40 EXPLORATIONS IN TURKESTAN.
the Aralo-Caspian region is concerned, the expansion of the sea was the cause rather than the consequence of local climatic changes. Petrusevitch (1880) ascribes the shrinkage of the sea and the withering of the Murg-ab and the Tejen from an inferred connection with the Amu, to an assumed destruction of forests in the neighboring mountains. The probability of repeated Quaternary expansions of the sea does not appear to have been considered ; but in this statement I may be doing injustice to Russian observers, whose more recent articles I have not been able to
THE PIEDMONT PLAINS.
Since the withdrawal of the Pliocene sea, the eastern and southern borders of the plains of southern Turkestan appear to have been aggraded by the rivers that flow out upon them from the mountains. That a certain measure of such constructive action has taken place is announced by the Russian geologists, but it is not apparent that the full measure of river action has been recognized. Some of the strata of the plains are said to be not fluviatile but lacustrine, because they are of fine texture and uniform structure, without the variable layers of gravel that are by implication supposed to be always indicative of river work ; but this seems to be a simpler solution than the problem deserves. There are many rivers that do not carry gravel, and there are many river plains whose smooth surface must receive very even and uniform deposits of flood-laid silts over large areas. Records of borings are quoted by Walther (1888, 21o) which show river muds on sand and loess to a depth of nearly 5o meters beneath the bed of the Ainu River at Charjui, where the great railroad bridge was built. The record of a well boring at Askhabad, quoted by the same author (190o, 105) shows variable piedmont deposits over 2,000 feet deep. It seems, indeed, as if we had in the 'plains of Turkestan and the Great Plains of our West one of the most striking of the many physiographic resemblances between Eurasia and North America ; and that there as well as here an increasing share may be given to the action of aggrading rivers in forming the plains, as observations are extended. It is well known that the tide of geological opinion in this country has in recent years turned more and more toward a fluviatile origin for the strata of the Great Plains that slope eastward from the Rocky Mountains, and the traditional lacustrine origin of the plains strata has been repeatedly questioned ; so we may expect, as closer attention is given to the details of river-laid formations, that a larger and larger share of the fresh-water strata that slope westward from the mountains of Central Asia may be interpreted as fluviatile rather than as lacustrine.
In one respect, however, the comparison between the two continents reveals a contrast. In North America the rivers that flow eastward from the Rocky Mountains are now dissecting the plains that they once built up, as has been so well shown by Johnson ; while in Turkestan the rivers that emerge from the mountains, heavily silt-laden, are still engaged in building up the plains. This is notably the case with the Murg-ab and the Tejen, as will be more fully stated below, for these rivers wither away without reaching the sea, and every particle of sand and silt that they bring from their headwater valleys in the mountains must be laid down as they dwindle to dryness on the plains. Moreover, while the rivers at present bring abundant gravels out from the mountains and spread them on the nearer parts of