ORIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 371 



topaz, and apatite, the most abundant source being the first- 

 mentioned.* 



Tourmaline is an important constituent of very large areas 

 within our district; in granite and its various modifications 

 (schorlyte, luxullyanyite, &c.); in tourmaline schist and in tin 

 capel. Probably at least one-tenth of the entire district, in 

 round numbers some 200 square miles, is thus permeated with 

 tourmaline, and to the average extent of one-tenth of the entire 

 mass. Now the rocks of this district weigh very closely upon 

 two tons per cubic yard, so that each yard in depth of this large 

 area will weigh near 1,240 millions of tons, and by our estimate 

 one tenth of this, or 124 millons of tons, consist of tourmaline. 

 Tourmaline, by analysis, yields from 1'49 up to 2-70 per cent. 

 of its weight of fluorine. If we assume an average of 2 per 

 cent., our 124 millions of tons contain no less than 2,480,000 

 tons of fluorine. This be it remembered for each yard in depth. 



Fluor spar is another, though far less important, source of 

 fluorine. It has been found in many of the copper lodes and 

 lead lodes of the district, as well as, sparingly, in a few of the 

 tin lodes. Also it has been met with occasionally as a constituent 

 of the granite (as at Wheal Daniel) and of the modified and 

 kaolinized granite rock known as China Stone or Petuntzyte, 

 notably in the parishes of St Stephens and St. Dennis. The 

 absolute quantity of this fluor spar must be very difficult to 

 estimate. It may however be worth while to make the attempt, 

 however roughly. If we assume the existence of 500 veins, 

 averaging one yard wide and continuous for an average of 500 

 yards in length, we have a total area of 250,000 square yards, 

 or a little less than one-twelfth of a square mile. If in this 

 area we assume further that the fluor spar forms one-twentieth 

 of the whole vein contents, probably a reasonable estimate, then 

 as fluor spar contains about 47 per cent, of fluorine, there will 

 be thus indicated 11,750 tons per yard of depth. 



It is probable that fluor spar as a rock-constituent occurs 

 over a much larger area than as a veinstone, perhaps even over 

 a square mile in all. But it exists there in much less proportions, 

 probably not over one per cent., and this would give us a further 

 quantity of fluorine of something like 29,000 tons per yard of 

 depth. 



* The chief localites for this and other mineral substances mentioned arp 

 given in the Author's " Hand-book to The Mineralogy of Cornwall and Devon," 

 Truro, 1876. 



