MINERAL VEINS AND ORE-DEPOSITS. 779 



(5.) Bed-impregnations. — They are beds in a stratified rock which 

 have become impregnated with ore by some method distinct from that 

 of superficial accumulation. They thus differ from iron-ore beds, 

 which are simply marsh deposits. They occur in many parts of west- 

 ern America, and are worked for silver and other ores. The follow- 

 ing sections illustrate a case of this kind. They are from a report 

 made in 1879, by Roth well and Crouch, on a district in Utah, on Virgin 

 River, 250 miles south of Salt Lake. The formation containing the 

 ore-beds, o, is probably Cretaceous (see Gilbert's Rep., 158, 171, 1875). 



Fig. 1132. Fig. 1133. 





The ore is chiefly silver chlorid or horn-silver. The rocks are sand- 

 stone, argillaceous sandstone, and shale, and are more or less upturned. 

 The ore-beds are usually clayey layers or shales, or combinations of 

 layers of sand and clay, and appear to have a wide extent ; and the 

 ore is most abundant when the clays contain vegetable remains. Since 

 the silver ore in these beds is the chlorid, it is probable that salt 

 waters were in some way concerned in the ore-making process ; and 

 as eruptive rocks are not far away, it is also probable, as J. E. Clay- 

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

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

 eruptive movements, were another factor. The vapors may have pen- 

 etrated directly the wet rocks from fissures underneath ; but it seems 

 to be far more probable that they rendered hot, and in places metal- 

 liferous, the waters of salt lakes, and that these waters, passing through 

 the sandy beds, made their deposits in the less pervious clayey beds. 

 Fossil remains are always the first portions of a bed to become pene- 

 trated by ore. The beds were impregnated with ores after they were 

 made ; and yet it is not certain that there were not repetitions of the 

 process for the several ore-beds during the course of the era in which 

 the rocks were in progress. 



In other cases of bed-impregnations, the waters may be those of 

 ordinary fresh-water ponds ; but over most of the Great Basin, salt is 

 common in the soil and waters. The regions of hot springs and lakes 

 in California and Nevada illustrate some of the conditions. 



The mines of cinnabar, of California, as the facts and reasonings of 

 J. D. Whitney show, are probably other examples of this mode of ore- 

 deposition, and the fissures which brought up the vapors were appar- 



