266 abstracts: geology 



GEOLOGY. — Placer deposits of the Manhattan district, Nevada. Henry 

 G. Ferguson. U. S. Geological Survey Bulletin 640-J. Pp. 

 163-193. 1917. 

 The Manhattan district is in part a region of intensely contorted 

 Paleozoic sediments and in part covered by Tertiary volcanics. The 

 lodes from which the placer gold is derived occur principally in the 

 Cambrian schist, but are of Tertiary age. Manhattan Gulch, drain- 

 ing the central part of the district, has yielded a large amount of placer 

 gold. The bed-rock gravels from which the bulk of the placer gold has 

 been obtained were laid down in Pleistocene time, and the accumu- 

 lation of gold is the result of successive concentrations. Of particular 

 interest is the regular increase in purity of the placer gold with the dis- 

 tance from its source. In a distance of two miles down the gulch the 

 fineness of the gold changes from 700 to 740 parts per 1000. 



H. G. F. 



GEOLOGY. — Economic geology of Gilpin County and adjacent parts of 

 Clear Creek and Boulder Counties, Colorado. Edson S. Bastin and 

 James M. Hill. U. S. Geological Survey Professional Paper 94. 

 Pp. 379, with 23 plates and 79 figures. 1917. 



Gilpin County and the adjacent portions of Boulder and Clear Creek 

 Counties, Colorado, lie nearly west of Denver in the heart of the Front 

 Range of the Rockies. Central City, the seat of Gilpin County, is 

 the oldest lode-mining camp in Colorado and in total production one 

 of the most important. 



Most of the rocks of the region are pre-Cambrian in age. Some of 

 them have undergone severe dynamic metamorphism ; others are prac- 

 tically unmetamorphosed. The pre-Cambrian rocks are intruded by 

 a great variety of much younger igneous rocks, most of them por- 

 phyritic in texture. Though they show some diversity in age, all are 

 believed to have been intruded in Tertiary time. They occur as stocks 

 and dikes, and are abundant in all parts of the region. 



The region forms part of a broad mineralized belt which embraces 

 most of the economically important mining camps of Colorado. The 

 ' ores of the region may be classed, according to the metals which give 

 them their predominant value, as (1) gold-silver ores, which constitute 

 the main economic resource of the region; (2) uranium ores, which 

 occur in a few places only but are of much interest as a source of ra- 

 dium; (3) tungsten ores, which form the basis of the tungsten industry 

 of Boulder County, the most productive center for this metal in the 



