OEIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 73 



Half a square mile of " effective area " yielding 



per yard of depth, of black tin . . . . 40,000 tons. 



Five square miles of '' stanniferous 'penumbra," 



yielding per yard of depth . . . . 40,000 ,, 



Two hundred square miles of " slightly impreg- 

 nated area," yielding per yard of depth . ,• 16,000 ,, 



Total . . . . 96,000 „ 

 This total of 96,000 tons being derived from 205J square 

 miles of stanniferous rocks for each yard of denudation. Now, 

 supposing two thirds of this tin to have been collected and 

 retained in the 340 square miles of gravel deposit as above esti- 

 mated, the remainder being ground into "slime" and carried 

 away during the denuding process, an average of about 40 yards 

 of denudation would have been required to yield the 2 J millions of 

 tons originally contained in them, and this large amount of denu- 

 dation must have been effected between the "Pliocene" and 

 " Prehistoric " (Postglacial) times. A similar series of operations 

 must have gone on during many previous ages, the results of 

 which are no longer within our ken. But it is evident that 

 stanniferous debris must have been very widely distributed over 

 the sea bottom. The imagination is lost in trying to follow such 

 residues, which have probably been again and again denuded 

 and re-consolidated, but it is clear that the sediments of the seas 

 around the West of England must contain traces of tin spread 

 over many thousands of square miles. 



Sec. 8. Relation of lodes to detrital deposits. 



In a former section (Chap, in. Sec. 13,) the workable tin 

 contained in fifty stockworks, averaging 250 yards long and 20 

 yards wide, has been estimated rather to exceed that contained in 

 1000 lodes (with their branches) each 1000 yards long and one 

 yard wide. A rough calculation will enable us to compare the 

 relative contents of definite lodes, and of what may be called 

 stanniferous zones of disseminated tin, in another light. 



The productive part of the Dolcoath lode may be taken at 

 2500 fathoms long, with an average depth for the tin producing 

 part, so far as it has yet been explored, of 200 fathoms, and an 

 average thickness of one fathom. Thus we have 2500 x 200 x 



