372 ORIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 



If we double these last last figures for the fluorine existing 

 in gilbertite, topaz, apatite, and other rare or widely scattered 

 minerals, whether occurring as rock-constituents, or in the veins, 

 we reach in round numbers a total of two millions five hundred 

 and fifty thousand tons of fiuorine per yard of depth. Thus it 

 is evident : 



(a). — That fluorine is an important constituent of the 

 Cornwall mining region. 



(h). — That it is localized as regards tourmaline in one-tenth, 

 and as regards fluor spar and other minerals in about 

 two-thousandths of the entire area. 



There are indeed traces of fluorine to be found by careful 

 analyses in many other parts of the mining district, but the 

 element for all practical and most theoretical purposes may be 

 regarded as absent. 



Boron. This element is with us confined to the tourmaline, 

 of which, like the fluorine, it constitutes about 2 per cent. 

 Consequently we may estimate the boron present at about two 

 and a half millions of tons per yard of depth. This element 

 hardly occurs in the West of England excepting in tourmaline, 

 and it is probably entirely absent from the rocks outside the 

 tourmaline area. 



Tin has hardly been found in the district, or indeed in any 

 district except as cassiterite.* It is far less abundant than 

 either fluorine or boron. The actually impregnated area may 

 be roughly estimated to contain, or to have contained, tin as 

 follows ; of course not counting the stream tin, which represents 

 the results of a very large denudation : 



One thousand lodes (with their branches, carbonas, or other 

 adjuncts) averaging 1,000 yards long and one yard 

 wide is equal to one million yards of surface area. 

 And this reckoning one per cent, of (metallic) tin 

 would give 20,000 tons per yard of depth. 



* The tin occurring in stannite (tin pyrites) may be finely divided cassiterite, 

 at any rate it is not, as was once supposed, a true sulphide. 



