Ch, XXXVIIL] different kinds of mineral veins. 619 



contemporary, Dr. Hutton, in his speculations as to the origin of granite.* 

 According to him, the plutonic formations, as well as the crystalline 

 schists, were substances precipitated from a chaotic fluid in some prime- 

 val or nascent condition of the planet ; and the metals, therefore, being 

 closely connected with them, had partaken, according to him, of a like 

 mysterious origin. He also held that the trap rocks were aqueous de- 

 posits, and that dikes of porphyry, greenstone, and basalt, were fissures 

 filled with their several contents from above. Hence he naturally infer- 

 red that mineral veins had derived their component materials from an 

 incumbent ocean, rather than from a subterranean source ; that these 

 materials had been first dissolved in the waters above, instead of having 

 risen up by sublimation from lakes and seas of igneous matter below. 



In proportion as the hypothesis of a primeval fluid, or " chaotic men- 

 struum," was abandoned, in reference to the plutonic formations, and 

 when all geologists had come to be of one mind as to the true relation 

 of the volcanic and trappean rocks, reasonable hopes began to be enter- 

 tained that the phenomena of mineral veins might be explained by known 

 causes, or by chemical, thermal, and electrical agency still at work in 

 the interior of the earth. The grounds of this conclusion will be better 

 understood when the geological facts brought to light by mining opera- 

 tions have been described and explained. 



On different kinds of mineral veins. — Every geologist is familiarly 

 acquainted with those veins of quartz which abound in hypogene strata, 

 forming lenticular masses of limited extent. They are sometimes ob- 

 served, also, in sandstones and shales. Veins of carbonate of lime are 

 equally common in fossiliferous rocks, especially in limestones. Such 

 veins appear to have once been chinks or small cavities, caused, like 

 cracks in clay, by the shrinking of the mass, which has consolidated 

 from a fluid state, or has simply contracted its dimensions in passing 

 from a higher to a lower temperature. Siliceous, calcareous, and occa- 

 sionally metallic matters, have sometimes found their way simultaneously 

 into such empty spaces, by infiltration from the surrounding rocks, or 

 by segregation, as it is often termed. Mixed with hot water and steam, 

 metallic ores may have permeated a pasty matrix until they reached 

 those receptacles formed by shrinkage, and thus gave rise to that irregu- 

 lar assemblage of veins, called by the Germans a " stockwerk," in allu- 

 sion to the different floors on which the mining operations are in such 

 cases carried on. 



The more ordinary or regular veins are usually worked in vertical 

 shafts, and have evidently been fissures produced by mechanical violence. 

 They traverse all kinds of rocks, both hypogene and fossiliferous, and 

 extend downwards to indefinite or unknown depths. We may assume 

 that they correspond with such rents as we see caused from time to time 

 by the shock of an earthquake. Metalliferous veins, referable to such 

 agency, are occasionally a few inches wide, but more commonly 3 or 4 



* Principles, &c, chap. iv. 



