OEIGlN AND DEVELOPMENT OF OEE-DEPOSlTS. 71 



killas now bordering the granite is richer than that situated at a 

 greater distance, so too in all probability was that which 

 formerly covered it as with a blanket, and which has since been 

 denuded away. 



Sec. 6. Situation and extent of the West of England tin-gravels. 



It is certain that during the past seven centuries the tin- 

 gravels were found to be more extensive and richer in the central 

 and western parts of Cornwall, than in the rest of the West of 

 England mining district, and there is much reason to believe that 

 this was so from the first. And as the western gravels were 

 probably the first to be worked, so they have continued in work 

 right up to the present, though on a very small scale, being now 

 in fact almost exhausted. Of over 20 distinct stream-works 

 visited by Mr. Henwood while in work, all were situated 

 to the West of Bodmin. In nearly all of these more than 

 one tin-bed existed, and whenever this was the case the 

 lower bed, separated from the upper by " false shelf," peat, or 

 other stratified material, was by far the richer, and often it 

 was the only bed worth working. 



We may, perhaps, form a rough estimate of the quantity of 

 tin originally present in these gravels from the following data : — 



1. Area of tin-ground above present sea-level in Cornwall. 



120 



Do. in Devon : — 

 Dartmoor and its surroundings. . 70 



Beneath the sea-level, extensions \ 

 of the Pentewan and other > 150 

 valleys ) 



340 square miles. 



