124 ORIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 



killas in connexion with a lode exists at Wheal Coates, on the 

 hanging-wall side of the Towan-rath lode, and, when worked 

 some years since yielded about 25 lbs. of tin to the ton of stuff.* 

 Pednandrea. The underground stockwork here was worked 

 to a very considerable extent, about a quarter of a century ago. 

 It was thus described by Mr. H. C. Salmon, in 1862: — -'This 

 great deposit, which in the old working was, I believe, called the 

 Great Carbona, is what the Grermans would call a Stockwerk. 

 For a length of 25 fathoms at the 68 fathoms level, the tin made 

 in branches in the killas " country," by the side of the lode for 

 11 fathoms wide, the lode itself being only 4 feet wide."f 



I have been informed that the average produce of this belt 

 of tin-ground was about 25 lbs. of tin to the ton, which was 

 scarcely enough to cover the expenses of excavation, hauling, 

 crushing, and dressing, with the additional cost of pumping and 

 timbering at that depth. Had it been at the surface it would 

 of course have paid handsomely. 



The aggregate of tin-ground removed and treated in the 

 killas stockworks just described cannot be less than 375,000 cubic 

 fathoms, or say 6 million tons, and is probably much more. The 

 average tin produce has been about 6 lbs. to the ton in those 

 works which are unconnected with definite lodes, and 18 lbs. to 

 the ton in the others. At least 20,000 tons of black tin must 

 have been obtained from them in the aggregate, besides that 

 lost in the tailings, to which reference will be made later on. 



3. — TIN STOCKWORKS IN GRANITE UNCONNECTED WITH LODES. 



The granite in which disseminated tin-ore occurs is almost 

 invariably altered into greisen, schorlyte, or zwitter. I proceed 

 to give examples of each. 



Carrigan. This mine is on the left side of the turnpike road, 

 going from Bugle to Lanivet. The open-work here is in a 

 mass of greisen (essentially quartz and white mica), and is 100 

 yards long, 50 yards wide, and 20 deep. " On the S.E side it is 

 bounded by a large clay vein or flucan, and on the north it dis- 

 appears under the alluvium of the neighbouring valley. The 



*Foster, Trans. R. G. S. C, ix., p. 212. 



f Mining and Smelting Magazine, 1862, pp. 143-4. 



