TOPOGRAPHIC FEATURES OF THE DESERT BASINS. 7 



streams, and it is probable that few new alluvial dams were formed. 

 But with the advancing aridity which has caused the disappearance 

 of the lakes many valleys once freely open have been barred by 

 alluvial dams and converted into areas of inclosed drainage. Ob- 

 viously this has great importance from the present viewpoint. A 

 valley where inclosed drainage is a condition of recent origin can not 

 reasonably be expected to have retained important quantities of 

 salts. In cases, therefore, where the boundaries of valleys are 

 alluvial dams it is necessary to determine so far as may be possible 

 the age of the dams, and whether they are sufficiently old and per- 

 manent to have retained behind them the more plentiful waters of 

 the lake period. 



The building of alluvial dams has been accompanied by another 

 basin-creating process — the decay of the drainage systems due to 

 an excess of evaporation over rainfall and the consequent failure of 

 streams to maintain themselves over their whole length. In this 

 way local depressions in the valleys become cut-off lakes, and chan- 

 nels or flood-plains become alkaline flats, even without the formation 

 of important alluvial dams. Very much of the West is not so much 

 an area of inclosed drainage as one of no drainage, but thousands of 

 dry stream beds furrow its surface and scores of greater channels 

 bear witness to a tune when rivers were not all of sand. Occasional 

 floods may fill these channels for a day; there may be still some 

 constant drainage through them as underflow, but essentially they 

 are dead and the alkali flats which dot their courses mark the places 

 of their burial. 



Alluvial damming and stream decay mean two things; first that 

 many new and recent basins have been produced, and second, that a 

 large part of the drainage and salt supply of the earlier basins has 

 been cut off; for these processes have been just as active in the 

 regions tributary to the greater basins as in regions once tributary 

 to the sea, and the areas from which salt and water now reach those 

 basins are often but a small fraction of what was once their compass. 

 This, however, is not a matter of great importance. The answer 

 to it is the same as to the statement — frequently made as an objec- 

 tion to the general potash theory — that the desert basins are too 

 arid for the occurrence of rock decay and the freeing of potash. 

 The basins were not always so arid. The lake period was one of 

 considerable humidity, and we may be sure that during it plenty of 

 potash was freed and carried to the central sinks. The doubt is 

 not whether there is any potash, but where it is and whether it has 

 been sufficiently segregated. 



There remains to notice one more aspect of the history of the 

 region. It has already been noted that extensive salt deposits are 

 very rare on the surfaces of the present basins. In many of the 



