1890. ] Erosive Agents in the Arid Regions. 459 
proved by the great number of new holes in every “ dog town,” 
and also by the number of holes in old roads passing through 
sections inhabited by the prairie dog. Deserted burrows can 
always be told by the destruction of the mound, and widening of 
the hole under the influence of subaerial denudation. The towns 
are generally built on gentle sloping hills, where there is some 
drainage, consequently the fresh clay and earth brought to the 
surface is gradually drifted away. Not only is the work of 
erosion aided by the bringing up of material to the surface, but 
also by the opening of tunnels, by which water is allowed free 
underground passage. With a mound around the hole this in- 
flowing of water is reduced to a minimum; but when the hole is 
abandoned, and the mound destroyed, water freely enters, and in 
such cases I have frequently noticed small runlets leading to the 
hole. 
Thus it will be seen that the great agent of erosion, water, is 
of particular importance in the arid regions on account of its in- 
tensified action during brief periods; that the direct effect of 
aerial currents is a powerful factor in the erosion of these pla- 
teaux; that the sun’s heat must also play an important part; that 
plants are only slightly conservative and slightly destructive; 
and that animals, by tramping down the soil or by burrowing and 
tunneling into the earth, are important aids in the great work of 
subaerial denudation by which the plateaux of the west are being 
gradually eaten away. 
