8 BULLETIN 54, U. S. DEPARTMENT OF AGEICULTUKE. 



basins no salts at all are visible. There can be no reasonable doubt 

 tbat large amounts of salts have entered these basins and remained 

 there. Where are they now? To meet this dilemma Gilbert and 

 Russell devised the theory of salt burial and of "freshening by 

 desiccation." Essentially this theory says that when a body of salt 

 is left behind by a desiccated lake on a playa or its topographic 

 equivalent, this salt body may ultimately be covered by inwashed 

 clay and sand without solution, and if a second lake comes later to 

 occupy the basin the buried salt deposit will be protected by its 

 alluvial seal and wWl. remain undissolved. Certain stages of this 

 process have actually been observed, and there is little doubt of the 

 essential correctness of the theory or of its apphcability to the present 

 problem. We can assume quite safely that the salt which must have 

 been in the great Quaternary lakes is now buried beneath the floors 

 of their basins. 



There arises at once the question of the horizon at which these salts 

 are to be found, and the duplicity of the lake period seems to furnish 

 at least a suggestion along this line. Periods of lake expansion and 

 stream vigor are periods of salt accumulation. It should be con- 

 centrated and deposited when the lakes evaporate. There are, there- 

 fore, at least two horizons at which salt deposits are to be looked for: 

 (1) That corresponding to the drying of the first great lake (the 

 ''interlake arid period") and (2) that corresponding to the drying of 

 the second great lake; that is, the arid period of the present and 

 the recent past. The few surface salt deposits known in the desert 

 basins are beheved to belong (with perhaps one exception) to this 

 second period of accumulation. The "interlake" salt — probably 

 far larger in amount — is beheved to be everjnvhere more or less 

 deeply underground. 



The various undrained areas outside the Great Basin have had their 

 own structural histories, sometimes analogous to that of the basin 

 but more often not. Where necessary these structural histories will 

 be noted briefly in the detailed chapter which follows. The climatic 

 history, however, has been everywhere the same. In particular the 

 processes of alluvial damming and of stream decay have been as 

 active outside the Great Basin as within it, and indeed most of the 

 undrained areas external thereto have originated in this way. The 

 contraction and mutilation of the great drainage systems have left 

 tremendous areas now without seaward drainage and split into 

 inclosed basins of larger or smaller area. The following chapter will 

 furnish numerous illustrations. 



A brief word as to nomenclature is perhaps necessary. The double 

 period of lake expansion has been variously referred to as ''Quat- 

 ernary," "Pleistocene," "Glacial," etc. All of these terms carry 

 suggestions of chronology and correlation, the discussion of which 



i 



