§ 15. DERBYSHIRE LEAD-MIXES. 681 



surrounded these bodies, having resisted the chemical action 

 that subsequently destroyed the calcareous structure of 

 the originals.* 



15. Derbyshire lead-mines. — It is in the mountain 

 limestone that the principal British lead-mines are situated, 

 namely, those of Somersetshire, Derbyshire, York, Durham, 

 and Northumberland. In Derbyshire the metal occurs in 

 numerous veins which traverse the strata, and extend in 

 some instances into the toad-stone ; a volcanic rock we 

 shall describe in the sequel. The perpendicular, or rake- 

 veins as they are termed, are from two to forty feet wide ; 

 and there are chasms or hollows in the rock, several hun- 

 dred feet in width, which also contain metallic ores and 

 spars. Manganese, copper, zinc, and iron, are found in the 

 limestone ; but the predominating metalliferous ore is the 

 sulphuret of lead, or galena, as it is called by minera- 

 logists. This substance is of a bluish-grey colour, and 

 often occurs in cubic and octahedral crystals ; it is also 

 disposed in thin layers, as well as in veins. It is accom- 

 panied by fluor and calcareous spar, sulphate and carbonate 

 of barytes, iron pyrites, &c. The variety termed specular 

 galena, or slickensides, is a thin coating of lead on the sides 

 of the veins, and appears to have arisen from one wall of 

 the fissure having slipped along the face of the other, so as 

 to give it a polished or slicken surface. 



The beautiful mineral known by the name of Derby- 

 shire spar, is a fluate of lime, and occurs in crystals, and 

 also in nodules. The celebrated spar, provincially called 

 Blue-John, so much in request for vases, and other orna- 

 mental purposes, is found in the state of veins, and in 

 large irregular masses from three inches to a foot in thick- 

 ness, in the Odin mines, near Castleton. 



The structure of the country around Matlock, and the 



* See the pulley-stone, Lign. U5,jig. 1, which is a siliceous cast of 

 an encrinital column. 



