcanic eruptions seem to have taken place, Banks' Peninsula rising afterwards a few hundred feet and maintaining with the exception of small oscillations, its present level; at the same time it remained surrounded by the sea for a considerable period. There is clear evidence, as I shall point out in the sequel, that even during its occupation by an autochthonic population, the straits round the island had not yet been closed by the driftsands and shingle advancing from the south. During this period, without doubt of long duration, the deposits of loess were formed on its slopes of which I shall treat in one of the next chapters.page 350
|No.||Name, according to Dr. Hector.||Sp. G.||Silica.||Alumina||Iron, oxides.||Manganese. oxides.||Lime.||Magnesia.||Potash.||Soda.||Loss by Ignition||Name, according to Geological Position and Lithological Character.|
|180||Trachytic porphyry (vesicular, with iron salts in cavities||2·374||65·57||15·67||5·98||traces||2·88||traces||(9·||28)||·62||Tufaceous lava|
|218||Porphyritic claystone (porphyry or phonolite)||2.453||62·15||22·11||5·37||1·20||1·20||·40||6·38||1·19||Trachyte dyke from centre|
|234c||Scoriaceous trachyte||2·507||61·99||13·08||8·65||4·42||2·21||traces||1·61||4·22||3·82||Vesicular trachyte, fragment from an agglomerate bed|
|29b||Trachyte||2·590||61·38||20·60||2·57||1·19||2·18||·40||—||9·70||1·98||Trachyte dyke, from side|
|29a||Porphyritic trachyte||2·374||60·69||17·75||3·83||1·21||1·20||1·43||Traces||13·10||·79||Trachyte dyke, from centre|
|227||Ferrocalciferous phonolite||2·303||54·18||19·25||8·99||3.16||3·13||2·07||3·77||3·98||1·47||Phonolitic boulder from an agglomerate bed|
|55||Porphyritic basalt …||2·608||53·55||13·79||15·41||—||3·45||2·64||(10·||35)||·81||Porphyritic basalt|
|183||Amygdaloidal claystone||2·352||53·47||16·35||9·23||traces.||3·65||1·73||4·42||5·77||5·38||Trachydolerite dyke|
|196||Porphyritic dolerite||2·797||53·03||18·01||(5·||18)||7·24||3·10||(11·||79)||1·65||Porphyritic basalt|
|191a||Vesicular dolerite||2·314||47·24||14·23||16·60||—||7·71||3·16||1·97||2·37||6·72||Porphyritic basalt|
|10||Vesicular claystone (ferrocalciferous)||2·723||46·79||17·46||15·52||2·22||7·33||3·17||·44||5·05||2·02||Porphyritic basalt|
|5lA||Bole||2·089||44·78||15·66||16·87||·60||2·02||5.02||(2·||69)||12.36||Bole, partly altered to a laterite|
|No.||Name, according to Dr. Hector.||Sp. Gr.||Silica.||Alumina||Iron||Manganese.||Lime||Magnesia.||Potash.||Soda.||Loss by Ignition||Name, according to Geological Position.|
|218||Porphyritic claystone (porphyry or phonolite)||2·453||52·78||12·34||16·87||—||7·87||traces||—||3·40||6·74||Trachyte dyke|
|29a||Porphyritic trachyte||2·374||44·41||11·10||19·53||4·14||6·95||1·39||—||6·93||5·55||Trachyte dyke, from side|
|29b||Trachyte||2·590||37·15||25·63||6·41||5·11||7·70||traces||—||5·18||12·82||Trachyte dyke, from centre|
|10||Vesicular claystone (ferrocalciferous)||2·723||28·97||9·67||37·94||7·59||4·15||4·84||—||—||6·84||Porphyritic basalt|
|No.||Name, according to Dr. Hector.||Sp. Gr.||Silica.||Alumina||Iron. oxides.||Manganese. oxides.||Lime.||Magnesia.||Potash.||Soda.||Name, according to Geological Position.|
|29b||Trachyte||2·590||65·81||19·68||1·87||·47||1·17||·48||—||10·52||Trachyte dyke, from centre|
|218||Porphyritic claystone (porphyry or phonolite)||2·453||64·16||24·20||2·90||1·46||—||·26||—||7·02||Trachyte dyke|
|28||Trachydolorite …||2·129||63·98||10·75||2·42||traces||6·73||4·03||—||12·09||Porphyritic dolerite|
|29a||Porphyritic trachyte||2·374||63·39||18·85||1·23||·72||·25||1·43||—||14·13||Trachyte dyke, from side|
|10||Vesicular claystone (ferrocalciferous)||2·723||54·18||20·70||6·20||—||8·67||2·48||·62||7·15||Porphyritic basalt|
The specimens analysed were taken from the following localities:—
- No. 180 Tufaceons lava, from a stream about 10 feet thick from Lyttelton end.
