-GEOLOGY. 



Art. XLVI. — On the Geological Structure of the Thames Gold Fields. 

 By Capt. F. W. Hutton, F.G.S. 

 [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 structui'e 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. 



In 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 1859, before any auiuferous 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 lai'ge blocks of wood 

 silicified and changed into wood opal. By local geologists those ti'achytic 

 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 

 palseozoic 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 

 ari-anged more or less vertically, their ii-regular upturned edges affording the 

 most convenient and abundant pockets for the detention and storage for the 



