194 abstracts: geology 



and 1000 feet in length, have a similar vertical range, and an average 

 width of 10 inches to 5 feet. They show a wide range of strike and dip 

 at steep angles. They comprise three closely related mineralogic types. 

 The first contains pyrite, galena, sphalerite, and chalcopyrite with sub- 

 ordinate tetrahedrite, in a quartz gangue with some barite and rhodo- 

 chrosite, and yields silver and lead with subordinate copper and little 

 gold. The second contains galena, sphalerite, and tetrahedrite, with 

 subordinate chalcopyrite and pyrite, in a gangue of quartz, barite, and 

 rhodochrosite, and yields chiefly silver and lead. The third contains 

 petzite, tetrahedrite, and minor quantities of other sulphides, in a 

 gangue of fine-grained quartz carrying in places some hinsdalite, a new 

 mineral; this type yields silver and gold in proportion by value of 1:1 

 and is characterized by high tenor in both metals. The first two groups 

 include most of the Lake City lodes, as there is but one productive 

 telluride vein. The lodes are of late Miocene or early Pliocene age. 

 Their materials are believed to have been emitted from a magma of 

 monzonite whose apophyses as intrusions are scattered through this 

 general region in considerable numbers. Alfred H. Brooks. 



GEOLOGY. — Some ore deposits in noTthivestern Custer County, Idaho. 

 J. B. Umpleby. U. S. Geological Survey Bulletin 539. Pp. 104, 

 with maps, figures and views. 1913. 



The rock formations exposed in this area are schists, slates, and 

 quartzites of Algonkian age. Unconformably on these rocks in parts 

 of the area lies a great series, at least 9000 feet thick, of Paleozoic quartz- 

 ites, slates, and dolomitic limestones. During the late or early Creta- 

 ceous Eocene epoch these rocks were invaded by granite, quartz diorite, 

 and diorite, which are probably outliers of the great central Idaho bath- 

 olith. Dikes of granite, granite porphyry, and diorite porphyry, closely 

 related in age to the granite rocks, are locally abundant in the western 

 part of the area. In the central portion and along the eastern margin 

 occur vast accumulations of Miocene lava and tuff, which occupy old 

 erosion valleys. Morainic material covers much of the highlands. 



The ore deposits comprise gold placers, and lodes carrying gold-silver, 

 gold-copper, silver-copper, and lead-silver ores. The first three types 

 of lodes are typically fissure fillings, but the fourth might equally well 

 be considered of replacement origin. The gold-silver deposits are char- 

 acterized by a highly siliceous gangue; the others by a siderite gangue. 



Two distinct periods of mineralization are recognized in the area, 

 the deposits of each being distinct in character and their age relations 



