24:6 STRUCTURE COMMON TO ALL ROCKS. 



This view is powerfully supported by the phenomena of hot alka- 

 line springs in California and Nevada. The Steamboat Springs, near 

 Virginia City, Nevada (so called from the periodic eruption of hot 

 water and steam), come up through fissures in comparatively recent 

 volcanic rock. The waters are strongly alkaline, and deposit silica in 

 abundance. By this deposit the fissures are gradually filling up and 

 forming veins. Some fissures are now partially and some entirely filled. 

 The ribbon-structure in some cases is perfect. Moreover, sulphides of 

 several of the metals, viz., iron, lead, mercury, copper, and zinc, have 

 been found in the quartz-vein stuff. Here, then, we have true metalli- 

 ferous veins forming under our very eyes.* So also at Sulphur Bank, 

 Lake County, California, hot alkaline sulphide waters,! coming up from 

 beneath, deposit both silica and cinnabar in small, irregular fissures 

 and cavities, forming quartz- veins containing cinnabar. The deposit 

 is so recent that the silica is sometimes still in a soft, hydrated con- 

 dition, which cuts like cheese. £ 



After this general discussion of the theory of metalliferous veins, 

 we are now in position to state more clearly their mode of formation. 

 Meteoric waters, circulating in the interior of the earth in any direc- 

 tion — downward, upward, or laterally — deposit slightly soluble matters 

 in their course, in cracks, cavities, or great fissures, forming fossil casts, 

 geodes, amygdules, infiltration-veins, and fissure- veins. As to direction, 

 the up-coming waters, especially in metamorphic and volcanic regions, 

 deposit most freely, and are most metalliferous, because they are hot and 

 often alkaline, and therefore most powerful solvents, and, of course, cool 

 gradually on approaching the surface. But that downward percolating 

 waters may also deposit metallic ores is proved by the fact that these 

 are sometimes found depending, like stalactites, from the roofs of cavi- 

 ties.* As to the different kinds of veins, those of great fissures are 

 most prolific, because these fissures are the highways of water from 

 the heated depths. But every kind of water-way will receive deposits ; 

 and, as the kinds of these are infinitely various and pass by insensible 

 gradations into each other, so also will be the veins which fill them. 

 The open fissure is the easiest and therefore the most traveled high- 

 way. In these, therefore, we have the most perfect type of veins, with 

 their banded structure and their selvages, their great size and conti- 

 nuity. But in many cases crust-movements produce only incipient fis- 

 sures, i. e., a loosening of the rock-cohesion, along planes affected with 



* Arthur Phillips. American Journal of Science, vol. xlvii, p. 194; and Philosophical 

 Magazine, 1872, vol. xlii, p. 401. 



f The water in this mine is 176° Fahr. — Becker. 



\ Le Conte, American Journal of Science, vol. xxiv, p. 23, 1882. 



# Schmidt, American Journal of Science, vol. xxi, p. 502, 1881. Chamberlin's Geol- 

 ogy of Wisconsin, vol. iii, p. 495. 



