Other formats

    TEI XML file   ePub eBook file  

Connect

    mail icontwitter iconBlogspot iconrss icon

Geology of the Provinces of Canterbury and Westland, New Zealand : a report comprising the results of official explorations

The Christchurch and Lyttelton Railway Tunnel

page 355

The Christchurch and Lyttelton Railway Tunnel.

In the Historical Notes on pages 62 and 156, I have alluded to the geological survey undertaken by me during the progress of the interesting and instructive work of passing through the caldera wall of a large extinct volcano. I prepared at the time a section at the scale of 20 feet to 1 inch, and have reduced, for this report, some of its most interesting portions to half that size.—Vide coloured Plate III.

The direction of the tunnel is N. 14° W. The first trial shaft was commenced in January, 1860, and the permanent works under contract with Messrs Holmes and Co., began in July, 1861. The tunnel was laid-out, and its execution solely superintended by Mr. Edward Dobson, C.E., Provincial Engineer. It was brought to a successful termination on May 25, 1866, when both adits met near the centre. The opening for railway traffic took place on December 9, 1867. The total length of the tunnel is 8598 feet, and if we deduct from this 365 feet on the northern or outer side, and 105 feet on the southern or inner side, formed by slope deposits and loess, there remains 8128 feet of rock of volcanic origin, of which the caldera wall has been built up. Classifying the rocks according to their lithological character, we find that the crater above the present water-line consists of—

  • 61 lava-streams, having the character of a stony compact or porphyritic basalt.
  • 54 lava-streams, formed of a scoriaceous basaltic and doleritic lava, some of them changing so gradually into agglomeratic beds that the line between them cannot be clearly defined.
  • 39 beds of agglomerates, a few of them changing into scoriaceous lava, but most of them consisting of scoriæ, lapilli, and other ejecta, imbedded in ashes. A. few of them have a brecciated appearance.
  • 19 beds of laterite, clays, and slope deposits, partly or wholly burnt by overlying lava-streams, and
  • 1 small layer of bolus—together 174.

These beds are intersected by 32 dykes, 18 consisting of trachytelava (of which five do not reach to the roof of the tunnel), and 14 of a basic nature (five of them being intermediate in character, trachy dolerites). One of them comes from the top of the tunnel.

page 356

Beginning at the southern or Lyttleton side of the tunnel, we observe that a large bed of loam has been deposited upon the volcanic rocks, being thickest on the lower portion of the caldera wall. This peculiar rock, which, when in small pieces, is easily pulverised between the fingers, has a remarkable consistency and solidity when in large masses, and is of subærial origin. It may be designated as loess, an expression now extensively used in Europe for similar deposits. It owes its origin to various processes, of which rain, wind and vegetation are the principal factors. This bed of loess, which in some localities, is more than 100 feet thick, changes gradually before we reach the volcanic rock, to a true slope deposit, consisting of fragments of rock more or less rounded, the lines of junction being often impossible to trace, owing to the decomposition of the volcanic rocks immediately below the slope deposits. The greatest amount of agglomerate, consisting of scoriæ, lapilli, and ashes is, as might be expected, congregated on the inner side of the caldera wall, not far from the focus of eruption. These more or less incoherent beds, of which each was probably formed during one eruption, have generally an inward as well as an outward dip, of which the beds 232 to 241 close to the entrance of the tunnel in Section No. 1, and extending 20 feet still more to the south, than shown in that section, form a notable instance. They were without doubt deposited on the lip of the crater. Near the Lyttelton end they are much disturbed. Two stony lava-streams cross these agglomerate beds, and we have to assume that after No. 231 was formed, the lava-stream 233, ascending from the mouth of the crater, had consolidated over it, being in its turn covered by a new talus of ejecta sloping inwards to the crater's mouth. After these latter beds, 234 and 234a were formed, a new stony lava stream, No. 237, ascended, in which case Nos. 231, 234, and 238 to 241 were three distinct agglomerate beds, covered and preserved on their inner slope by stony lava-streams, consolidated during their ascent. Or, to offer another explanation, we might regard these two stony lava streams, 233 and 237, as having broken through the huge accumulations of ejecta which were heaped up all around the crater's mouth—a phenomenon frequently observed during violent volcanic eruptions, when a huge cinder cone is formed in a short time. A similar occurrence seems to have taken place more towards the centre of the tunnel (Section No 4), about 60 chains from the Lyttelton side, where a large stony lava stream, No. 167, is seen to ascend through the agglomerate bed or beds, Nos. 166 to 168. The lava stream, 163, in close page 357proximity, might be considered to be the continuation of the former, which here flows down the steep side of the cinder cone. Gradually, as we retreat from the focus of eruption, the agglomerate beds decrease in number and size, but they still are occasionally present even close to the mouth of the tunnel near to the Heathcote entrance. Some of them (as, for instance, No. 11, 11 chains from the Heathcote end, Section No. 6), consist in their lower portion of fine ashes, or lava d'aqua, and above of scoriæ and lapilli, so as to suggest, that first fine ashes had been thrown out or had been brought down the side in the form of a mud stream, on the top of which large ejecta were afterwards deposited. Another agglomerate bed having an anticlinal or saddle arrangement is 22a, 17 to 20 chains from the Lyttelton end.; it was evidently deposited on the rim of the crater, of which the uneven surface is well visible in its lower portion. After its formation, two more agglomerate beds were deposited over it, 216 and 227, and 211 and 228 in the section, each being separated from the other by a bed of laterite. Moreover, it is clear that, whatever may have been its origin, the lowest portion of this and several other agglomerate beds must have been deposited when in a state of high temperature, as the argillaceous bed below it has been burnt red, so as to take all the characteristics of a laterite. All round Banks' Peninsula, agglomerate and ash beds are visible in the cliffs, but they are like the lava streams of small vertical extent only, and we have to approach more towards the centre of eruption, when we wish to see them in their greatest dimensions.

