364 OEIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 



Let us take another illustration. In fig. 15, PI. ix, whieli 

 represents the case of many small and rich tin-lodes in granite, 

 the tin might be supposed to have come up or along the fissure 

 until it was quite full, the stanniferous solutions meanwhile 

 acting upon the solid granite, converting it into kaolin, depositing 

 quartz and carrying off alkali, at the same time. But we may 

 equally well suppose, in accordance with the circulating theory, 

 that the solvent percolating through the mass of the granite 

 dissolves out the tin from the original stanniferous silicates, or 

 from any other combination in which it may exist, and on 

 reaching the fissure deposits it there ; only kaolinizing the granite 

 in the neighbourhood of the fissure and not throughout the mass 

 of the rock, because there only was it possible for the alkalies 

 to be freely carried off and for the liberated silica to be deposited. 

 It seems to me that this view is strongly supported by the 

 phenomena of the carbonas, stock-works, and capels, and by 

 what are called impregnations generally. 



The great carbona in St. Ives Consols (fig. 2, pi. viii) was 

 720 feet long, and though very variable in width and height, 

 averaged perhaps 30 feet. At a moderate computation 60,000 

 tons of tin-impregnated granite must have been extracted from 

 this carbona, yet its only communication with the standard lode 

 was but a few inches in width and height, and those with other 

 lodes much smaller still. Such small channels might very well 

 serve for the continuous discharge of what may be called spent 

 liquors coming from all sides, but could hardly serve for the 

 entry of enriching solutions from the lode fissures to what was 

 really a kind of blind alley. 



The argument is still still stronger in the case of impregnation 

 at the South Wendron Mine, (fig. 6, -gl. viii) for here the 

 traversing fissure is a mere crack which rarely contains tin at all. 

 The wide, even, and sparing distribution of the tin in the killas 

 stockworks, which are not associated with definite lodes, seems 

 to afford strong reasons for regarding them as originally 

 stanniferous beds, as already indicated. This hypothesis helps 

 us to understand how it is that so large a quantity of tin could 

 be localized without the aid of any lode sufficiently large or 

 continuous to serve as a channel from considerable depths. It 



