66 ORIGIN AND DEVELOPMENT OF OEE- DEPOSITS. 



Sec. 2. — Classification of tin-gravels. 



Detrital tin-ore occurs in the West of England under the 

 following conditions : — 



a. As a constituent of river-gravels and sea-beaches now 

 in process of formation. The tin-ore here referred to is generally 

 in very small particles, often angular, or sub-angular. Where 

 it is the result of natural processes of denudation, and conse- 

 sequently has been liberated from its matrix with extreme 

 slowness, as on some of the isolated beaches near St. Agnes, it is 

 very pure ; but where it results from the more rapid operations 

 of the miner, as in the bed and at the mouth of the noted lied 

 River which divides the parish of Illogan from Camborne, it 

 is extremely impure from the presence of pyrites, mispickel, 

 wolfram, chalcopyrite, and other more or less readily decompos- 

 able minerals. These impurities no doubt existed to some extent 

 in the veins from which the true " stream-tin " appears to have 

 been derived, but have been removed gradually by natural 

 chemical as well as mechanical processes acting through immense 

 periods of time.* 



ib.) The elevated stanniferous quartz gravels, such as those 

 of the "Blue Pool" in Crowan, referred to by Mr. Tyack,t and 

 those of St. Agnes BeaeoD, mentioned by Messrs. Kitto and 

 Davies.J 



* Tbe stanniferous refuse from our mines would, indeed, form rich and 

 important detrital deposits in the Red River and other valleys, and in the shallows 

 at their mouths if they were left to themselves. But such deposits would differ 

 as largely in character as in origin from the true tin-gravels which were formed 

 entirely by natural agencies without help from human operations. Tin-streaming 

 in the proper sense of the word is now so nearly extinct in the West of England, that 

 the term has come to be applied to the very different mode of working adopted in 

 the Red River, the Wheal Vor valley, and other places with the object of 

 recovering some of the tin which escapes from the mines as " slimes " and 

 "rows." Some modern writers have been entirely misled by the modern use of 

 tliis term. Thus, Prof. Blake writes, " The larger part of the tin of commerce 

 is obtained from " stream-tin. In Cornwall at first the tin ore was obtained solely 

 from the alluvions of the tin-region, and ei en so late as 1876, it is reported that 

 800 perso7is were engaged in mining for stream-tin on the Red River." — W. P. 

 Blake, Tin Ore of the Black Hills, Eng. and Min. Jour., New York, Sep. 1883. 

 The italics are mine, and of course the statement so marked is an absolute error 

 of the Professor's. 



f Trans. Roy. Geo. Soc. Corn., ix, p. 177. 



J Ibid, p. 196. . . . , 



