Absence of rock in place--that is, of ridges or strata of rock rising through the soil above or nearly to the surface--has determined the character not only of the Plains but of much of the roll of the great rivers east and south of them. Even at the very base of the Rocky Mountains, the Chugwater shows a milky though rapid current, while the North Platte brings a considerable amount of earthy sediment from the heart of that Alpine region. After fairly entering upon the Plains, every stream begins to burrow and to wash, growing more and more turbid, until it is lost in 'Big Muddy,' the most opaque and sedimentary of all great rivers. I suspect that all the other rivers of this continent convey in the aggregate less earthy matter to the ocean than the Missouri pours into the previously transparent Mississippi, thenceforth an unfailing testimony that evil company corrupts and defiles. Louisiana is the spoil of the Plains, which have in process of time been denuded to an average depth of not less than fifty and perhaps to that of two or three hundred feet. I passed hills along the eastern base of the Rocky Mountains where this process is less complete and more active than is usual,--hills which are the remaining vestiges of a former average level of the plain adjacent, and which have happened to wear away so steeply and sharply that very little vegetation ever finds support on their sides, which every rain is still abrading. At a single point only do I remember a phenomenon presented by some other mountain bases,--that of a water-course (dry perhaps half the year, but evidently a heady torrent at times), which had gradually built up a bed and banks of boulders, pebbles and gravel, washed down from a higher portion of its headlong course, so that its current, when it had a current, was considerably above the general surface on either side of it. Away from the mountains, however, boulders or loose stones of any size are rarely seen in the beds of even the largest and deepest channeled streams, which are usually swift, but never broken by a fall, because never down to the subjacent rock in place, assuming that such rock must be.