Ch. XXXVm.] SUCCESSIVE ENLARGEMENTS OF VEINS. ffl 



shock, after which tliey remained uninjured, and without any opening, 

 although the line of each crack was still visible. "When all movement 

 had ceased, there were seen on the floor of the house, at the bottom 

 of each rent, small heaps of fine brickdust, evidently produced by 

 trituration. 



In some of the veins in the mountain limestone of Derbyshire, con- 

 taining lead, the vein-stuff, which is nearly compact, is occasionally 

 traversed by what may be called a vertical crack passing down the 

 middle of the vein. The two faces in contact are slicken-sides, well 

 polished and fluted, and sometimes covered by a thin coating of lead- 

 ore. "When one side of the vein-stuff is removed, the other . side 

 cracks, especially if sinah holes be made in it, and fragments fly off 

 with loud explosions, and continue to do so for some days. The 

 miner, availing himself of this circumstance, makes with his pick small 

 boles about 6 inches apart and 4 inches deep, and on his return in a 

 few hours finds every part ready broken to his hand.* These phe- 

 nomena and their causes (probably connected with electrical action) 

 seem scarcely to have attracted the notice which they deserve. 



That a great many veins communicated originally with the surface 

 of the country above, or with the bed of the sea, is proved by the 

 occurrence in them of well-rounded pebbles, agreeing with those in 

 superficial alluviums, as in Auvergne and Saxony. In Bohemia, such 

 pebbles have been met with at the depth of 180 fathoms. In Corn- 

 wall, Mr. Carne mentions true pebbles of quartz and slate in a tin lode 

 of the Relistran Mine, at the depth of 600 feet below the surface. 

 They were cemented by oxide of tin and bisulphuret of copper, and 

 were traced over a space more than 12 feet long and as many wide.f 

 Marine fossil shells, also, have been found at great depths, having 

 probably been engulfed during submarine earthquakes. Thus, a gry- 

 phsea is stated by M. Virlet to have been met with in a lead-mine near 

 Semur, in France, and a madrepore in a compact vein of cinnabar in 

 Hungary.J 



When different sets or systems of veins occur in the same country, 

 those which are supposed to be of contemporaneous origin, and which 

 are filled with the same kind of metals, often maintain a general 

 parallelism of direction. Thus, for example, both the tin and copper 

 veins in Cornwall run nearly east and west, while the lead-veins run 

 north and south ; but there is no general law of direction common to 

 different mining districts. The parallelism of the veins is another 

 reason for regarding them as ordinary fissures, for we observe that 

 contemporaneous trap dikes, admitted by all to be masses of melted 

 matter which have filled rents, are often parallel. Assuming, then, 

 that veins are simply fissures in which chemical and mechanical de- 



* Conyb. and Phil. Geol., p. 401 ; and Farey's Derbysh., p. 243. 

 | Came, Trans, of Geol. Soc. Cornwall, vol. iii. p. 238. 

 \ Fournet, Etudes sur les Depots Metallif eres. 



