368 ORIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 



The mineralized bands are worked away as far as they are 

 found to yield mineral enough to pay the cost of working. 

 Consequently, as the price of tin varies within wide limits, the 

 estimated or stated width of the paying ground known as 'Hhe 

 lode," may also vary at different times, hut in the Camborne and 

 lUogan districts the average width may be taken at from 4 to 8 

 feet.* 



Since in all cases it is the workable part of the eapel 

 (including the "leader" when present) which is described as 

 the lode by the miners, many wrong impressions have resulted 

 as to the width of the ore-bearing fissures in the West of 

 England. Thus the table of lode-widths already given from 

 Mr. Henwood's "Address," though fairly accurate as regards 

 copper lodes, where workable capels have always been either 

 very rare or altogether absent, is to a considerable extent 

 misleading as regards tin lodes or lodes containing both tin and 

 copper, unless the distinction pointed out be borne in mind. 



The most noteworthy point in connection with these capels 

 is the enormous amount of silica present. Doubtless we might 

 suppose the interior of the earth — whatever that phrase may be 

 taken to mean — quite capable of yielding this silica, but the 

 circulating waters of these fissures could hardly bring up so 

 much. The first effort would probably be to coat the wall with 

 an almost impermeable layer, after which the silicia would make 

 its way outwards still in s<jlution. If we suppose the silica 

 to be derived from the county rock, our difficulty is lessened 

 if not altogether vanishing. 



There is much reason to believe that the quartz of capels 

 and of quartz veins is, as already indicated, the result of solution 

 in and deposition from alkaline solutions.! 



*In this width, however, the tin is still disseminated somewhat irregularly, 

 so that it is often necessary to break a great deal of ground that will not pay for 

 subsequent treatment, and even to raise such " deads " to surface. Of the whole 

 bulk of the lode-stuff broken in the Camborne district, probably not more than 

 one half is actually treated in the stamps, and the average produce of this 

 portion in "black tin," as finally "cleaned" and sold to the smelter, does not 

 exceed 2 per cent. 



f At New Rosewarne and other mines in Cornwall quartz has been frequently 

 found deposited on chalybite, dolomite, and even on calcite crystals, the surfaces 

 of which are entirely uninjured from corrosion. This could hardly be the case if 

 the quartz were deposited from acid solutions. 



