198 REVIEWS 



Geology and Coal Resources of North Park, Colorado. By A. L. 



Beekly. U.S. Geol. Survey, Bull. 596, 1915. Pp. 118, 



pis. 12. 

 North Park is described as a synclinal basin limited on the east and 

 west by anticlinal mountain ranges, on the south by a high ridge com- 

 posed of Tertiary extrusives, and on the north by an area of pre-Cambrian 

 crystalline rocks faulted up into contact with the Paleozoic and later 

 sediments of the Park. The latter comprise two sharply distinct groups: 

 the lower, which rests upon pre-Cambrian crystallines, begins with a 

 few feet of limestone of doubtful age, at the base of the Red Beds, and 

 ends with the Pierre shale — 7,400 feet of beds in all, apparently con- 

 formable throughout; the upper group comprises some 5,000 feet of 

 uppermost Cretaceous or Eocene strata included in the Coalmont for- 

 mation. The folding of the region took place in part prior to the 

 deposition of the Coalmont and in part later. Moderately extensive 

 faulting accompanied or followed the later folding, and therefore 

 affected the Coalmont in common with the older rocks. Lying upon 

 the Coalmont in uncertain relationship is the North Park formation, 

 a 600-foot series of clastic sediments interbedded with sheets of pyro- 

 clastic volcanics. The Tertiary igneous rocks near the southern border 

 of North Park occur in both intrusive and extrusive relations. Ande- 

 sine basalt and volcanic agglomerate are the chief types. Granite is the 

 chief constituent of the pre-Cambrian complex exposed in the mountains 

 east, west, and north of the Park. 



Coal occurring in the lower part of the Coalmont formation is the 

 one mineral resource of this area which is now being exploited. More 

 than two billion tons of sub-bituminous coal are estimated as the avail- 

 able reserve. The coal seams are of unusual thickness, one bed reaching 

 a thickness of more than 50 feet, and maintaining an average thickness 



of so feet along a 15-mile outcrop. 



C. W. T. 



Common Minerals and Rocks: Their Occurrence and Uses. By 



R. D. George. Bull. Colo. State Geol. Survey No. 6, 1913. 



Pp. 406, pis. 5. 



The main purpose of this bulletin is to describe the commoner 



minerals and rocks, and furnish the means of recognizing them and 



knowing their uses. 



V. 0. T. 



