74 ORIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 



1 =:five hundred thousand cubic fathoms, and reckoning 5 cwt. of 

 the fathom, a total of 125,000 tons for the depth specified, or 50 

 tons per fathom of length, which is, perhaps, not far from the 

 truth. But a single stockwork 250 fathoms long X 10 fathoms 

 wide, if it yields only 5 lbs. of tin to the ton, or say 80 lbs. to 

 the cubic fathom will, if worked to the same depth of 200 fathoms 

 yield 17,850 tons, or an average of over 70 tons per fathom of 

 length. 



A reasonable consideration of the foregoing figures and 

 estimates will shew that it is a great error to suppose, as many 

 have done, that rich detrital deposits imply the former, if not the 

 present, existence of rich lodes within the denudation area. 

 Eather, it would appear that the reverse is the case, and in our 

 estimates we have only supposed about 40 per cent, of the tin to 

 have been derived from regular lodes, and this is probably 

 too high an estimate. 



On the other hand, the existence of large masses of ore in 

 the detritals may generally be taken to indicate the near exist- 

 ence of large deposits. 



Sec. 9. Summary. 



a. The tin-gravels, whether near to or below the present 

 sea-level, as at Carnon and Pentewan ; or at considerable eleva- 

 tions as at Tregoss Moor or near Princetown ; occupy ground 

 which is relatively low, i.e. lower than much of that which is 

 adjacent. 



h. They consist of matter denuded from those higher 

 grounds or from others not far off, and at still greater elevations. 



c. The richness of the gravels has little or no direct rela- 

 tion to that of the principal lodes of the near neighbourhood. 



d. They represent actual concentrations, i.e. they are 

 largely composed of the heavier portions of masses of rocks 

 whose denudation must have occupied long periods, 



e. They seem mostly to have been derived from granitic 

 rocks, with some admixture of such altered killas as may be seen 

 near the junctions. 



