ORIGIN AND DEVELOPMENT OP OEE-DEPOSITS. 163 



At Trevenen Mine in the Carn Marth district, the lode is 

 separated from the compact granite by pot granite {i.e. granite 

 of which the felspar is changed to kaolin). At Wheal Dream 

 in the Helston district, a shaft was put down, throixgh hard 

 ground to cut a shoot of tin which dipj)ed away west from the 

 old Trumpet Mine. On reaching the " lode " it was found to be 

 barren until about the 60 fathom level, when the tin was reached 

 " in a softer channel of ground." Erom this point to the 180 

 fathom level the mine was productive.* It would be easy to 

 multiply instances of this general softening. In elvans the rock 

 is often softened from the kaolinization of the felspar but 

 hardened by silicification. 



Copper. In slate, softish strata are considered favourable, 

 especially if light yellow, brown or red. Hard dark blue slate 

 is always regarded as an unfavourable sign, so too is a hard 

 siliceous elvan or hard granite. In fact silicification cannot be 

 regared as favourable to the production of deposits of copper, 

 except very locally and in very special cases. f 



The special relations of gozzans to deposits of copper ore 

 will be dealt with hereafter. 



Lead. At West Chiverton the "productive rock " is of a 

 light greyish yellow colour near the surface, and of a greyish 

 blue a few fathoms down — fine grained and smooth to the touch. 

 The unproductive rock on the contrary is dark blue — very hard 

 and coarse-grained — passing into a sandstone or conglomerate. 

 Similar observations apply to the rocks at East Wheal Rose, 

 Wheal Shepherds and South Cargoll ; also to Wheal Eose and 

 Wheal Penrose near Helston. | On the whole however it would 

 appeal that with lead — as in a less degree with copper — the 

 infiltrations which have brought the minerals into the fissures 

 have been more modified by the character of the original rock 

 than seems to be the case with tin deposits. 



*See Notes of Excursions, Report of the Miners Association, 1864. 



f See Henwood, Trans. V, and M.A Reports of Excursion, 1864, &c. 



J See T. Clark, Report M.A., 1875, in which he gives good sections of these 

 two districts. I know of no instance where any of the lead veins passing into 

 what I have elsewhere called the Ladock sandstones, have continued to be pro- 

 ductive. 



