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DYNAMICAL GEOLOGY. 



Under this process of infiltration the vein-material becomes firmly 

 adherent to the walls, and sometimes graduates into them ; and the 

 country rock is sometimes penetrated, more or less deeply, with crys- 

 tallizations of some of the vein-materials. 



When quartz and granite veins cross one another, the former are 

 the younger ; and they were formed either in a later part of the epoch 

 in which the granite was made, when the heat had diminished, or at 

 some subsequent epoch. 



It is remarkable that quartz veins are far oftener metalliferous than 

 granite veins. Nearly all gold-bearing veins are of quartz ; and the 

 same vein may contain also ores of lead, zinc, copper, and other metals, 

 besides the universal pyrite. The veins of copper ore in metamor- 

 phic regions are usually quartz veins containing chalcopyrite (or cop- 

 per-pyrites). Tin ore (cassiterite, or oxyd of tin), besides occurring 

 frequently in granite veins, is common in quartz, but it is often a 

 micaceous quartz, showing a higher temperature for its origin than is 

 needed for ordinary quartz veins, and indicating this also in being 

 often accompanied by crystals of topaz, tourmaline (schorl), ores of 

 tungsten (wolfram, especially), and other minerals. 



(2.) The ore-veins and local ore-deposits of non-metamorphic regions, 

 which occur in limestone formations, are among the most remarkable. 

 They include some of the largest lead mines of the world, as those in 

 the Subcarboniferous limestone of the Cumberland and Derbyshire 

 regions in England, in the Devonian (Eifel) limestones of Westphalia, 

 in the Triassic (Muschelkalk) of Upper Silesia, the Trenton (Galena) 

 limestone of Wisconsin and northern Illinois, the Calciferous (Lower 

 magnesian) and Subcarboniferous limestones of Missouri; and they 

 also hold in some places deposits of ores of zinc, copper, nickel, cobalt, 

 and silver ; and the lead-ore is occasionally valuable for silver as well 

 as lead, though seldom containing as much as that from veins in met- 

 amorphic rocks. The ores occur partly in veins, but largely in local 

 ore deposits, occupying fissures, passages and chambers, at different 

 levels, along certain courses. 



In the great lead regions of Wisconsin and Missouri there are, as 

 shown by J. D. Whitney, no lead veins. The passages or cavernous 

 spaces containing ore, owe their forms and size largely to erosion of the 

 limestone by water or acid solutions. With the lead ore occurs barite, 

 calcite, broken chert and limestone, a large amount of zinc ores, and 

 sometimes valuable nickel and cobalt ores. Adolf Schmidt, in his 

 account of the Missouri lead mines, observes that the same solvent 

 waters that made the caves and horizontal fissures or openings, may 

 have held the various associated ores and other minerals in solution. 

 The source of the ores is supposed to have been superficial The po- 



