280 Transactions.—Geology. 
Mr. Davis has brought forward two cases of what he supposes to be 
unconformity between the two formations. One of these is at Omaru Bay, 
near Coromandel (/.с., p. 97), and the other at Tapu. In both cases a brown 
tufa or breccia is supposed to lie on a water-worn surface of blue tufa or 
breccia respectively. With the example at Tapu I am quite familiar; the one 
at Omaru I have not seen, but my acquaintance with these rocks in other 
districts leaves no doubt on my mind that the appearance at Omaru is owing 
to the same cause as the one at Tapu, viz, the decomposition of the upper 
parts of the beds, a distinct and undulating line often being seen between the 
decomposed and undecomposed portions of the same rock. 
Dr. Hector also mentions what he considers a case of unconformity in the 
Ohinemuri district. He says (Geological Reports, 1870-71, p- 102) “the 
sudden alteration in the form of the hills, and the marked change in the 
mineral composition of the rock, and other circumstances, indicate that b is 
unconformably superimposed on d, and that the two formations are 
distinct.” On this I would remark that “the alteration in the form of the 
hills" is probably caused by the “ marked change in the mineral composition 
of the rock," so that the evidence of unconformity is simply the alteration in 
the mineral composition of the rock “and other circumstances” which are not 
specified, and I submit that no geologist would consider this unconformity as 
proved. But apart from this, unconformity among voleanic rocks can by no 
means be taken as a proof of two formations, for the products of a volcanic 
eruption generally lie more or less unconformably on those of the last. I think, 
therefore, that the evidence adduced in favour of two distinct volcanic 
formations at the Thames completely breaks down on every point. 
Having at last, * Deo juvante," finished my criticisms, I now enter on the 
more pleasing task of giving the evidence on which I rely for proving that 
Dr. Hector's *greenstone tufa" formation is simply the older and central 
portion of the * trachytic tufa” formation, of which the coast accumulations of 
scoriaceous agglomerates are but the last dying efforts. I ought, however, 
first to define that I mean by a volcanic formation, or period, the whole length 
of time from the first volcanic outbreaks in a district to their final extinction, 
provided that the series of outbursts are not interrupted by a period of repose 
so great in extent as to be comparable in duration with a geological formation 
or period. Е 
1. Lithological Evidence —The rocks at the Thames are, as I have already 
said, very variable in appearance, not more so, however, than is usual in 
trachytic districts. The chemical composition also remains essentially the 
same in all the varieties, and corresponds with the same class of trachytic 
rocks in other parts of the world, such as Hungary, Styria, Teneriffe, the 
Siebengebirge Mountains, etc., as the following table will show :— 
