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 time 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; ‘rs that 
many new and recent basins have been produced, and second, that a 
large part of the drainage and salt supply of the earlier ome 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 smks. 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 
