752 REVIEWS 



In a region of concentrated rainfall and long drouths the river mouths become 

 temporarily closed, and the abundant aquatic and other life in the lagoons thus 

 formed contributes to the organic acid of the waters which, upon penetrating 

 the wall or dam of beach sand, first dissolves the lime, and then redeposits it when 

 it comes in contact with the dense sea-water on the ocean side. In this manner 

 some portions of the beaches have been hardened, while others have remained 

 incoherent. 



The density of the ocean water is in all probability considerably greater during 

 the dry than during the rainy season, and this would still further hasten the con- 

 solidation of the beaches during dry seasons. 



The process of beach-hardening is not a continuous one, but varies with 

 geographic and climatic conditions. New reefs may be fornied behind the older 

 ones on the shores of the estuaries and embayments. 



The monograph is closed by a chapter on the associated coral reefs. 



T. C. C. 



The Copper Deposits of the Encampment District of Wyoming. By 

 Arthur C. Spencer. (Professional Paper No. 25, 1904.) Pp. 

 107 ; 2 plates and 49 figures. 



The district of which Mr. Spencer treats in this report comprises some 

 450 square miles in southern Wyoming, so situated that the southern 

 boundary of the area lies close to the Colorado line, while the one hundred 

 and seventh meridian bisects it. The encampment district is crossed 

 diagonally from southeast to northwest by the irregular line of the Conti- 

 nental divide, which is the crest of the Sierra Madre Mountains. The 

 maximum elevation is 11,007 ^^^^ (Bridger Peak), and the minimum 

 6,650 feet. 



The structure is essentially a low arch or anticline whose axis is parallel 

 with the mountain crest, that is, east-west. The core of this arch is made 

 up of a mass of hornblende schists and sedimentary rocks, closely folded and 

 overturned to the north, the whole forming a complex east-west synclino- 

 rium. These rocks are said to be pre-Cambrian, though the reasons for 

 this conclusion are not given. They were invaded at a later period by 

 basic igneous rocks related to gabbros. The hornblende schists appear 

 everywhere to be the basement upon which the sediments were laid down. 



Flanking these pre-Cambrian rocks is a series of Mesozoic and Cenozoic 

 sediments which are now confined to the southwestern part of the area, as a 

 remnant of a great mantle of sediments that formerly covered the whole 

 arch. These formations dip away to the south beneath the surrounding 

 prairie. The oldest of these formations is the "Red Beds" series of the 



