REVIEWS 99 



the presence of bodies of water in all the large plain-like valleys in the 

 region of the present continental divide in the latitude of the Mexican 

 border. They niay have been more or less isolated by the low divides 

 connecting the mountain ranges, but were probably at or near sea 

 level. 



During the deposition of the beds and following them flows of lava 

 occurred in places, and then the uplift and arching of the crust along 

 the present divide. 



This differential uplift gave rise to the conditions necessary for 

 the excavation of the great canyons of the Plateau region and per- 

 haps originated the Colorado river. The lower portion of this stream 

 as well as the Mohave river has been superimposed upon an old topog- 

 raphy as shown at numerous points. 



Some Gypsum Deposits of Northwestern Nevada. By Geo. D. 



LOUDEKBOCK, RcHO, NcV. 



A DEPOSIT of gypsum occurs some six or seven miles south of 

 Virginia City. It is an isolated mass inclosed in limestone walls, the 

 Avhole surrounded, in the main, by diorite. The principal bodv gives 

 a surface exposure of over loo yards in width and over 150 vards in 

 length. This is practically all gypsum almost vertical in attitude. The 

 mineral occurs as a milk-white holocrystalline granular aggregate, with 

 some soft secondary earthy material at the surface and along a 

 gullv draining the area. The granular variety is quite pure, running 

 generally over 90 per cent. CaSOj,.2H2 0, the chief impurity being 

 CaCOg. 



Another large deposit occurs in the Humboldt range some five 

 miles east of Lovelock. It occurs interstratified with limestone and a 

 little quartzite. While undisturbed by intrusion, it is faulted and 

 folded. Surface exposures of 300 yards or more in width occur, some 

 two-thirds of which is gypsum, the rest mainly limestone. The thick- 

 est gypsum bed is about 150-175 feet thick. The mineral occurs as a 

 holocrystalline granular aggregate except on the surface and lower 

 slopes down which it has washed. It runs from 95-97 per cent. 

 CaS0^.2H2 0, practically the only impurity being calcite. 



Reasons are given for believing that both of these gypsum deposits 

 are original members of the stratigraphic series — /. <?., that they were 

 formed by deposition from a saline water-bodv, and are probablv of 

 Middle or Upper Triassic age. 