The largest and most numerous stony lava-streams are met with towards the centre of the tunnel, where the basalt of which they are composed possesses the greatest hardness and crystalline texture More towards the boundaries of the volcanic system, the lava-streams are much thinner and, at the same time, more porphyritic, amygdaloidal or scoriaceous, and it is very instructive to follow some of the lava-streams which form clear sections in the deep valleys radiating round the Peninsula, from the summit of the caldera wall to their termination at its foot, and to note the gradual change in their size, and in the texture of the rocks of which they are composed. I have already alluded to the lava-stream 237, nine chains from the Lyttelton end (Section No. 1), but in connection with it, I may here mention that the first shaft sunk by Messrs. Smith and Knight, the English contractors, unfortunately reached it soon below the surface of the ground, and continued all the way through it to the roof of the tunnel. page 358This was one of the principal causes that the firm, being unacquainted with the formation of the crater wall, abandoned the contract so soon. The first stony lava in the tunnel, flowing down the slopes of the crater wall, is a small stream, No. 214, about 22 chains from the Lyttelton end. Several others of similar dimensions follow, till we reach stream 206 (Section No. 2), which might be the continuation of No. 237 (in Section No 1), 11 1/2 chains from the Lyttelton end. This stream throws a great deal of light, by its configuration, on the manner of the flow of liquid lava. After flowing down the slopes, we see it shortly afterward ascend again (No. 202) over a bed of agglomerate, and, after having reached the apes of the latter, descend again (No. 200), diminishing rapidly in size, the rock now, becoming highly porphyritic and lighter in colour. The largest stony lava-stream of the whole series, of which Section No. 3 gives a portion, begins about 41 chains from the Lyttelton end, and continues without interruption to 52 1/2 chains. Consequently, taking its angle of dip into account, it is more than 500 feet thick. More or less porphyritie on both sides, the whole central portion consists of a very hard basaltic rock, ringing to the hammer, irregularly jointed, with here and there a tendency towards spheroidal structure. This huge stream gains an additional interest from the existence of three caves in its centre, which, however, have partly been filled up with thin plates of basalt of the same texture as the lava-stream, and which lie more or less horizontal. They are coated over and often cemented together by sphærosiderite. Sometimes they lie in such regular order, and so loosely upon each other as if they had been artificially placed in that position. The open space, or cave proper, is always on the southern side of each cavity. The only explanation I can offer as to their formation is that gases have been enclosed in this portion of the lava-stream, which, in course of time, were absorbed, and that liquid matter from the upper portion of the stream found access to the cavities, gradually filling them up, but that the channels of communication were stopped before the whole of the gases still remaining in the southern parts of each had been absorbed.

