278 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 pi'oves 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 Cororaandel) 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 qiute 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 scoriae 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. Miiieralogical 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 coiild not have been sufiiciently 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" foi'mation, 

 and occur also at several places in the Shortland district, as for instance the 

 Karaka Creek. 



3. SlTatigrafhical 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 



