628 SOIL BELOW ROCK. 
once formed a part of that gulf. This northern extension of 
the gulf appears to have been cut off from the main body by 
deposits brought down by the great river Colorado, at no very 
distant period. These deposits at the same time turned the 
course of the river to the south, and it now enters the gulf at a 
point twenty miles distant from its original outlet. 
When this northern arm of the gulf was cut off from the 
sea, and the river which once discharged itself into it was di- 
verted, it was speedily laid dry by evaporation, and now yields 
no vapor to be condensed into fog, rain, and snow on the neigh- 
boring mountains, which are now parched and almost bare of 
vegetation. 
The ancient bed of the river may still be traced, and in 
floods the Colorado still sends a part of its overflowing supply 
into its old channel, and for a time waters a portion of the 
desert. It is believed that the river might easily be turned 
back into its original course, and indeed nature herself seems 
to be now tending, by various spontaneous processes, to accom- 
plish that object. The waters of the Colorado, though perhaps 
not sufficient to fill the basin and keep it at the sea-level in 
spite of the rapid evaporation in that climate,* would at least 
create a permanent lake in the lower part of the depression, the 
evaporation from which, Dr. Widney suggests, might sensibly 
increase the humidity and lower the temperature of an exten- 
sive region which is now an arid and desolate wilderness. 
Soil below Lock. 
One of the most singular changes of natural surface effected 
by man is that observed by Beechey and by Barth at Lin 
Tefla, and near Gebel Gentines, in the district of Ben Gasi, in 
Northern Africa. In this region the superficial stratum origi- 
nally consisted of a thin sheet of rock covering a layer of fer- 
tile earth. This rock has been broken up, and, when not prac- 
* The thermometer sometimes rises to 120° F, at Fort Yuma, at the S. E. 
angleof Californiain N. L. 33°. 
