374 GEOLOGICAL EXCURSION TO THE ROCKY MOUNTAIN'S. 



structure exists. An acid lava (rhyolite) forms the bed of the stream 

 and is overlain by a sheet of basalt. 



A short distance above Shoshone falls are the Upper or Twin falls, 

 about 180 feet high, which drop from a cliff formed by the two lower 

 sheets of basalt. Red tuff and volcanic agglomerates are found in 

 places between the successive sheets of lava. 



A hundred miles or more further down the stream the basalts have 

 been observed to rest directly on sedimentary beds of recent age 

 probably deposited in some inclosed fresh- water lake. Other cascades 

 and falls are known to exist along the course of the Snake River, but 

 its canyon has never, so far as known, been followed continuously, and 

 their number and its extent is unknown. The stream is unusually 

 rapid for its size, in Spite of these many leaps in its course, and i.s eut- 

 ting down its bed very fast, though yet far from reaching a baselcvl 

 Of erosion. From longitude 112° W. to longitude 117° W. its total 

 descent has been about 2,500 leet (762 m.). 



GllEAT SALT LAKE AND LAKE BONNEVILLE. 4950 

 By <L K. GlLBBRT. 



A large district of interior drainage, lying to the west of the Wasatch 

 mountains and the Plateau region, is known as the Great basin. 

 It includes nearly the whole of the state of Nevada, the western half of 

 Utah, and smaller portions of Idaho, Oregon, ami California It is 

 naturally subdivided into a number of smaller basins, from each of 

 which the entire product of precipitation is evaporated, so that there 

 is no discharge to the ocean. In most of the basins there are no per- 

 manent lakes, but temporary lakes are produced by the waters of each 

 great storm. In a few basins there are permanent lakes with saline 

 waters. The largest of these is Groat Salt lake, which receives the 

 waters of the Bear, Weber, and Jordan rivers, ami has an area of about 

 1,800 square miles (4,500 sq. km). The extent and depth of the lake 

 are determined by the balance between inflow and evaporation. In 

 years of great rainfall the surface of the lake rises, and in dry years the 

 waters recede. During the past thirty-five years the water height has 

 several times oscillated through a , range of 11 feet (3*3 m.), and it is now 

 (1891) near its lowest observed stage. The salinity undergoes corres- 

 ponding changes, being greatest when the lake is low. The solid eon 

 tents now amount to about 20 per cent, of which four-fifths is sodium 

 chloride. Sodium sulphate is naturally precipitated by the cold of each 

 winter, and afterwards redissolved. The lake is very shallow, having a 

 mean depth of 13 feet (1 m.) and a maximum depth of less than 10 feet 

 (12 m.). It is inhabited by a brine shrimp and the larva of a fly. 



