page 337

A Chapter on Geology.

Having taken some part in the examination of the geology of New Zealand, it will not be out of place if I now attempt to give a general view of the formation of the islands.

The leading feature of the geological structure is extremely simple, viz., a great fracture and elevation, forming a range of high, and in many places alpine mountains (culminating in Mount Cook at a height exceeding 13,000 feet), which runs from end to end of the islands in about a north-east and south-west direction. This range lies nearest the west coast of the South Island, but in the North Island is found towards the eastern side, forming the chain, or series of chains which stretches from Wellington to near the longitude of the East Cape. Thus, in the latitude of Cook Strait the main chain appears to curve.

These ranges are composed of palæozoic rocks, with some admixture of triassic. In the South Island the older rocks—say lower silurian (mica and chlorite schists)—appear, as a general rule, to crop out to the westward, the higher rocks (upper page 338silurian to trias) appearing in succession to the eastward. The oldest rocks (gneiss, &c.) lie in the south-west corner of the South Island; granite and syenite are found at various places on the west coast of the same island and in Adêle and Stewart's Islands, but no granite whatever has been discovered in the North Island. Outlying ranges of palaeozoic rocks are found on both sides of the main range of mountains already described, but these are on a much smaller scale as regards elevation and area.

This description will give an idea of the ancient skeleton of the islands. This has been covered up by perhaps nearly, if not quite, as complete a series of mesozoic and tertiary rocks as are to be found in Great Britain.

Mesozoic Rocks.—Our knowledge of these rocks has been greatly increased during the last few years. In the North Island, formations ranging from trias to what is called cretaceo-tertiary have been found extensively on both sides of the island. In the South Island most interesting discoveries have been made. The coal of the west coast, and indeed the good coal of New Zealand generally, appears to belong to the middle cretaceous period. Numerous remains of extinct saurians have been found in the neighbourhood of the Kaikoura Peninsula, showing a former liassic period; and a very extensive series of mesozoic rocks has lately been examined by Mr. Herbert Cox, C.E., and Mr. Mackay, both officers of the Geological Survey. These have proved rich in fossils, and range from what is page 339perhaps provisionally termed permio-carboniferous up to cretaceo-tertiary.

The above is the most perfect continuous section of the mesozoic rocks which has yet been found in New Zealand. The thickness is estimated at 21,000 feet from upper jurassic to carboniferous. Part of this, no doubt, may be classed as palæozoic, but still leaving a great thickness of secondary rocks.

Tertiary or Neozoic Rocks.—These rocks are very largely developed in New Zealand, particularly in the North Island, and are found in the three stages of Eocene, Meiocene, and Pleiocene. The greater part of the area of the North Island is certainly page 340occupied by tertiary rocks, as also a large part of the South Island.

In the illustration given at page 124 of the ascent of the Moawhanga river, a good idea may be obtained of the general appearance of the tertiary strata, and of the mode in which the rivers cut channels (canons) through them, when circumstances are favourable for that purpose.

Recent Formations.—Among the chief of these are pumice deposits in the North Island, gravels deposited by rivers, moraines, sandhills, and swamps, in which moa bones are found.

The deposits of pumice in the interior of the North Island cover an enormous area. The deposits of gravel are most extensive in the South Island. The presence of immense quantities of moa bones in swamps and sand deposits is one of the most interest ing facts in the natural history of the islands. These birds must have inhabited the country from a very remote period, from the geological time at which the islands were united, for we cannot conceive the possibility of these non-flying birds, with others still in existence, such as the kiwi and the weka, passing across a broad strait.

I have devoted several papers in the Transactions of the New Zealand Institute to showing the results which would follow were the islands united, one of which would be the necessity for a great Cook Strait river. That the moa only became extinct at a very late date there can be no reasonable doubt, as the Maoris have many traditions as to its habits and page 341the mode of its capture. Remains of the bird have also been found with the feathers still undecomposed. The locality of a moa skeleton is generally indicated by a heap of quartz pebbles—the gizzard stones of the bird. A search underneath is generally successful in finding at least some remains.

Inclination of Strata.—The stratification of the palaeozoic rocks is found always highly inclined and more or less approaching the vertical, thus showing that these rocks must have been subjected to great pressure. The mesozoic rocks are found more or less inclined, and sometimes vertical. The tertiary strata approach the horizontal.

Volcanic Rocks.—The more ancient volcanic rocks of New Zealand are perhaps found in the South Island, appearing on Banks' Peninsula, the peninsula at Otago, and at some intermediate points, as also in some places along the front ranges of the mountains which bound the Canterbury plains. These rocks are, I think, supposed to have been formed by submarine eruptions. The whole of Banks' Peninsula is composed of trachytes, dolerites, basalts, laterites, and other volcanic rocks, and the same may be said of a considerable area near Dunedin.

In the northern half of the North Island, however, lies the district where volcanic action has taken place on the most magnificent scale, forming mountains, some of which are from 8000 to 9715 feet in height. Mount Egmont, or Taranaki, is second to no volcanic cone in the world in grace of outline, and rises to a height of over 8000 feet, while the page 342volcanic group of the centre, consisting of Tongariro, Ngauruhoe, and Ruapehu, is truly a magnificent sight Ngauruhoe, which is a lateral cone on the glacis of Tongariro, is the present cone of eruption, but hot springs and vapours still rise from Tongariro, and I am told also from Ruapehu, although I did not perceive them.

