ORIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 125 



rock is a mixture of quartz and (white) mica with a good deal of 

 schorl, some gilbertite, and a little iron-pyrites, fluor, and 

 cassiterite. The mass is traversed by a number of so-called 

 leaders, which are quartz veins with tin-ore, schorl, gilbertite, and 

 clay, dipping 85° N., and running E. 7° N. Very often they are 

 an inch or two inches wide, and from 1 foot to 6 feet apart. 

 Occasionally the leader adheres to the enclosing rock by one side 

 only, and has a clay vein on the other. On washing the clay 

 broken crystals of cassiterite are generally found, proving, I 

 think, that since the deposition of tin- ore in the fissures there 

 has been a movement of the walls. 27,500 tons of rock were 

 stamped {i.e. a few years before 1876), and yielded 64 tons of 

 tin-ore, or 5-2 lbs. of tin per ton, say \ per cent. It was expected 

 that the wholly virgin ground would produce 8 lbs. of tin-ore 

 per ton."* 



Oligga. The interesting tin deposits at Cligga have long 

 yielded a little tin to men washing the beach-sands, and picking 

 a little here and there on the cliff face, but have never been 

 systematically worked on a large scale. The remarkable alter- 

 ation of the granite into parallel bands of quartz, stanniferous 

 and schorlaceous greisen, and kaolinized granite with unaltered 

 or little altered granite between, was well described and illustrated 

 by Dr. Foster, in 1876. "The cliff section exhibits a countless 

 number of these veins, varying from ^ inch to 6 inches in width, 

 and from a few inches to a few feet apart." There are some 

 pseudomorphs of chlorite or schorl after orthoclase, and the killas 

 a little inland is converted into tourmaline schist at its contact 

 with the granite. Dr. Foster suggests that the original fissures 

 here were "contraction fissures," and that subsequently these 

 were "penetrated and altered by metalliferous solutions arising 

 from below, "f and it would seem that the direction of these 

 fissures was determined by the foliation previously produced by 

 the lateral pressure which has contorted the neighbouring killas. 

 The more extensive conversion of the granite into greisen, at 

 Carrigan, just referred to, is a change of the same character. 



* Foster, Quart. Journ. Geol. Soc, 1876, p. 657. 



fSee " Tin lodes of the St. Agnes district." Trana. Eoy. Geol. Soc, ix, pp. 

 213, 219, 



