174 Professor H. D, Rogers, on the Geology and [Feb, 8, 



D. The Great Western Desert Plateau. 



Between the Rocky Mountains on the east, and the Pacific Alps, 

 Cascade Chain, and Sierra Nevada on the west, there stretches 

 from the Gulf of California to the Arctic Ocean, an elevated 

 desert zone of great breadth and height, especially in its central 

 section, between latitudes 35^ and 45°, where its mean level above 

 the sea is about 5000 feet. It consists of three principal regions, — 

 the Central Desert Plateau, now indicated ; a semi-desert area, 

 south-east and south of this, the basin of the Rio Colorado, divided 

 from it by the Wahsatch Mountains ; and a northern region, that 

 of the Columbia and Frazer's Rivers, sterile and rugged, separated 

 from the central by the mountains south of the Columbia. 



The Central Plateau, or Great Utah Desert, is an almost 

 rainless region, parched by dry winds. The evaporation balancing 

 the rain-fall, none of its scanty drainage passes to the sea, but each 

 attenuated stream ends in a closed lake, the water of which is more 

 or less salt. It is avast elevated arid waste, containing wide plains 

 encrusted with salt, a soil generally more or less saline, and large 

 tracts of light volcanic scoriae and ashes. Through its centre there 

 runs, southward, a broad belt of straight mountain ridges, the 

 Humboldt Mountains, composed in part of crystalline rocks, in part 

 of carboniferous limestone, and other palaeozoic strata. The chief 

 stream is the Humboldt River, which, flowing west, dies away in 

 the salt plain before it reaches the foot of the Sierra Nevada. 

 There are twelve or fifteen salt lakes of considerable magnitude in 

 this high insulated basin. The largest of these is the Great Mor- 

 mon, or Utah Salt Lake, the waters of whicli are impregnated 

 almost to saturation, containing 20 per cent, of common salt, and 

 2 per cent, only of other salts. Its length is 75 miles, and mean 

 breadth about 30 miles. 



The SouUiern Desert Basin, or that of the Rio Colorado, lying 

 between the Rocky Mountains and the Wahsatch and Californian 

 Mountains, is, like the Utah Desert, a succession of arid table lands 

 and plains, intersected by rugged mountains. Its north-eastern por- 

 tion is better fed with rain than its western ; those tracts most under 

 the lee of the intercepting barrier of the Pacific Mountains being 

 more desiccated than the districts further removed. The Pacific 

 Mountains are also less elevated in this latitude than further north, 

 opposite the Central Desert. This Southern or Colorado Desert 

 slopes gently southward to the ocean level at the head of the Gulf 

 of California, north of which there is an arid tract of the surface, 

 which is actually lower than the surface of the sea, believed to have 

 a depression in its centre of 300 feet, — a region of continental 

 depression, from which the dry winds have lapped up the remnant 

 waters of the Gulf of California, and annually drink away every 

 trace of the back waters of the Rio Colorado, at the season of its 

 rise from the rains in the mountains. In some localities along the 



