618 MINERAL VEINS. [Cii. XXXVIII. 



we bear in mind how often fossils are obliterated, wliolly or in part, even 

 in tertiary formations — bow often vast masses of sandstone and shale 

 of different ages, and thousands of feet thick, are devoid of fossils- - 

 how certain strata may first have been deprived of a portion of their 

 fossils when they became semi-crystalline, or assumed the transition state 

 of Werner — and how the remaining portion may have been eiTaced 

 when they were rendered metamorphic. Rocks of the last-mentioned 

 class, moreover, must have sometimes been exposed again and again to 

 renewed plutonic action. 



CHAPTER XXXVin. 



MINERAL VEINS. 



Werner's doctrine that mineral veins were fissures filled from above — Veins of 

 segregation — Ordinary metalliferous veins or lodes — Their frequent coincidence 

 with faults — Proofs that they originated in fissures in solid rock — ^Veins shifting 

 other veins — Polishing of their walls or " slicken-sides" — Shells and pebbles in 

 lodes — Evidence of the successive enlargement and reopening of veins — Four- 

 net's observations in Auvergne — Dimensions of veins — Why some alternately 

 swell out and contract — Filling of lodes by sublimation from below — Chemical 

 and electrical action — Relative age of the precious metals — Copper and lead 

 veins in Ireland older than Cornish tin — Lead vein in lias, Glamorganshire — 

 Gold in Russia, California, and Australia — Connection of hot springs and min- 

 eral veins — Concluding remarks. 



The manner in which metallic substances are distributed through the 

 earth's crust, and more especially the phenomena of those nearly verti- 

 cal and tabular masses of ore called mineral veins, from which the larger 

 part of the precious metals used by man are obtained, — these are sub- 

 jects of the highest practical importance to the miner, and of no less 

 theoretical interest to the geologist. 



The views entertained respecting metalliferous veins have been modi- 

 fied, or rather, have undergone an almost complete revolution, since the 

 middle of the last century, when Werner, as director of the School of 

 Mines, at Freiburg in Saxony, first attempted to generalize the facts then 

 known. He taught that mineral veins had originally been open fissures 

 which were gradually filled up with crystalline and metallic matter, and 

 that many of them, after being once filled, had been again enlarged or 

 reopened. He also pointed out that veins thus formed are not all refera- 

 ble to one era, but are of various geological dates. 



Such opinions, although slightly hinted at by earlier writers, had never 

 before been generally received, and their announcement by one of high 

 authority and great experience constituted an era in the science. JSTever- 

 theless, I have shown, when tracing in another work, the history and 

 progress of geology, that Werner was far behind some of his predeces- 

 sors in his theory of the volcanic rocks, and less enlightened than his 



