Phenomena with the Origin of Mineral Veins. 405 



tals of pyrites, together with crystalline cinnabar, although the 

 latter mineral has generally been deposited in an amorphous 

 state*. A specimen of chalcedony taken from one of the fis- 

 sures in the sulphur bank, which on being first broken was ex- 

 teriorly so soft as readily to receive an impression of the nail, had 

 on reaching this country become hardened, and had assumed the 

 ordinary characteristics of that mineral. Thin sections of this 

 specimen show a structure resembling fine-grained fortification 

 agate, and are traversed by numerous fissures filled by opaque 

 oxide of iron. Fig 2 is intended to show the appearance of a 

 section of this substance magnified 30 diameters. When ex- 

 amined between crossed prisms, brilliant colours are obtained, 

 and the crystalline structure becomes exceedingly distinct. 



The vein -matter of lodes on the Pacific Coast of North Ame- 

 rica has generally so many characteristics in common with de- 

 posits produced by solfatara action, that various geologists, who 

 have examined that region, have arrived at the conclusion that 

 the two are the result of similar causes. 



Von Richthofen remarks that "the process which immediately 

 follows the opening of fissures at or near active volcanoes, is the 

 violent emission of steam. A crevice in this state is called a sol- 

 fatara. It has been proved by Bunsen for the volcanoes of Ice- 

 land, by Boussingault for those of the South American Andes, 

 by St. Claire-Deville for those of the Canary Islands, and by 

 myself for the tertiary volcanoes of Hungary and Transylvania, 

 that every solfatara, in the course of time, passes through two 

 stages, in the first of which the steam is accompanied by gaseous 

 combinations of fluorine and chlorine, in the second by those of 

 sulphur, while a third one is ordinarily marked by the emission 

 of carbonic acid and combinations of hydrogen and carbon; at 

 which time the term solfatara is no longer applicable. We have 

 in the elements evolved during the first two periods, all the con- 

 ditions required for filling the Comstock fissure with such sub- 

 stances as those of which the vein is composed "f. 



W. P. Blake describes the lodes at Bodie Bluff, nine miles 

 west of the town of Aurora, as being of a character favourable 

 for the production of gold, and then goes on to say : — " They 

 are all alike in this respect, and doubtless had the same origin. 

 The quartz, instead of being a solid homogeneous mass, is formed 

 in thin layers or coats, one over the other, like sheets of paper 

 or pasteboard, with irregular thin seams or openings between. 

 This structure, with other peculiarities, indicates that the veins 

 were deposited gradually in the fissures by thermal springs, 



* Jahrbuchfur Mineralogie, &c, 1871, vol. iii. p. 291. 

 t On the Comstock Lode, its character, and the probable mode of its 

 continuance in depth, p. 47. San Francisco : 1865. 



