No. l.J GEOLOGY AND MINERALOGT. 117 



above the strata of cooled lavn. The banks of the Dos Chutes 

 River and Coluiiib:a afford splendid sections of these lake deposits, 

 where the history I have so hastily sketched may be read as from 

 an open book. 



But, it will be said that there are portions of the great central 

 plateau which have not been drained in the manner I have described. 

 For here are basins which have no outlets, and which still hold 

 sheets of water of greater or less area, such as those of Pyramid 

 Lake, Salt Lake, etc. The history of these basins is very difft^rent 

 from that of those already mentioned, but not less interesting nor 

 easily read. By the coniplete draimige of the northern and 

 southern thirds of the plateau through the channels of the Columbia 

 and Colorado, the water surface ot this great area was reduced to 

 the tenth or one-hundredth part of the space it previously occupied. 

 Hence, the moisture suspended in the atmosphere was diminished 

 in like deg-ree, and the dry hot air, sweeping over the plains, licked 

 up the water i'rom the undrained lakes until they were reduced to 

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

 constant flow of the streams that drain into them from the moun- 

 tains on the east and west, but the evaporation is so rapid that their 

 dimensions are not only not increased thereby, but are steadily 

 diminishing from year to year. Around niany of tliese lakes, as 

 Salt Lake for example, ju.st as around the margins of the old 

 drained lakes, we can trace former shore lines and measure the 

 depression of the water level. Muny 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 diy 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 always 

 contains a portion of saline matter, is concentrated by the sun 

 and wind until it becomes a saturated solution, and deposits its 

 surplus salt upon the bottom. 'i^ ^ jl? ^ ^ 

 The pictures which geology holds up to our view, of North 

 America during the Tertiary ages, are, in all respects but one, 

 moreattraclive and interesting than could be drawn from its present 

 aspects. Then a warm and lienial climate prevailed from the 

 Gulf to the Arctic Sea; the Canadian highlands U'cre higl.er, but 

 the Rocky Mountains lower and le.ss broad. 3lwst of the conti- 

 nent cxliibited an uadulating surface ; rounded hiils and broad 



