ORIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 121 



6. Copper stockworks in killas. Example, Wheal Music. 



7. Copper stockworks in granite. Example, Wheal 

 Yyvyan. 



1. — TIN STOCKWORKS IN KILLAS UNCONNECTED WITH LODES. 



Mulberry. This is one of the most ancient open tin workings 

 in Cornwall. It is situated on an elevated "down" (Mulberry- 

 Down) about 2 miles to the N.W. of Lanivet Church. The 

 excavation is at the bottom about 400 yards long and 30 wide, 

 with a depth varying from 80 to 120 ft., but more tinny ground 

 still stands on the east side of the pit. The then condition of the 

 workings was described by Dr. C. Le Neve Foster in 1876 as 

 follows : — " The killas, which is of an ash-grey colour, dips at an 

 angle of about 45° in a direction N. 22° W. {true). It is tra- 

 versed by numerous branches or veins running N. 7° W., dipping 

 about from 80° W. to 90° (vertical), and varying from mere joints 

 to veins 4 or 5 inches in width, rarely more than a foot apart — in 

 fact generally only a few inches (fig. 1., plate viii). Many of the 

 veins preserve their independence for a considerable distance 

 without intersecting other branches ; but at the same time it is 

 easy to find junctions both in the dip and in strike ; sometimes 

 also two adjacent strings may be connected by a " floor " or vein 

 of tin following the stratification. In addition to tin the veins 

 contain quartz and a little arsenical pyrites and wolfram."*' 



The average result of the operations is stated at that time 

 to have been 7 lbs. of tin-oxide to the ton of stuff, which at the 

 then prevailing low price paid expenses and a little more. 



Wheal Prosper and Michell is half-a-mile westward of Lanivet 

 Church, and was also described by Dr. Foster in 1876. The 

 workings here are also in killas. The pit is 800 yards long, 30 

 yards wide at the bottom, and averages 90 ft. deep. The killas 

 is soft and light-coloured (white, grey, yellow, brown), and is 

 full of little veins running E. 7° N., and containing quartz, 

 gilbertite and cassiterite, the impregnated mass being wider and 

 the veins somewhat more productive where certain stanniferous 

 caunters cross the pit (or rather, as I think, certain non-stannif- 

 erous caunter veins become stanniferous in crossing the pit). 



* Quart. Journ. Geol. Soc, 1876, p. 655. 



