340 DYNAMICAL GEOLOGY. 



Rep., 158, 171, 1875). The ore is chiefly silver chloride or horn-silver. The 

 rocks are sandstone, argillaceous sandstone, and shale. The ore-beds are 

 usually clayey layers or shales, and the ore is most abundant when the 

 clays contain vegetable remains. Eruptive rocks are not far away, and J. E. 

 Clayton, in the same report, urges that hot vapors, derived either from the 

 fissures of eruption, or from other wide-spread fracturings made by the erup- 

 tive movements, were the chief source of the distributed ores. 



In southern Utah and in Colorado, according to J. S. Newberry, veins exist made of 

 coarse gravel and stones, m wliich the stones liave become coated with argentiferous galena 

 and other ores, including silver chloride, that were received from below. They are worked 

 for the silver. Examples are the Bassick and Bull Domingo mines near Silver Cliff, Col., 

 and the Carbonate mine at Frisco, Utah. The large fissures were opened near the base of 

 the mountains, where they became filled with the pebbles, stones, and bowlders of all kinds 

 there accumulated, and yet received the ascending metallic solutions, and also siliceous 

 solutions, which deposited at the Bassick mine much chalcedony among the stones. 



2. The intersected rocks of easy corrosion. — Many of the richest ore- 

 deposits of the world occupy cavities in limestone made by the corroding 

 action of solutions or vapors. The cavities were eroded usually along joints 

 or fractures of the limestone. Examples occur in the Leadville region, Col- 

 orado ; in the Wasatch and Oquirrh mountains, Utah ; at the Eureka mine, 

 ISTevada ; in Lake Valley, New Mexico ; in the Los Carlos Mountains, Mexico ; 

 and elsewhere. The ores of these mines, as generally of others, are of two 

 classes : (1) the original, and (2) the secondary — mainly the latter. The 

 original ores include galena (PbS), containing some silver and chalcopyrite, 

 with sometimes pyrite and sphalerite (ZnS). Some of the secondary are 

 silver chloride and bromo-chloride, made from the silver of the galena; 

 lead sulphate, carbonate, phosphate (and less commonly vanadate and mo- 

 lybdate), made from the galena; zinc silicate, made from sphalerite; and 

 also iron oxide (hematite or limonite), made from pyrite and from iron in 

 the limestone ; and manganese oxides, probably from the limestones. 



The following figures show the forms, at Leadville, of some of the cavities 

 of ore in the corroded limestone (a blue Carboniferous limestone) underneath 

 a sheet of porphyry, the latter being the igneous rock which carried up with 

 it the ore and heated vapors. They are from the very valuable Report of 

 S. F. Emmons (1886). The porphyry is also usually altered and often pene- 

 trated for some distance with ore, and its decomposition has afforded part of 

 the ore for the limestone cavities. Although the ore deposits are usu^ally in 

 a Carboniferous limestone at Leadville, the time of the outflow of the por- 

 phyry and of the making of the cavities was not earlier than the Cretaceous 

 period (Emmons). The similar silver-lead mines of all western America are 

 probably likewise Cretaceous (chiefly the Laramie or later Cretaceous), or 

 else Tertiary. 



At the famous Eureka Mine, eastern Nevada, where the rocks are all 

 Paleozoic, the eruptions were Tertiary, according to Hague (1892), and 

 mostly late Tertiary ; they were partly along old fault-planes of post-Carbonif- 



