72 ORIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 



Taking tlie average thickness of the tin-ground at eighteen 

 inches, this would give 526,592,000 cubic yards, or say 

 1,000,000,000 tons. Supposing one four-hundredth of this 

 (one-fourth of one per cent.) to be oxide of tin, this would give 

 two and a half million tons of black tin. Of this large amount 

 probably four-sevenths have been almost completely worked 

 away, while the remaining three-sevenths are practically in- 

 accessible. 



I have shewn elsewhere* that the black tin produced in the 

 West of England during the past seven centuries has been not 

 less than two millions of tons of which the stream-works 

 must have yielded about three-eighths, or say 750,000 tons. 

 Allowing only half as much for the preceding (perhaps 

 more than) 30 centuries ; allowing for small patches of tin- 

 ground still unworked ; and for the tin washed away in the 

 streaming process, as well as for the inaccessible submarine 

 deposits still unworked, it will be seen that there is still a sufiB.- 

 cient margin in the estimate given above. 



Sec. 7. Amount of denudation indicated. 



Assuming that the tin-gravels before they began to be 

 worked, contained 2^ millions of tons of black tin, we have now 

 to enquire how much denudation is thus indicated. In Chap, iii 

 Sec. 13, I have estimated that what may be called the " eifective 

 tin-bearing area " of the lodes at a given level is equal to 

 half a square mile, and capable of yielding 40,000 tons of 

 black tin per yard of depth. Let us assume farther an area 

 of "stanniferous penumbra" formerly existing but removed 

 by denudation during the formation of the tin-gravels ten times 

 as large (viz., 5 square miles) but only one-tenth as rich, 

 this would give us another 40,000 tons. Add to this a " slightly 

 impregnated area " of 200 square miles, but only capable of 

 of yielding one-hundrtdth as much of grain-tin as the penumbra 

 area, we should then have : — 



* Seven Centuries of Tin Production, 1892. 



