ORIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 67 



{c.) The stanniferous "head" of angular debris which 

 forms occasional portions of the " overburden " of the China 

 Clay districts. This is often found at elevations, not only con- 

 siderable in themselves, but relatively as compared with the 

 surrounding country. The components differ as a whole from 

 those of the quartz gravels, above referred to, in being much less 

 water-worn. Some of the fragments are very large, and the 

 whole mass is, so to speak, cemented together with an earthy or 

 clayey mass of filling, which has considerable tenacity. It 

 occurs covering killas as well as granite. Grains of tin-ore are 

 often found widely distributed through the subsoil in similar 

 localities, even when there is no stony " head " properly so-called. 

 Such subsoils may or may not be of the same age as the " head" 

 proper. 



{d.) The true stanniferous valley gravels, sometimes, as 

 already stated, consisting of two distinct beds separated by 

 layers of different material, indicating a very considerable lapse 

 of intervening time. The true gravels occur at all elevations up 

 to nearly 700 feet, but always in situations relatively low, i.e. in 

 the immediate neighbourhood of ground of greater elevation. 

 These are the deposits now practically worked out, which were 

 so fully described by Mr. Henwood. 



Sec. 3. Geological age of the gravels. 



In 1859, Mr. S. E. Pattison endeavoured to correlate these 

 various deposits in some degree, and to connect them with the 

 traces of man in the district, just as Dr. Winn had done so long 

 ago as 1839.* The order of origin of the various beds as stated by 

 him, agrees pretty nearly with that given above, and he places 

 the important Forest bed between c and d. Much more recently 

 Mr. Ussher after a close study of the superficial geology of the 

 county, has come to the conclusion that both the " head " and 

 the principal stream-tin gravels were formed during a period of 

 elevation, of subsequent subsidence, and of sub-aerial waste 

 corresponding to the second glacial period, f while he finds the 

 first traces of man in the immediately succeeding Forest-bed. 

 But how long ago this was, we do not at all know. Some geolo- 



* See the Eep. Roy. Inst. Corn, for 1860, p. 5, et seq. 

 f Port-tertiary Geology of Cornwall, 1879, p. 50. 



