OEIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 365 



also agrees with the fact that, however numerous the individual 

 tin " strings " may be, and they are sometimes a score or more 

 to the yard (fig. 1, pi. viii), yet that the killas between always 

 contains traces at least of tin, and sometimes actually more than 

 the aggregrate of the " strings," which alone it is possible to 

 save by economic treatment of the whole mass owing to the 

 extreme fineness of the tin in the rock between the strings. 



These strings appear to be no more than filled shrinkage 

 cracks, those which are now more than capillary in size having 

 been enlarged by the crystallizing forces. It does not seem 

 likely that any of them were ever really open fissures in the 

 ordinary sense of the term. 



The minerals associated with the tin in these stock-works 

 are precisely what they are in so many other situations, viz. : 

 gilbertite, schorl and quartz ; with, rarely, topaz and wolfram. 

 Fluor and apatite are either very rare, or altogether absent. 

 Usually in these little " strings " the cassiterite is more abundant 

 than all the rest of the components taken together, so that as 

 the term is usually applied there is no veinstone. It seems 

 impossible to believe that a band of rock 20 or 30 fathoms wide 

 can have been impregnated with so large a quantity of tin, 

 brought from so far by solutions flowing through these compara- 

 tively trivial channels. Can we imagine that such a solution 

 passing through a veinlet, often less than one sixteenth of an 

 inch in thickness and only extending a few yards in length or 

 depth, has impregnated the country rock for a foot or more in 

 some instances on either side ? Eather it would appear that a 

 sufiicient solvent power had existed in the interstitial fluid 

 soaking through rocks already saturated with tin particles ; in 

 fact that in these instances at least the old sediments which now 

 form the bulk of Cornwall were, in their deeper portions (which 

 are now uplifted on the flanks of the granite), already stanniferous 

 before the granite began to be uplifted. Thus as regards these 

 stockworks in killas which are unconnected with distinct fissure 

 veins, it seems that we must look to what has been called lateral 

 segregation for the concentration in the strings ; and as to the 

 tin itself, although it may originally have been brought to the 

 surface and poured into the ancient seas or lakes by means of 

 thermal springs, yet that the springs were not less ancient than 



