280 Transactio'os. — 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 {I.e., p. 97), and the other at Tapu. In both cases a brown 

 tufa or breccia is supposed to lie on a water-woi'n 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 i"0cks 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 h 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 volcanic 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 accumiilations of 

 scoriaceous agglomerates are but the last dying efibi-ts. 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 compai-able in duration with a geological formation 

 or period. 



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 woi-ld, such as Hungary, Styi-ia, Teneriffe, the 

 Siebengebirge Mountains, etc., as the following table will show : — 



