122 ORIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 



The produce here did not exceed 3 lbs. to the ton in 1876, which 

 then paid expenses.* 



At Minear Downs, a little to the N.E. of St. Austell, is another 

 of these stanniferous channels of killas, lying between the famous 

 Charlestown mines and the granite. The works were visited by 

 the Miners' Association in 1870, and a short account was given 

 in the report of proceedings of that year.f Dr. Foster described 

 it six years later as follows — "The great open quarry.... is 

 about 200 yards long at the top and 60 or 70 yards wide, but only 

 90 yards long and 20 or 30 yards wide at the bottom. The 



greatest depth can scarcely be less than 120 feet „ The 



tin-ore occurs in a series of more or less parallel veins in the 

 killas, striking about E. 7^^ S. and dipping N. at an angle of 

 about 70°; the strings are often mere cracks but occasionally 7 to 8 

 inches wide, and lie from 2 inches to 12 inches apart.. ..ten 

 strings in one place in a width of 6 feet. They generally keep 

 their own course without much interlacing in dip and strike. 

 The killas itself dips S.S.E. at an angle of 20° to 25°, so that 

 the strings intersect it almost at right angles. At the sides of the 

 strings the killas is often stained red and yellow, and is occasion- 

 ally altered into tourmaline schist. On the S.W. side of the pit 

 is a so-called lode which is merely a mass of tourmaline schist 6 

 or 8 inches wide between two tin branches. "J 



These works are still being profitably carried on, but the 

 pit is very much larger than at the time of Dr. Foster's visit. 

 It must now be at least 500 yards long and 150 feet deep. I 

 visited it in the present year (1892), and found over sixty heads 

 of stamps at work. The stuff is said to yield about 4 lbs. of 

 tin to the ton. 



* On this point Dr. Foster remarks as follows : — 



" Some of the reasons why the stuff can be treated so cheaply are : — 



1. The rock is soft and friable and easily stamped. 



2. The tin-ore is in large grains (crystals), consequently the rock need 3iot 

 be stamped fine, and the subsequent washing operations are greatly 

 facilitated. 



3. The substances mixed with the tin-ore are specifically very much lighter 



and easily separated by washing. There is no pyrites to necessitate cal- 

 cination. 



4. There is water power at command, 

 tp. 39. 



Jlbid, p. 656. 



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