218 Transactions. —Geology. 
The reduction of the rates of the base to the silica in a heterogeneous 
group of widely dissimilar rocks, as are those given on page 16 of the same 
report (Lab. Report, 1871), can have no possible scientific value, but even 
here, if it proves anything, it proves that the Thames rocks are quite different 
from those of the Dun Mountain and Gympie, as they contain considerably 
more silica. 
Dr. Hector has remarked (Geological Reports, 1870-71, p. 93) that “ the 
country occupied by the volcanic rocks (at Coromandel) has a very distinct 
appearance from the central portion of this part of the range, which is 
composed of the tufaceous and porphyritic sandstones and felspathic slates.” 
In this I quite concur, but it no more proves that the two are distinct 
formations than does the difference between the central scoria-cone of an 
active volcano and the lava streams round its base prove that the scoriz and 
lava belong to two geological formations ; for it is a phenomenon common to 
all large trachytic districts that the later outbursts are always more basic in 
character than the earlier ones, and have almost invariably occurred on the 
flanks of the mountains. We have, in New Zealand, another example of this 
in the Malvern Hills, in which case the imbedded fossils enable us to prove 
that the central trachytic mass of Mount Misery belongs to the same formation 
as the basaltic lava streams of the outskirting Harper Hills. 
Both Professor Dana and Mr. Darwin have attempted explanations of 
these phenomena, but whether their explanations be true or not the fact still 
remains that large trachytic ranges almost always have a centre of solid felspar 
rock, and basaltic lavas with scoriaceous agglomerates on their flanks, 
2. Mineralogical Evidence.—Dr. Hector has dwelt upon the fact that gold 
is found in some creeks and not in others ; but this, by itself, proves nothing, 
for not only do all metals occur locally, but gold could not be expected to 
occur in quantity in those superficial portions of the formation which both 
Dr. Hector and myself call trachytic agglomerates, for the heat in these 
portions could not have been sufficiently long continued for the formation of 
metallic veins, and these rocks are generally so porous that the percolating 
water would not be compelled to keep in distinct channels. 
Jasper and chalcedony are said by Dr. Hector (Report on Thames Gold 
Fields, p. 2), to be characteristic of the younger formation, but they are both 
found abundantly at Tapu in Dr. Hector's « greenstone porphyry” formation, 
and occur also at several places in the Shortland district, as for instance the 
Karaka Creek, 
3. Stratigraphical Evidence.—Dr. Hector asserts that his older auriferous 
formation is only found in narrow belts. He says (Geological Reports, 
1870-71, p. 92) that “the shafts and drives on the Tokatea Hill, and also a 
few of the road cuttings which penetrate the hard rock, show it to be the same 