The chief cone of eruption in New Zealand is, perhaps, White Island in the Bay of Plenty, where sulphur is deposited in large quantity. The neighbourhood of Auckland is dotted with small extinct craters, among which the island of Rangitoto guards the entrance to the harbour, and many of these are found in the Northern Peninsula and elsewhere. The hot springs have been so often described, that I may be excused from commenting upon them.

Volcanic rocks in the North Island are almost confined to the provincial districts of Auckland and Taranaki. None are found within the districts of Wellington and Hawkes' Bay except Ruapehu and Tongariro, which just come within the boundary of the former.

Lakes.—The lakes of Otago and Canterbury offer material for a geological problem. They are rock basins on a very large scale, Lake Wakatipu, for instance, being sixty miles long. It will be observed by reference to the map that they mostly lie along a straight line in the central axis of the island. An attempt was made to prove that these rock basins had been formed by the scooping action of glaciers; but as the surface of Lake Wakatipu is page 3431000 feet above the sea-level, and its depth 1300 feet, Or 300 feet below sea-level, this theory has been abandoned as untenable. The true solution would seem to be that the beds of the lakes in general show the lines of parallel faults, although this theory will not apply in all cases.

Glaciers.—The glaciers of the South Island, of which the largest are in the vicinity of Mount Cook, are on a very large scale, and I have heard it asserted that they exceed those of the European Alps in magnitude.^* In former ages the glaciers were much more extensive. Moraines are found in many places where the glaciers have disappeared or retreated. It is now generally admitted that there is no evidence in New Zealand of the "glacial period." A greater number and extension of glaciers may have been the consequence of a greater extent of elevated land, but would not require a colder climate. There is no evidence of an ice sheet covering the islands. As New Zealand lies in a comparatively low latitude, the absence of any proof of a "glacial epoch" there does not prove its non-existence in the southern hemisphere.

Gold.—Gold is found in New Zealand, as elsewhere, in the older palaeozoic rocks, chiefly upper and lower silurian, and consisting chiefly of mica and chlorite schists. A reference to the geological map will therefore show in what districts this metal is prevalent. The principal areas for gold working lie in Otago, Westland, and Nelson. At the page 344Thames mines in the north, the mineral veins traverse slate rocks and a tuff resting upon them. The tuff is said to be of tertiary age, and the veins are, I believe, found to be rich in the tuff, and pinched and poor in the slate. In the mountains in the neighbourhood of Wellington small quantities of fine gold have been found in rocks which certainly seem not to be older than "carboniferous." The value of the gold varies with the locality. The gold of Otago and Westland is about equal in value to that of Victoria, viz., exceeding £4 an ounce; that of the Thames contains much silver, and is, I think, not worth £3 an ounce. The gold found at Wellington is also of low value.

Copper.—Ores of copper have been found in many parts of both islands, from the Barrier Island in the north, to D'Urville's Island in Cook Strait and Dusky Bay and Stewart's Island in the south. None of these mines have as yet proved financially successful, but some of them have only lately been opened, and they may pay well in the future, being generally in the vicinity of water carriage.

Iron.—Dr. Hector reports as follows:— "Almost every known variety of iron ore has been discovered in the colony." The greater proportion of these ores, whether granular or massive, appears to belong to the class which, although rich in iron, and yielding first-rate metal, is hard to reduce, and requires the application of both capital and skill for its working.

Coal.—The coal measures of New Zealand are page 345of extreme geological interest, as here we find seams of coal of excellent quality, from three or four to perhaps fifty feet thick, as high up in the scale as middle cretaceous. To indicate the quality of New-Zealand coal, Dr. Hector has divided the classification into hydrous and anhydrous—the latter containing a minimum of water, and the former, I think, anything above six or seven per cent.

The position of the New Zealand coal beds is as follows:—If we draw a line from the Thames through the Waikato Valley to the Upper Mokau, and thence to the Tangarakau, a tributary of the Whan-ganui river, we find a line of strike of the North Island coal from which the seams dip to the westward. All the North Island coal as yet worked or discovered lies to the westward of this line, although indications have been found to the eastward. There is no reason, however, why the coal seams should not yet be found to the eastward of the line of strike, and I still hope that they will be made out.

In the South Island the chief outcrops of coal lie to the westward of the main range at West Whan-ganui, the Buller, and Grey rivers, and very thick and valuable seams are found there. Coal is found at the Malvern Hills in Canterbury and at Kaitan-gata in Otago, but the west coast is the great coal held.

There are numerous small fields of tertiary brown coal in Otago and elsewhere, very useful for local purposes, but of inferior quality, but these are of different age to the main coal seams of the country.

page 346

An important theoretical point is this, that the New Zealand coal seams, some of them forty and fifty feet thick, are of cretaceous age! Does not this fact affect the theories as to the secular cooling of the earth? The theory, I think, is this, that in the "carboniferous" period the earth was of itself so warm, the vapour so dense, and the supply of carbonic acid in the atmosphere so great, that vegetation was excessive, and thus gave an immense supply of plant life to form the coal measures. Now, this theory seems to me to be somewhat damaged if we find a similar growth in much more modern periods, as in the cretaceous rocks of New Zealand, and I suppose we may say the triassic rocks of Australia. It is curious that coal of the carboniferous period has not yet been found in New Zealand, as the rocks and fossils of that epoch are represented. Possibly these coal seams may yet be discovered.