622 SUCCESSIVE ENLAHGEMENTS OF VEINS. [Oh. XXXVIIL 



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, contain- 

 ing 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 small 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 holes 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 phenomena 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 occur- 

 rence 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 Cornwall, Mr. 

 Came 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 en- 

 gulfed during submarine earthquakes. Thus, a gryphaea is stated by 

 M. Virlet to have been met with in a lead-mine near Seniur, 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 paral- 

 lelism 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 deposits have accumulated, 

 we may next consider the proofs of their having been filled gradually 

 and often during successive enlargements. I have already spoken of 

 parallel layers of clay, quartz, and ore. Werner himself observed, in a 

 vein near Gersdorff, in Saxony, no less than thirteen beds of different 



* Conyb. and PkiL GeoL p. 401 ; and Farey's Derbysh. p. 243. 

 f Carne, Trans, of GeoL Soc. Cornwall, voL iii p. 238. 

 j Fournet, Eludes sur les Depots Metalliferes. 