- No. 218 Trachyte dyke, rapidly thinning out in the tunnel, 21½ chains from Lyttelton end.
- No. 234c Fragment from agglomerate bed No. 334, vesicular trachyte, section No. 1.
- No. 29b Trachyte dyke, from side, tabular jointing, section No. 5.
- No. 29a Same dyke, from centre.
- No. 227 Phonolitic rock, fragment from an agglomerate bed, 16½ chains from Lyttelton end.
- No. 55 Porphyritic basalt, from a large stream, 42 chains from Heathcote Valley end.
- No. 44 Trachydolerite dyke, 18 feet thick, 13½ chains from Heathcote end.
- No. 183 Trachydolerite dyke, about three feet thick, 38½ chains from Lyttelton end.
- No. 196 Porphyritic basalt, from a lava-stream about 10 feet thick section No. 2.
- No. 7 Scoriaceous dolerite, section No. 6.
- No. 28 Porphyritic dolerite, section No. 5.
- No. 23 Laterite, section No. 5.
- No. 191a. Porphyritic basalt, from the centre of a small lava-stream, about 7 feet thick, 35 chains from Lyttelton end.
- No. 10 Porphyritic basalt, section No. 6.
- No. 51a Bole, partly altered to laterite, 40 chains from Heathcote end.
Analysis made at the Laboratory of the Imperial Geological Institute of Austria, Vienna, by Carl Ritter von Hauer.
Flaky vesicular trachyte.—Banks' Peninsula, from the large dyke near the Bridle-path, in which Messrs. F. Thompson and Co.'s quarry is situated (No. 29 of Tunnel Section.)
|Protoxyde of iron||2·00|
The result of the analysis of this rock, of which the specimen sent was beginning to show a little decomposition, is very striking, as alkalis are entirely missing, and lime is present in such large quantities.
This rock deserves, perhaps, another name, as the perfect resemblance with the sanidine trachyte of Kühlenbronn, is only one of a superficial character.
The following analyses were made in the Laboratory of the Imperial Polytechnical Institute of Vienna, under the direction of Prof. Dr. A. Sehrotter, at the request of my friend Prof. Dr. F. von Hochstetter, by the different chemists, the names of whom are appended to each.
|Sesquioxyde of iron||9·492|
|Protoxyde of iron||6·050|
|Carbonate of Lime||2·852|
|Protoxyde of manganese||0·521|
|Oxyde of titanium||0·791|
The specimen was dried in a dessicator at a temperature of 110 deg. Centigrade (Celsius) and lost by that operation 0·860 water.
|Alumina with traces of oxyde of titanium||18·042|
|Sesquioxyde of iron||7·053|
|Protoxyde of iron||4·192|
|Protoxyde of manganese||0·97l|
The water was expelled at a temperature of 100 deg. Centigrade (Celsius).page 354
These two specimens A and B, taken from the same lava-stream in the tunnel, show by their analysis that they are more closely allied to the dolerites than the trachytes, as, according to Bunsen, a normal pyroxenic dolerite ought to have only 48.47 silica, whilst this lava from the tunnel has 54.616, and has at the same time all the other ingredients which constitute a normal dolerite in lesser proportion than it ought to have. They are the two prevailing rocks occurring near the northern end of the tunnel, and a comparison of their analyses proves well, although so very different in their lithological character, that they possess nearly the same constituents.
|Sesquioxyde of iron||1·68|
No loss by drying the specimen in the dessicator.
Analysis made by Professor A. W. Bickerton, F.C.S., at the Laboratory of Canterbury College.
|Loss on ignition||·900|
|Silica and insoluble matter||73·170|
|Lime (Ca. 0.)||1·635|
|Soda, loss and undetermined||776|
* Ia some of the tufas, augite in well formed crystals, often of large size, is not of unfrequent occurrence.