52 LEAD 



powerful they are, in the portions which traverse the calcareous rocks, than in the beds 

 of sandstone, and more particularly the schistose rocks. It is rare in the rock called 

 plate (a solid slaty clay) for the vein to include any ore ; it is commonly filled with a 

 species of potter's earth. The upper calcareous beds are also in general more produc- 

 tive than the lower ones. In most of these mines, the veins -were not worked till 

 lately below the fifth calcareous bed (the four-fathom limestone), which is 307 yards 

 beneath the Millstone-Grit ; and as the first limestone stratum is 108 yards beneath 

 it, it follows that the thickness of the part of the ground where the veins are rich in 

 lead does not in general exceed 200 yards. It appears, however, that veins have been 

 mined in the neighbourhood of Alston Moor downwards to the eleventh calcareous 

 stratum, or Tyne bottom limestone, which is 418 yards under the Millstone-Grit of the 

 coal formation, immediately above the whinsill ; and that they have been followed 

 above the first limestone stratum, as high as the grindstone sill, which is only 83 yards 

 below the same stratum of Millstone-Grit ; so that in the total thickness of the plumbi- 

 ferous formation is there more than 836 yards. It has been asserted that lead veins 

 have been traced even further down, into the Memerby scar-limestone ; but they have 

 not been mined. 



The greatest enrichment of a vein takes place commonly in the points where its 

 two sides, being not far asunder, belong to the same rock ; and its impoverishment 

 occurs when one side is calcareous and the other a schistose clay. The minerals which 

 most frequently accompany the galena are carbonate of lime, fluoride of calcium, sul- 

 phate of baryta, quartz, and pyrites. 



The pipe-veins (amas in French) are seldom of great length; but some have a 

 considerable width ; their composition being somewhat similar to that of the rake-veins. 

 They meet commonly in the neighbourhood of the two systems, sometimes being in 

 evident communication together ; they are occasionally barren ; but when a wide pipe- 

 vein is metalliferous, it is said to be very productive. 



The flat-veins, or strata-veins, seem to be nothing else than expansions of the matter 

 of the vein between the planes of the strata ; and contain the same ores as the veins 

 in their vicinity. When they are metalliferous, they are worked along with the ad- 

 jacent rake-vein, and are productive to only a certain distance from that vein, unless 

 they get enriched by crossing a rake-vein. Some examples have been adduced of ad- 

 vantageous workings in flat-veins in the great limestone of Cumberland, particularly in 

 the mines of Coalcleugh and Nenthead. The rake-veins, however, furnish the greater 

 part of the lead which Cumberland and the adjacent counties send every year into 

 the market. 



The metalliferous limestone occupies, in Derbyshire, a length of about 25 miles from 

 north-west to south-east, under a very variable breadth, which towards the south 

 amounts to 25 miles. Castleton to the north, Buxton to the north-west, and Matlock 

 to the south-east, lie nearly upon its limits. It is surrounded on almost all sides by 

 the Millstone-Grit, which covers it, and which is, in its turn, covered by the coal strata. 

 The nature of the rocks beneath the Kmestone is not known. In Cumberland the 

 metalliferous limestone includes a bed of trap, designated under the name of whinsill. 

 In Derbyshire the trap is much more abundant, and it is thrice interposed between the 

 limestone. These two rocks constitute of themselves the whole mineral mass, through 

 a thickness of about 550 yards, measuring from the Millstone-Grit ; only in the upper 

 portion, that is near the Millstone-Grit, there is a pretty considerable thickness of 

 argillo-calcareous schists. 



Four great bodies or beds of limestone are distinguishable, which alternate with 

 three masses of trap, called toadstone. The lead veins exist in the calcareous strata, 

 but disappear at the limits of the toadstone. It has, however, been ascertained that 

 they recur in the limestone underneath. See MINES and MINING. 



METAIXTTBGY OF LEAD. 



Although lead forms an essential element in a large number of minerals, the ores of 

 this metal are, strictly speaking, far from numerous. Of these the most important is 

 sulphide of lead, or galena. This mineral, which possesses a metallic brilliancy, and 

 has a lighter colour than metallic lead, presents, in its cleavage, all the variations 

 from large facettes and laminae indicating a cubic crystallisation to a most minutely 

 granular structure. It is extremely brittle, and its powder presents a brilliant black- 

 ish-grey appearance. 



The specific gravity of galena is 7"5 to 7'8, and its composition, when absolutely 

 pure, is 



Lead 86-55 



Sulphur 13-45 



100-00 



