104 BOTANICAL FEATURES OF NORTH AMERICAN DESERTS. 
direction. According to Russell (I. C. Russell, Geological History of Lake 
Lahontan, Monograph 11, U. S. Geological Survey, 1886), the Great 
Basin region of Nevada underwent a period of desiccation before it 
received the waters of Lake Lahontan, which, after reaching a maximum 
height without overflowing, receded to leave the region even more com- 
pletely desert than at the present time. The next oscillation toward 
increase of the water-supply carried the lake back to a level higher than 
the first, and then receded to leave a desert for the third time, which, so 
far as weight may be given to some of the evidence, reached its greatest 
aridity a few hundred years ago; so that the Nevada desert is now not so 
dry as it has been within the period witnessing the advent of man. 
Climatological and geological changes of this kind work a double 
effect upon a region. The decreasing rainfall results in a general aridity 
of the entire region affected, and the reduction of the streams is followed 
by a recession of lakes and other sheets of water by which basins with the 
soil highly charged with salts are left to specialized types of vegetation. 
Comparatively local causes, such as the cutting off of the head of the Gulf 
of California by the silt of the Colorado River, may leave a basin to become 
a part of the contiguous desert, as is also the case with the Pattie Basin 
to the southward. Changes of level by both upheaval and subsidence 
may also contribute to the formation and extension of deserts, as indi- 
cated in a preceding section. 
The great Lake Eyre basin in southern central Australia is taken by 
some geologists to have had a complicated history, by which the region 
in which it was included was slowly depressed until flooded by the sea, then 
emerged to undergo the resultant changes in the muds that covered its 
floor. In a second subsidence the surface was carried down to a level 
where its floor was 39 feet below sea-level. Two rivers carried such vol- 
ume of water into it that it became the bed of a fresh-water lake, with a 
luxuriant development of plant and animal life which found agreeable 
conditions for existence. The entrance from the sea appears to have 
become filled, the amount of river flow decreased, the annual evaporation 
in the basin being about 86 or 88 inches per year, and a desiccation set in 
probably early in the Pleistocene, which continued to the present time, 
although, as in all deserts, small bodies of water occasionally collect 
in the lower parts of the basin. (J. W. Gregory, The Dead Heart of 
Australia, London, 1906.) 
Lastly, many writers hold to the opinion that distinct changes in 
climate with increasing aridity have taken place on various parts of the 
earth’s surface long since the advent of man and well within historic 
time. The most pronounced of such views is held by Huntington con- 
cerning central Asia. He says: 
To sum up the history of Lop-Nor during the last 2,000 years: We have first 
a comparatively large lake, said to measure 75 miles each way, in spite of the fact 
that the populous towns of Lulan and of more remote regions diverted much more 
water than now. Next, during the early centuries of the Christian era, there is 
