V.—GEOLOG Y. 
Art. XLVI.—On the Geological Structure of the Thames Gold Fields. 
By Capt. F. W. Hurron, F.G.8. _. ; 
[Read before the Auckland Institute, 9th June, 1873.] 
THE extraordinary amount of gold that has been obtained from some of the 
reefs at the Thames gives a great importance to these gold fields, and a correct 
knowledge of the geological structure of the district cannot fail to be of great 
interest to science. But at present a considerable difference of opinion on this 
point exists among geologists, and as it is only by discussion that a more 
Satisfactory state of things can be brought about, no apology is, I think, 
necessary for bringing the subject before the members of the Auckland 
Institute. 
Tn order to make clear the points on which different opinions are held, it is 
necessary, in the first place, to give a short historical summary of our present 
knowledge of the geology of the district. Professor Hochstetter was the first 
geologist who visited these gold fields, and he, after a short examination of the 
country about Coromandel, in 185 9, before any auriferous reef had been found, 
said that “The coast consists of nothing but trachytic breccia and tuff, in the 
most varying colours, and in the most different states of decomposition, from 
the hardest rock to a soft clayish mass, and in various places broken through 
by doleritic and” basaltic dykes.  Siliceous concretions, in the shape of 
chalcedony, carnelian, agate, jasper, and the like, are of very frequent 
occurrence in these tuffs and conglomerates, likewise large blocks of wood 
silicified and changed into wood opal. By local geologists those trachytic 
rocks were erroneously taken for granite and porphyry, and, by a gross 
mistake, the most sanguine hopes were based- upon the notion that these 
siliceous secretions might be auriferous quartz veins, The Coromandel gold 
originates from quartz reefs of crystalline structure, belonging to a clay-slate 
palzozoic formation, of which, under the cover of trachytic tuff, and 
conglomerate, the mountain range of Cape Colville Peninsula consists. The 
mountains are so densely wooded that it is only here and there in the gorges 
of the streams that sections of these slates may be examined. In these sections 
the clay-slates are frequently found to resemble. Lydian stone. They are 
arranged more or less vertically, their irregular upturned edges affording the 
most convenient and abundant pockets for the detention and Storage for the 
