CONDITIONS OF PRODUCTIVENESS. 411 



by miners as affording reason to expect an alteration for better or 

 wor.-e, suggesting secretion of minerals from the rocks. 



Metalliferous Veins in Silesia. Remarkable instances of this 

 relation are given by Von Dechen. 1 The numerous veins which 

 cross the steeply-inclined strata of greywacke in the Liegen district, 

 are metalliferous in narrow bands parallel to the inclined beds of 

 slate. The veins of the Kupferberg, in Silesia, bear ore only in 

 the hornblende schist, and are impoverished in mica schist. At 

 Joachimsthal the mica schist is traversed by quartzose porphyry in 

 veins, which, as well as the contiguous rock, hold pyrites in mica 

 slate. The rothegang of Elias consists of loam, and holds only 

 uranite ; where it runs between mica schist and a porphyry vein, and 

 where it traverses the latter, its substance is a red hornstone, and it 

 bears vitreous silver, native silver, arsenical cobalt, bismuth glance, 

 kupfernickel, arsenic, and bismuth ; but red silver, elsewhere abun- 

 dant, is entirely wanting. 



Condition of Veins in North of England. In the lead veins 

 of the north of England, which are situated in the Carboniferous 

 limestone tract, a singular dependence is observed between the con- 

 tents of the vein and the nature of the adjacent rock. The vein 

 divides limestones, sandstones, and shales, and these are brought 

 variously into apposition by the dislocations which accompany almost 

 all the veins. The vein is sometimes productive of lead ore under 

 every case of apposition in rocks. Where limestone, or schist, or 

 solid sandstone forms the walls, its productiveness is at the maximum, 

 but generally it is contracted in breadth and impoverished in its 

 metallic contents, wherever it is included between walls of shale ; and 

 even where only one side is occupied by shale, the same effect is fre- 

 quently observed. It would appear that the impoverishing influence 

 of the shale is referable to mechanical causes. In the same way as 

 the shales in a coal-pit swell out from the undisturbed parts to fill 

 the excavated vacuities, so we may conceive them to have expanded 

 into the natural fissure ; this will account for the contraction of the 

 vein. In the process of crystallisation, to which all the contents of a 

 vein are subject, it seems conformable to analogy to suppose that the 

 permanent walls of limestone and gritstone would permit a more early 

 growth of sparry and metallic crystals than the crumbling edges of 

 shale ; a supposition, perhaps, confirmed by the occasional mixture of 

 shale in the sparry mass of a vein, where it is " nipped," as the miner 

 says, in beds of shale. 



Quantity of Lead Ore from Different Beds of Limestone. 

 From some or all of these causes it happens in the north of England 

 that certain limestones are very much more productive than the 

 others ; in different mining districts, different limestones are thus 

 favourably distinguished, but in the country of Alston Moor, Tees- 

 dale, arid Swaledale, the uppermost thick limestone is by far the most 

 rich in lead. 



Upon the whole there is no sufficient evidence to show that the 

 1 "De la Beche's Manual," German Trans., 594. 



