OEIGIlSr AND DEVELOPMENT OF ORE-DEPOSITS. 75 



/. The absence of auriferous gravels, workable as such, in 

 Cornwall, supports the conclusions in Chap, in, Sec. 13, that 

 Cornwall was never an auriferous region in any proper economic 

 sense. 



g. The valleys in which the gravels lie, after leaving the 

 granite, were eroded at a period -when the sea-level was, rela- 

 tively to the land, far lower than it is now. 



h. The valleys seem to have been swept clean before the 

 tin-gravels found their way into them in many cases. 



i. Although there are in a few cases, evidences of a second, 

 or even of a third stanniferous layer of gravel, yet the lowest is 

 always by far the most important. 



y. Though the physical changes in the West of England 

 have been so considerable since the tin-gravels were laid down, 

 they must yet be regarded as of Post-tertiary age. 



h. The occurreDce of mining implements and other remains 

 of the ancient tinners at great depths in the valley deposits 

 agrees with the historical indications of the great antiquity of 

 mining in the West of England. 



I. There is distinct evidence that in some instances at least 

 very considerable thicknesses of '' overburden " resting upon the 

 tin-gravels have been accumulated within comparatively short 

 periods. 



m. The actual proportion of oxide of tin present in the 

 valley deposits is very small. 



n. It represents, nevertheless, a very large amount of 

 denudation. 



\To he continued.'] 



Note. — Pages 327 — 377 in the Journal, No. 39, passed tdrougli the press 

 during the absence of the author in America, consequently there are several 

 misprints of minor importance, and a few more serious errors, which should be 

 corrected, as follows : — 



p. 327, line 7 from bottom, for marhed read masked. 



p. 349, line 10 from top, for radule „ radicle. 



p. 352, line 22 ,, for 7iautical ,, natural. 



p. 360, line 2 ,, for crumblings „ constituents. 



p. 371, line 21 ,, for Daniel „ Lamsel, 



