334 GEOLOGICAL SURVEY OF THE TERRITORIES. 



mentioned, and not less interesting or easily read. By the complete 

 drainage of the northern and southern thirds of the plateau through 

 the channels of the Columbia and Colorado, the water surface of this 

 great area was reduced to the tenth or hundredth part of the space it 

 previously occupied. Hence the moisture suspended in the atmosphere 

 was diminished in like degree, and the dry hot air, sweeping over the 

 plains, licked up the water. from the undrained lakes until they were re- 

 duced to their present dimensions. Now, as formerly, they receive the 

 constant flow of the streams that drain into them from the mountains 

 on the east and west. But the evaporation is so rapid that their di- 

 mensions are not only not increased thereby, but are steadily diminish- 

 ing from year to year. Around many of these lakes, as Salt Lake, for 

 example, just as around the margins of the old drained lakes, we can 

 trace former shore lines and measure the depression of the water level. 

 Many of these lakes of the Great Basin have been completely dried up 

 by evaporation, and now their places are marked by alkaline plains or 

 "salt flats." Others exist as lakes only during a portion of the year, 

 and in the dry season are represented by sheets of glittering salt. Even 

 those that remain as lakes are necessarily salt, as they are but great 

 evaporating pans where the drainage from the mountains — which al- 

 ways contains a portion of saline matter — is concentrated by the sun and 

 wind until it becomes a saturated solution and deposits its surplus salts 

 upon the bottom. 



The southern portion of the great central table-land — that which has 

 been denominated the Colorado plateau — is almost without mountain 

 barriers or local basins, and we therefore find upon it fewer traces of 

 ancient lakes, though they are not entirely wanting. It is apparent, how- 

 ever, that thishigh plateau, which stretches away for several hundred miles 

 west of the Rocky Mountains, was once a beautiful and fertile district. 

 The Colorado draining then, as now, the western ranges of the Rocky 

 Mountains, spread over the surface of this plateau, enriching and vivi- 

 fying all parts of it. When it reached the western margin of the table- 

 land, however, it poured over a precipice or slope five thousand feet in 

 height, into the Gulf of California, which theu'reached several hundred 

 miles farther north than now. In process of time the power developed 

 by this stupendous fall cut away the rock beneath the flowing water, and 

 formed that remarkable gorge to which 1 have already referred. This 

 gorge is nearly one thousand miles in length and from three thousand 

 to six thousand feet in depth, and is cut through all the series of sedi- 

 mentary rocks from the tertiary to the granite, and has worn out the 

 granite to a depth of from six hundred to eight hundred feet. Just in 

 proportion as the Colorado deepened its channel, the region bordering 

 it became more dry, until ultimately the drainage from the mountains 

 passed through it, in what may be even termed underground channels, 

 and contributed almost nothing to the moisture of the surrounding 

 country. The reason why the walls of this cafion stand up in such 

 awful precipices of thousands of feet is, that the perennial flow of the 

 stream is derived in far distant mountains ; almost no rain falls upon 

 its banks, and when any portion of the bordering cliff has passed be- 

 yond the reach of the stream, it stands almost unaffected by atmospheric 

 influences. 



On the east of the Rocky Mountains lies the country of the "Plains," 

 a region not unlike in its topography to the great plateau of the West, 

 but differing in this, that it is not bordered on the east by a continuous 

 mountain chain ; that it slopes gently downward to the Mississippi, and 

 that its eastern half has been so well watered that its valleys have been 