Another stream of large dimensions is No. 14, beginning 20 chains from the Heathcote end (Section No. 6). It is over 100 feet thick, has a jointed structure, the central portion being spheroidal, with concentric layers. All the stony streams in the tunnel above the latter are very thin, but it is possible that the scoriaceous basaltic lava (the violet beds of the section) which overlie them, are only page 359their upper portion, the bottom of the streams, owing to their thinness and to the distance from the centre of eruption, not having been able to cool to the stony compact form. I may, however, observe that the boundary line between both kinds of rock is in many instances very distinct and clearly defined. These scoriaceous beds occur throughout the tunnel; they are sometimes of considerable dimensions, some of them being over 100 feet thick. In speaking of the formation of the Lyttelton caldera, I have already pointed out that it has been built up by volcanic rocks belonging to two distinct divisions, of which the basic rocks have furnished all the material for the lava-streams, agglomeratic and tufaceous beds, whilst the principal portion of the dykes owe their origin to the acidic division. As might be anticipated, the dykes are most numerous near the focus of eruption, thus we find the greater portion of them near the Lyttelton side, several of them not reaching to the roof of the tunnel. Of these dykes, No. 29 is the most important. It consists of a soft flaky and lustrous trachyte, and possesses, like most of the other acidic dykes, the characteristic feature that it is accompanied on both sides by a selvage of tachylite, sometime two or three inches thick. This change in the character of the bed rock is especially visible when the dykes pass through agglomeratic or tufaceous beds. It shows clearly that the volcanic matter ascending by these fissures was in such an intense state of fusion that it was able to alter the rocks on both sides so thoroughly for such a distance. In some instances the dyke rocks themselves have a selvage of tachylite, the bed rock being unaltered. It is worthy of notice that the basaltic dykes have not produced the same effect, the rocks on both sides being generally unaltered. Large beds of loess, similar to those deposited on the inner side of the caldera wall have also been passed through on the Heathcote side. Of minerals of secondary origin found in the tunnel, the most diffuse is sphærosiderite which usually coats the pores and cavities of scoriaceous lavas. Of others, calcareous spar and aragonite are the most conspicuous. The latter is younger than the former, having often been deposited upon the surface of the calcareous spar in the small geodes. In a few localities, hyalite fills small clefts, or is found in a stalactitic form.

I shall close this chapter by offering a few observations on some other physical features of the beds through which the tunnel has been excavated, and as I noted them on the large section during the survey Forty chains from the Heathcote end, a scoriaceous lava-stream fifteen feet thick, and accompanied on both sides by beds of laterite and page 360agglomerate was passed, which was so loose and full of water that the ground had at once to be heavily timbered. All the cavities in the lava are lined with sphærosiderite, on which crystals of calcareous spar have been deposited. At 40 3/4 chains on the same side, in a bed of laterite, four feet above the floor of the tunnel, a small spring was struct, drying up a few months after; 35 1/2 chains from the Lyttelton end, the lava-streams, when first passed through, were so wet that the workmen could scarcely continue the work. In these streams, all the cellular cavities were either lined with sphærosiderite or filled with calcite. Sixteen chains from the Lyttelton entrance, in the agglomerate bed No. 228, and from a fissure reaching from, the roof of the tunnel, a copious spring flows which has a constant temperature of 65·20 degrees Fahrenheit, consequently 12·20 degrees above the mean temperature of Christchurch—about 53 degrees. Several eels have been caught near this spring, in the drain which runs from here to the mouth of the tunnel There being no connection with any other water-course, these eels must have ascended by the spring; they belong to the species Anguilla Aucklandii Rich., and have properly developed eyes. During the construction of the tunnel, it was frequently observed in the north or Heathcote end, that the water rose in the floor before a south-west gale, and subsided before the gale lulled; no observations could be made to ascertain whether the state of the tide had anything to do with this. The height to which the water rose was somewhat under half an inch. After the earthquake of August 17, 1868, this spring in the tunnel increased to such an extent that it laid the rails slightly under water; after a few days it decreased again to its former volume.