124 Lecture on the Geology of [No. 9, new series. 



land, it is impossible to state the exact age : I am, however, of 

 opinion, that these argillaceous siliceous rocks will be found to 

 correspond to the oldest Silurian strata of Europe. 



The existence and great extent of this formation are of consi- 

 derable importance to this Province, as all the metalliferous veins 

 hitherto discovered, or likely to be hereafter found, occur in rocks 

 of this formation. 



To these rocks belong the Copper-pyrites, which has been worked 

 for some years at the Kaivau and Great Barrier — the Manganese 

 (Psilomelan) at Waihehi — and the Gold-bearing quartz at Coro- 

 mandel. 



The gold which is washed out from beds of quartz-gravel in the 

 rivers and creeks flowing down from both sides of Coroman- 

 del range, is derived from quartz veins s of cyrstalline character 

 and considerable thickness, running, in a general direction from 

 North to South, through the old primary rocks which form the 

 foundation of the Coromandel range. In some places these veins 

 stand up like a wall on the summit of the range to a height of 

 eight or ten feet. The clay-slate rock itself is exposed only at the 

 bottom of deep gorges which form the channels of the principal 

 trachytic tuff and breccia, of which the hills surrounding the Har- 

 bour of Coromandel are composed. The well-known " Castle 

 Hill" — which can be seen from Auckland — is a characteristic ex- 

 ample of the Trachytic Breccia formation. The magnetic iron- 

 sand which, in washing, is found with the gold, is derived from 

 the same source as all the magnetic iron-sand of New Zealand, 

 namely, from the decomposition of trachytic rocks. Small veins 

 of quartz of amorphous character that is, not crystalline, but in the 

 shape of chalcedony, cornelian agate, and jasper — are found in nu- 

 merous places on the shores of Coromandel. These veins occur- 

 ring in trachytic rocks, are quite different from the auriferous 

 quartz veins in the primary formation, — a fact, I think, of much 

 practical importance to state, to prevent the fruitless search for 

 gold where gold does not exist. All the gold-bearing gravel in 

 the creeks is derived, as I have already said, not from the veins in 

 the trachytic breccia, but from the much thicker and crystalline 

 veins in the primary rocks. The surface-deposit in those creeks 



