In the rare instances of rocky banks skirting the immediate valley of a stream, the seeming rock is evidently a modern concrete of clay and the usual sand or gravel composing the soil,--a concrete slowly formed by the action of sun and rain and wind, on a bank left nearly or quite perpendicular by the wearing action of the stream. In the neighborhood of Cheyenne Pass,--say for a distance of fifty to a hundred miles S.S.W. of Laramie,--this effect is exhibited on the grandest scale in repeated instances, and in two or three cases for an extent of miles. Along either bank of the Chugwater, at distances of twenty to forty miles, above its junction with the Laramie affluent of the North Platte, stretch perpendicular rocky terraces, thirty to forty feet high, looking, from a moderate distance, as regular and as artificial as the façade of any row of city edifices. I did not see 'Chimney Rock,' farther down the Platte; but I presume that this, too, is a relic of what was once the average level of the adjacent country, from which all around has been gradually washed away, while this 'spared monument' has been hardened by exposure and the action of the elements from earth to enduring rock--a gigantic natural _adobe_. The Plains attest God's wisdom in usually providing surface-rock in generous abundance as the only reliable conservative force against the insidious waste and wear of earth by water. Storms, rills, and rivers are constantly at work to carry off the soil of every island and continent, and lose it in the depths of seas and oceans. Rock in place impedes this tendency, by arresting the headlong course of streams, and depositing in their stiller depths the spoils that the current was hastening away; still more by the formation of swamps and marshes, which arrest the sweep of fires, and so protect the youth and growth of trees and forests. An uninhabited, moderately-rolling or nearly flat country, wherein no ridges of stubborn rock gave protection to fire-repelling marshes, would gradually be swept of trees by fires, and converted into prairie or desert.