EFFECT OF AN ARID CLIMATE. 
249 
ceive, is the state of atfairs in the Sahara and Arabian deserts. Vast ex- 
panses of drifting sand are formed which can be carried away only by the 
winds. In those deserts there are no rivers whatever, and transportation 
by streams is, therefore, at absolute zero. In the Plateau Country the 
streams, though few and mostl}^ spasmodic, are still sufficient to lay bare 
an unusual amount of rock-surface. And the country is also so high that 
even many of the spasmodic streams can comrade deeply. 
We have also mentioned in preceding chapters that another effect of an 
arid climate is to suppress the smaller streams, and to leave only a few of 
the most powerful ones which head in the highlands around the borders of 
the Colorado drainage basin. The intervals between these persistent streams 
are not corraded, but remain as flat surfaces These flat surfaces may be 
regarded as so many local base-levels maintaining a considerable amount of 
soil which protects them from erosion. The only way in which erosion can 
prosecute its attack at present is by sapping the faces of the walls laid bare 
by the few streams which are still able to corrade. The total process of 
erosion, therefore, resolves itself into the method which Powell has so aptly 
called the Recession of Cliffs. The study of some of the more striking de- 
tails of this process will be one of the themes of the following chapter ; and 
it is full of beauty and interest 
