768 DIFFERENT KINDS OF MINERAL VEINS. [Cu. XXXVIU. 



chaotic fluid in some primeval 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 deposits, and that dikes of por- 

 phyry, greenstone, and basalt, were fissures filled with their several 

 contents from above. Hence he naturally inferred 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 

 menstruum," 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 entertained that the phenomena of mineral veins might be ex- 

 plained' 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 operations have been described and ex- 

 plained. 



On Different Kinds of Mineral Veins. — Every geologist is famil- 

 iarly acquainted with those veins of quartz which abound in hypogene 

 strata, forming lenticular masses of limited extent. They are some- 

 times observed, also, in sandstones and shales. Veins of carbonate 

 of lime are equally common in fossiliferous rocks, especially in lime- 

 stones. 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, calca- 

 reous, and occasionally 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 irregular assemblage of veins, called by the 

 Germans a " stockwerk," in allusion. 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 vio- 

 lence. They traverse all kinds of rocks, both hypogene and fossilifer 

 ous, 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 three or four feet. They hold their course continuously in 

 a certain prevailing direction for miles or leagues, passing through 

 rocks varying in mineral composition. 



