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DYNAMICAL GEOLOGY. 





(4.) Contact Veins. — When eruptions of melted rock have taken 

 place, they have often brought not merely the heat of great depths 

 to the surface, but also, various mineral materials encountered on the 

 way up, and especially some of the metals or their ores. 



The fissures were in general deeper than those that gave origin to 

 veins of segregation, for the latter did not reach to where melted rock 

 could fill them, and hence had to be filled by what they could get 

 through the slower process. They consequently must have descended 

 to regions of very high temperature. As in a volcanic conduit, what- 

 ever at these depths, in the heated subterranean region adjoining the 

 opened passage-way, was ready to pass into a state either of vapor or 

 liquidity, would have been forced, by the pressure to which it was sub- 

 jected at those depths, to escape, if possible, by the way made for the 

 liquid rock, and would have ascended either along side of the latter, or 

 within its mass ; and at the same time, a portion would have been lia- 

 ble to be forced into the wall rock of the fissure wherever it was not 

 of too close a texture to receive it. The mineral material that could 

 take advantage of such an opportunity, or be aided in it by the heat 

 of the ascending melted rock, would be that, as just implied, which 

 was most easily fused or vaporized ; and this includes certain metals 

 and their ores, especially those of copper, silver, and antimonial, arsen- 

 ical, and sulphurous ores of lead, or of lead with silver or copper. The 

 fusing points of pure copper and silver are below 2,500° F., that of cop- 

 per being, according to Riemsdyk's experiments, at the Utrecht mint, 

 in 1869, 2426° F. and that of silver, 1904° F. ; and hence these might 

 have passed into the melted rock in the liquid state ; but whether this 

 was the fact, or whether they were in vapor, or in some vaporizable 

 or soluble compound, is not definitely known. 



The above is a general explanation of the initial movement in the 

 making of copper mines like those of Lake Superior, in which the 

 metal is in the native state, and the silver mines of Nevada, Mexico, 

 Bolivia, Chili, Transylvania, and of many other regions, which afford 

 various ores of silver with often some native silver. The igneous rock of 

 the Lake Superior region is largely doleryte, and copper is in fissures 

 and cavities in the igneous rock and in the sandstone of the walls. 

 The rock of the famous Comstock lode in Nevada consists of a horn- 

 blendic eruptive rock related to dioryte (called propylyte). (See Zir- 

 kel's Report (1876), in the Series on the Fortieth Parallel of the King 

 Expedition.) 



The materials carried into such fissures may react chemically on 

 one another, and so produce the occurring ores ; and successive changes 

 might take place from new accessions of ingredients at different levels. 



Besides the kinds of ore-deposits mentioned above, there are, also — 



