FRESHENING OF LAKES BY DESICCATION. 229 



are level-floored, and have the same general contour as many neighboring 

 valleys which are occupied by playas/' 



Applying this line of argument to all the inclosed lakes of the Great 

 Basin, we find, with the exception of Great Salt and Sevier Lakes — the 

 Soda lakes near Ragtown, Nevada, and Mono Lake, not being considered, 

 as they ai'e of an exceptional character — that there is not a lake among the 

 number that could have undergone the present rate ot concentration for 

 more than a very few centuries without being far more saline than we now 

 find it. By consulting Table C, it will be seen that with the exceptions we 

 have mentioned there is not a lake in the arid region of the West that con- 

 tains more than one-fiftieth of the quantity of the more common salts nec- 

 essary for saturation. This appears to be prima facie evidence that these 

 lakes have undergone some process by which their salts have been elimin- 

 ated within very i-ecent times. Li the case of Great Salt Lake we find an 

 exception not only in the amount of saline matter it holds in solution, but 

 also in its environment. With this exception, all the lakes of the Great 

 Basin occupy narrow valleys in which a lake on evaporating would deposit 

 its salts in a comparatively restricted area, thus favoring their burial by 

 playa deposits. The basin of Great Salt Lake, on the other hand, is not 

 only of broad extent, but receives its entire water supply from one side, and 

 is thus unfavorable in its topographical relations for the burial of products 

 of desiccation beneath playa deposits. The present density of this lake 

 ma)' therefore be due to the fact that its salts were not buried during a 

 time of desiccation which admitted of this result in smaller basins ; conse- 

 quently, when the valleys were reflooded, the lakes in the smaller basins 

 were fresh, while in the larger one the unburied saline deposits left by the 

 evaporation of the former lake were redissolved. 



The order in which a number of inclosed lakes in an arid reg-ion like 

 the Great Basin, will become dry during a time of more than usual desic- 

 cation, depends on many conditions ; one of the most important of which 

 is the ratio of evaporating surface to elevated catchment basin A lake 

 whose hydrographic basin is low will be extremely sensitive to climatic 



"A comparison of the lake basins of the Lahontaii region with the depressions holding the Lan- 

 rentiau lakes, shows that the bottoms of the former are more nearly horizontal and far more regular 

 than those of the latter. 



