8 BULLETIN 54, U. S. DEPARTMENT OF AGRICULTURE. 
basins no salts at all are visible. There can be no reasonable doubt 
that 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 will 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 applicability to the present 
problem. Wecan 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 lme. 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 
‘“Gnterlake 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 believed to belong (with perhaps one exception) to this 
second period of accumulation. The “interlake” salt—probably 
far larger in amount—is believed to be everywhere 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 
