654 C. LE NEVE FOSTER ON SOME 



38. On some Tin Stockworks in Cornwall. By C. Le Neve Poster, 

 Esq., B.A., D.Sc, E.G.S. (Read January 9, 1878.) 



The name Stockworh is usually applied to large masses of rock im- 

 pregnated with metallic ores or intersected by a number of mineral 

 veins at short distances apart, sometimes crossing one another in 

 all directions. Not having any word of our own, we have adopted 

 the term Stockwork from the German StocJcwerk. This expression 

 probably owes its origin to the method of working formerly often 

 adopted for such deposits, which were wrought by chambers arranged 

 in tiers or stories. 



The tin Stockworks of Cornwall may be divided into three classes, 

 according as they are found in clay- slate (killas), granite, or elvan. 

 I propose giving a short account of four of them which have never 

 been thoroughly described. 



Wheal Prosper and MicheU. — These are open workings about half 

 a mile west of Lanivet church, near Bodmin. For a distance of 

 upwards of 800 yards and a width of 30 yards the killas is inter- 

 sected by numerous little tin-veins running E. 7° N. (true); the 

 pit is usually 60 or 70 feet wide in the bottom, but in one part, 

 where a series of N. and S. branches go across, the width is double 

 that amount. In addition to the E. and "W". and N. and S. strings, 

 there are occasional caunters which are likewise stanniferous. 



The killas is usually very soft and generally light-coloured, i. e. 

 grey or white, or stained a yellowish brown by oxide of iron. In 

 some places near the veins the killas is converted into a soft tour- 

 maline-schist. The veins themselves are mere strings, rarely more 

 than | inch thick. In addition to quartz and cassiterite, I have 

 noticed a little gilbertite in them. The quantity of oxide of tin 

 contained in the stuff is exceedingly small, not more than 3 lbs. per 

 ton of rock on an average, say 0*13 per cent. ; and it is really 

 astonishing how such stuff can be worked without incurring a con- 

 siderable loss. Taking the oxide of tin, or " black tin," at <£42 per 

 ton, the pound is only worth 4%d. ; so that the ground as it stands 

 is only worth Is. l|c?. per ton. This amount has to cover all ex- 

 penses of blasting, breaking, tramming a quarter of a mile, besides 

 stamping and dressing. 



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, consequently the rock need not 

 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 calcination. 



4. There is water power at command. 



In spite of all these advantages there is much to marvel at in the 



