RECONNAISSANCE IN CENTRAL TURKESTAN. 165
existed ; but here the calcareous strata, which show other evidences of being marine, contain fossils in abundance. Above the limestones the strata are at first red, as though the shallowing of the sea allowed the very highly weathered soil of an old land mass to be washed farther and farther out into the area of deposition. The succeeding formations, the pink and brown sandstone and the brown conglomerate, show a nearer and nearer approach to present conditions. It appears as though, after the retirement of the sea, the land was covered with great playas, on which water first stood in thin sheets, forming ripple-marks in the mud (see fig. 123), and then retired or was evaporated, allowing the surface to become sun-cracked. As time went on streams began to flow across the playas, at first slow and broad and able to cut only shallow channels, which were afterwards filled and covered, assuming the form of very thin lenses of a material slightly different from that of the surrounding playa strata. Then, as the strength of the streams increased, sand was deposited over the whole area, and the channels, now deep and distinct, were filled with gravel. Lastly, gravel was deposited almost everywhere.
Some of these changes may be of climatic origin, some may be due to warping of the crust, and some seem to result from the lessening of relief by erosion. Thus in the earlier Mesozoic times the change from coarse conglomerate to the fine coal shales may be due entirely to the last-named cause, by which, as the mountains were worn away and the intermount basins were filled, relief became less and the size of the transported materials became smaller, until the coal-bearing shales were formed. The next change, from the coal measures to the red conglomerate and the cross-bedded sandstone, is much more sudden and probably indicates a warping and uplift by which previously base-leveled areas were raised and subjected to active erosion. The redness of the strata and the predominance of small pebbles of pure quartz in the gravel indicate that the materials were derived from a region that had long been subject to undisturbed weathering. It is possible, too, that the uplift was of such a nature as to cut off the supply of moisture which had been present during the formation of the coal and to convert the country into a desert, where the wind produced large-scale cross-bedding of the red sandstone. There is no positive evidence on the subject, and we can merely raise the question of the desert origin of this peculiar deposit. From the time of the cross-bedded sandstone to that of the limestones of the Cretaceous there appears to have been a steady sinking of the land or rising of the sea until at last the whole country was inundated. Through the succeeding period to the Tertiary there is no sign of climatic change. Everything points to a steady warping and lifting, by which relief was gradually increased in such a manner as to cause the deposits to change from marine to subaerial, and then gradually to change in texture from the fine silt of playas to coarse conglomerate. The warping seems to have been greatest along the borders of the present basins, for there we find the strata of the whole series considerably folded, while in the centers of the basins they are almost undisturbed. The older strata, too, seen to be more bent than the younger, so that the process seems to have gone on steadily almost from the beginning of deposition.