248 
THE GEAND OAS^ON DISTRICT. 
di’ainage, the absence of vegetation is accompanied by absence of soil. 
When a shower falls, nearly all the water runs off from the bare rock, and 
the little that is absorbed is rapidly reduced by evaporation. Solution be- 
comes a slow process for lack of a continuous supply of water, and frost 
accomplishes its work only when it closely follows the infrequent rain. 
Thus weathering is retarded. Transportation has its work concentrated 
by the quick gathering of showers into floods so as to compensate, in part 
at least, for the smallness of the total rainfall from which they derive their 
power.” (Geology of the Henry Mountains : G. K. Gilbert.) 
In this analysis of Mr. Gilbert I fully concur. It remains only to 
apply the principles he has developed. The effects which he deduces from 
an arid climate in a high country are a scanty soil, a diminished rate of 
weathering, and a great efficiency of transportation. We must further con- 
sider the efiFects of these varied conditions upon a country composed of hori- 
zontal strata which are vertically heterogeneous. The paucity of soil lays 
bare the edges of the rocks. The gentler slopes or taluses being found in 
the softer beds these are more readily weathered than they would be if the 
soil were more abundant. But harder beds are not so easily dissolved, and 
can be broken down only by the undermining resulting from the waste of 
underlying softer beds. Hence the hard strata form vertical ledges, while 
the softer beds form taluses or steep slopes, partially protected by debris 
and soil. In a word, the effect of an arid climate upon such a region as 
the Plateau Country is to increase the amount of bare rock, to sharpen the 
profiles and make them irregular, and to generate cliffs. To enforce this 
idea, let us imagine a moist climate returning to this region. The rate of 
weathering in the harder beds would be accelerated and the fragments and 
finer material would increase the amount of soil lying upon the sloped edges 
of the softer beds and the weathering of the latter would be retarded. Vege- 
tation would start into life and conserve this soil by clogging transportation, 
and the profiles would gradually lose their abrupt angular character and 
become softened and rounded, like those of the Appalachians. 
If the Plateau Country were at a much lower altitude the case would 
be very different. Transportation would then fail for want of declivity and 
soil would accumulate because it could not be carried away. This, I con- 
