422 LE NEVE FOSTER ON TIN LODES. 



and schorl, arranged in the original bedding of the killas with veins 

 of quartz, and intersected with strings of cassiterite and chlorite. 



The capel is usually on both sides of the leader, but sometimes on 

 one side only. It is never more than two feet thick. The deposit 

 of tin ore appears to have been subsequent to the formation of the 

 capel. The Wheal Kitty is a similar mine. The Wheal Coates is 

 especially interesting as yielding pseudomorphs of cassiterite after 

 orthoclase, showing how the metallic matter has replaced the felspar. 

 The Cligga promontory consists of granite intersected by more or less 

 parallel metallic veins running 20 to 30 north of east. Veins of 

 quartz run through the granite, and on each side of the quartz is a 

 band of greisen which consists of quartz and mica, though it often 

 contains schorl, gilbertite, and a little tin stone. There is no plane 

 of separation between the greisen and the granite as there is between 

 the greisen and the quartz. The cliff is intersected with a countless 

 number of little veins, which stand out because the greisen is more 

 durable than the granite. 1 



The Wheal Mary Ann. The lode at Wheal Mary Ann runs a 

 few degrees east of true north through the killas. In one working 

 the killas is lined with cab, which is a sort of chalcedony; next 

 succeeds crystallised quartz of the ordinary kind, which is lined with 

 galena ; and, finally, the centre is occupied by chalybite or spathose 

 iron. Sometimes the cab contains fragments of killas and galena, 

 while vitreous quartz fills up the centre part of the lode. Elsewhere 

 the cab on the eastern wall is traversed by crystallised quartz, then 

 succeeds a band of vitreous quartz, and, finally, on the foot wall a 

 breccia of killas and cab cemented with galena and calc spar. This 

 condition of the lode is explained by Dr. Foster, who supposes the 

 formation of the fissure as a slight fault, in the cavities of which the 

 cab was deposited, cementing the fragments of the rock into a breccia 

 on the foot walls. After this the fissure was reopened, the new 

 fracture sometimes passing along the middle of the cab and sometimes 

 cutting across it, and then quartz, galena, chalybite, and calc spar were 

 successively deposited in the open spaces. 2 



Stockworks near Bodmin. The term stockwork has been 

 adopted from the German, to designate large masses of rock impreg- 

 nated with metallic ores, or intersected with mineral veins which 

 cross each other in all directions, or at short distances apart. The 

 tin stockworks of Cornwall occur in the clay slate, granite, and 

 elvans. 



Such mines are found at "Wheal Prosper, near Bodmin, where a 

 distance of 800 yards of killas, thirty yards wide, is crossed by mul- 

 titudes of little veins, rarely more than one-eighth of an inch thick, 

 and it only contains three pounds of ore to the ton of rock. The 

 Mulberry mine in the same neighbourhood exhibits similar pheno- 



1 " Remarks on some Tin Lodes in the St. Agnes District," Le Neve Foster, 

 Trans. Roy. Geol. Soc. Cornwall, vol. ix. part III., 1877. 



2 Le Neve Foster, " The Lode at Wheal Mary Ann," Trans. Roy. Geol. Soc. 

 Cornwall, vol. ix. part I., 1875. 



