272 PROCEEDINGS OF SECTION C. 



number of metals are present in the micas, the augite, the horn- 

 blende, and the olivine of the crystalline rocks."* Mr. Becker 

 has also found gold and silver in the diabases that bound the 

 Comstock lode, most of it in the augites. This gold and silver is 

 in much the same relative proportions as the Comstock bullion ; 

 and he further found that the decomposed diabases contained only 

 about half, as much of the precious metals as the fresh rocks, f 

 That is to say, one-half of the gold and silver has passed out of the 

 decomposed rocks, and has, no doubt, been deposited elsewhere. If 

 therefore we assume that the pyroxenes of our volcanic rocks 

 contain gold and silver, that the conditions necessary for dissolving 

 them rai-ely obtain, but that one of the exceptions has been in the 

 Hauraki gold-tields, we have a hypothesis which will, I think, 

 explain most of the facts. 



The first change that took place in these rocks was, as I have 

 shewn, the conversion of the pyroxenes into chlorite and bastite 

 with the liberation of silica, lime, and some iron. If gold and 

 silver were partly removed with these substances we can conceive 

 that while the lime was altogether removed the silica and iron 

 might have been deposited with the precious metals in fissures and 

 the iron converted into pyrites by hydrogen sulphide. During 

 the second series of changes the whole of the chlorite with the 

 remaining gold would be removed and auriferous quartz would be 

 deposited in tfie veins. If the decomposition of the felspars took 

 longer than that of the clorites, which is very probable, pure crystal- 

 lised quartz might be deposited on the auriferous quartz. In the 

 thii'd series of changes the carbonates, which had been formed 

 during the second series of changes, were dissolved and part may 

 have been deposited occasionally on the quartz. 



This, it will be seen, gives a fair explanation of the principal 

 facts connected with the reefs, and also explains why tlie white 

 I'ock, from which the chlorite has been removed, is more favourable 

 for gold than the harder dark -green rocks in which the chlorite 

 still remains. But no reason is apparent why the sulphides of 

 antimony, zinc, arsenic, and copper, should have been formed 

 subsequently to the iron sulphide. Absence of gold in the well 

 crystallised quai'tz shews that silica continued to be removed after 

 all the gold had gone ; and we might account for the fine threads 

 and scales of gold between the points of quartz crystals by sup- 

 posing that during the second or third series of changes the 

 auriferous pyrites in the veins was in some places dissolved and 

 that the gold was redeposited, while the sulphur and most of the 

 iron were removed in solution, nothing but red stains being left 

 behind. If this hypothesis is the true one I should expect that, 

 as the whole of the gold in the veins in the hard dark rocks is due 



♦Green's Physical Geology, Eci. 1882, p. 560. 

 t U. S. Geol. Survej', 18S0-S1, p. 309. 



