70 ORIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 



The minerals found with the tin in the gravels are just such 

 as would result from the degradation of such associations. The 

 trituration, concentration and atmospheric exposure to which the 

 tin-bearing rocks have been exposed would naturally result : — 



{a) in separating to a certain extent all substances of lower 

 specific gravity. 



(b) in oxydizing all such sulphides as are liable to such a 

 change ; and this would be followed in most cases by the 

 removal of the oxydized products.* 



Sec. 5. Proximate origin of the gravel-tin. 



The remote sources of the tin have been discussed in chap. 

 Ill, sec. 14, we have now to consider the raore immediate sources 

 of the tin found in the gravels. The wide distribution of tin in 

 the rocks of the West of England has been already referred to in 

 the same chapter, sec. 1 3. It may be detected by careful chemical 

 analysis in most, probably in all of the granites and elvans ; it 

 is constantly present in tourmaline schist as well as in much of 

 the killas which abuts against the granite. With no more deli- 

 cate implement than a vanning shovel it may usually be found 

 in the mud of the parish roads in the tin districts ;f in the rock- 

 substance lying between the separate tin veinlets at Minear 

 Downs, Mulberry and Great Wheal Fortune ; in the overburden 

 at Tregoning Hill, Rock Hill, and Treviscoe ; and in the sands 

 of the sea-beaches at St. Agnes and Cape Cornwall. It is prob- 

 ably not too much to say that the whole of the granite and killas 

 lying within one mile of the granite junctions contains from 

 -Yo^wo ^^ 100000 *^f ^^^ weight of oxide of tin, exclusive of that 

 contained in well-defined veins. It is true that most of this exists 

 only as microscopically minute particles, yet there are occasionally 

 veinlets (themselves representing natural local concentrations of 

 such diffused particles) of a larger grain, and capable of supplying 

 such fragments as are met with in the tin-gravels. And as the 



*It is owing to this process that the true stream-tin is generally purer than 

 that obtained direct from the lodes. Conversely the tin from the modern stream 

 works (Red River, &c.) being the refuse from the veins is less pure than the 

 ordinary lode tin. 



1 1 have found 1 to 4 lbs. to the ton in the mud of the road between St. 

 Austell and Bodmin. 



