Art. XIX.—Notes on the Rock specimens collected by H. H. Travers, Esq., on the Chatham Islands.
[Read before the Philosophical Institute of Canterbury.]
The Chatham Islands consist of different formations, some of high geological interest; showing on the one hand, that in their geological relations they are nearly allied to New Zealand; and on the other hand, that they belong to a separate volcanic axis, of which we shall, in the course of time, be able to trace the continuation north and south.
The principal island is of volcanic origin, and consists chiefly of basaltic and doleritic rocks and tufas; although older rocks are certainly not wanting. Several cones with a crater-like character show us the different centres of eruption; whilst around them, and extending from one to the other, marine sands have formed barriers, enclosing tracts of low land favourable to the formation of peat swamps.
Pitt's Island is formed of the same volcanic rocks, but it is more hilly, and does not present the same strange aspect as the main island.
The rocks submitted to me, with the necessary explanations, by Mr. H. H. Travers, afford a clear insight into the structure and formation of these islands; and to them I shall refer when speaking of the different characters of the rocks.
The oldest rocks visible, occur near Kaingaroa, and stretch in a west and east direction, towards the north-eastern corner of Chatham Islands. They dip towards east at an angle of about 30°, and consist of micaceous clay-slates, silky, and of a pale green colour. They are traversed by veins of quartz, which has the appearance of being auriferous. Similar rocks occur in our Southern Alps of New Zealand, on the eastern slopes of the Moorhouse range, and on the south-western slopes of Mount Cook. Smaller outcrops of this rock occur at the north-western corner of the large salt-water lagoons, and at Wangaroa or Port Hutt. Some beds of limestone fringe the south-western shores of that lagoon, to which, judging by their mineral character, we may assign a very high age. This limestone is of a white colour, and very crystalline; its structure is somewhat vesicular, but as the specimens submitted to me are rather small, it is difficult to assign to this limestone its true age, although, in its general character, it has all the appearances of a palœzoic limestone.
As before stated, the main eminences of the principal island are formed by basaltic rocks, which contain often large concretions of Hornblende, Augite, and Chrysolite. At their base, basaltic rocks, which often form perfect cones, and tufa beds, are met with, which in lithological character are identical with those of the same age in New Zealand.
The collection of rocks from Pitt's Island is far more complete, enabling me to examine the fossils which they contained, and thus assign to them their true age.
The lowermost rocks resemble those from the main island, already described, of a basaltic and doleritic character. On them, and on the western side, repose Palagonite tufas, identical with those associated with our tertiary doleritic rocks in the Malvern hills, Mount Somers, etc. On them, again, we meet with calcareous fossiliferous tufas, changing by degrees from an agglomerate, containing large pieces of volcanic breccia cemented together by a sandy matrix, with a slight admixture of carbonate of lime and only traces of fossils, to a tuffaceous, whitish limestone enclosing great quantities of fossils. The latter are of the same species as those found in similar beds in the Province of Canterbury, namely, a Pecten, a large smooth Terebratula (Waldheimia), and some large corals allied to Pelagia, which prove them to be of older tertiary age. These beds are overlaid by very fine limestones, semi-crystalline, and identical with similar beds lying in the same horizon in New Zealand.
On the western side of the island, the occurrence of lignite beds amongst these strata, show, that during the formation of these marine deposits, oscillations of the ground took place, which favoured the growth of terrestrial plants, burying them afterwards below new marine deposits.
Amongst the specimens collected, is a very fine-grained limestone, closely resembling some coral-rag beds of Europe; also a specimen of Brown-iron ore, and another of Psilomelane.
Thus clear evidence is offered to us, that in an early part of the tertiary period, volcanic action took place in this part of the Pacific ocean; and although we meet, on the main island, some signs of the existence of old sedimentary rocks, there is no doubt that these volcanic eruptions gave birth to this Archipelago.