abstracts: geology 455 



The means and cost of obtaining supplies and the availabihty of wood, 

 water, and game are mentioned. 



R. W. Stone. 



GEOLOGY. — The geology and ore deposits of Ely, Nevada. Arthur C. 

 Spencer. U. S. Geol. Survey Prof. Paper 96. Pp. 189. 1917. 



The great bulk of the rocks of the Ely quadrangle are limestones, 

 quartzites, and shales, which range in age from Ordovician to Carbon- 

 iferous and which have an aggregate thickness of more than 9000 feet. 

 The sedimentary rocks have been classed under eight formations. They 

 have been greatly disturbed by folding and especially by faulting, so that 

 their areal distribution is very irregular. The six uppermost of the 

 eight formations have been invaded in one place or another by masses of 

 monzonite porphyry. 



The igneous rocks of the district include an older set of monzonite 

 porphj^-y intrusions and a younger set of tuffs, obsidians, and rhyolites. 

 The monzonitic rocks are of particular interest because the genesis of the 

 metallic ores of the district is closely connected with their geologic his- 

 tory. The conclusion is presented that all the coarse-grained intrusive 

 rocks are to be refprrcfd to a single epoch of igneous activity, and it is 

 shown that present differences in composition are due in the main to the 

 more or less intense metamorphism which in many places the rocks have 

 suffered. 



Two kinds of metamorphism are distinguished. Under igneous meta- 

 morphism are included all those alterations that attended or followed 

 the invasion of the sedimentary formations by the magma that eventu- 

 ally crystallized as monzonite porphyry. These alterations have affected 

 the invaded limestones and shales and also the igneous rocks themselves. 

 To this metamorphism is to be attributed the formation of the primary 

 metalhferous deposits of the district. The second kind — atmospheric 

 metamorphism — includes weathering, or decomposition and leaching by 

 oxidizing surface waters, and cementation, or changes involving the 

 deposition of material taken into solution during the process of 

 weathering. 



The enriched copper ores of the district have been formed as a result 

 of atmospheric metamorphism. By considering the amoimt of oxygen 

 that water can absorb by contact with the air under atmospheric pres- 

 sure at 7000 feet elevation and at the present mean annual tempera- 

 ture of the region, it is found that, even if precipitation in the past has 

 been 25 per cent greater than at present, and that as much as 60 per cent 

 of the rainfall could have penetrated to the ore body, the oxygen re- 



