166 ABSTpACTS: GEOLOGY 



Tertiary lakes. A study of some of the characteristic colemanite de- 

 posits has suggested the probable vein origin of this mineral, without 

 necessary association with dessication deposits. The vein character is 

 indicated in the physical structure of the deposits, which also give evi- 

 dence of the formation of colemanite by replacement of limestone. 

 The boric acid is assumed to have been derived from volcanic sources, 

 since volcanic activity has characterized the region in which these de- 

 posits occur, and intrusive or flow rocks are generally found in intimate 

 association with the ore bodies. H. S. G. 



GEOLOGY. — Geology and ore deposits of Lemhi County, Idaho. J. B. 

 Umpleby. U. S. Geological Survey Bulletin 528. Pp. 182, with 

 maps, sections and views. 1913. 



The rock formations of Lemhi County, Idaho, include: (1) a gneissoid 

 granite of Archean age, (2) a widespread series of Algonkian schists, 

 slates and quartzites, (3) about 6000 feet of Paleozoic strata which 

 include Cambrian quartzite, Ordovician, Silurian (?), and Devonian 

 dolomitic limestones, and Mississippian limestone, (4) large batho- 

 lithic masses of granite, and (5) Tertiary lavas and lake-beds. 



The ore deposits may be grouped as gold placers and lodes, lead-silver 

 veins and tabular replacements, copper-bearing gold veins, cobalt- 

 nickel deposits, and tungsten-bearing veins. Two epochs of minerali- 

 zation are recognized, the older late Cretaceous or early Eocene and the 

 younger late Miocene or early Pliocene. All of the deposits, except a 

 small group of gold-silver veins, belong to the earlier epoch. 



The gold-bearing veins are inclosed in many types of rock in Lemhi 

 County, but few of them are far distant from rocks of the granite rhyolite 

 family. Lead-silver deposits are recognized only in the southeastern 

 part of the county and at present are affording the principal production. 

 They are inclosed in Paleozoic formations and are thought to be geneti- 

 call}^ related to a quartz diorite facies of the granite intrusion. The 

 copper deposits have not proved of noteworthy commercial importance. 

 Cobalt-nickel deposits occur in only one of the nineteen mining districts 

 in the county. They occur as lens-like bodies and as bunches and 

 disseminations along fractured zones. Some of them are known to con- 

 tain about 2 per cent each of cobalt and nickel. Tungsten is mined in 

 one district where it is associated with zinc, copper, lead, molybdenum, 

 iron and silver minerals in lenticular quartz veins. J. B. U. 



