RECONNAISSANCE IN CENTRAL TURKESTAN. 171
the border of the Kashgar basin was i 5 or 20 miles north of its present location, and was a place of heavy gravel deposition. Then a small fold developed along the border, lifting up part of the gravels and causing the accelerated streams to deposit their load of pebbles farther toward the center of the basin, where playas had formerly deposited silt. Later another fold was developed and the gravels once more advanced, and so on by steps which were perhaps too slow to be noticed. The older gravels were compressed and hardened into conglomerates and their upper portions were worn down to the smooth grade of the Tertiary peneplain. A similar experience befell all the underlying formations. Each of them, and the peneplain as well, represents not a certain time, but a stage in development, and some of the stages are not yet completed.
THE TIAN SHAN PLATEAU.
The part of Central Turkestan traversed by the writer divides itself naturally into four provinces—namely, the Tian Shan plateau, the Alai Mountains, the Kashgar basin, and the Fergana basin. The first of these is generally termed the Tian Shan Mountains, but as far as the province was seen, it is not strictly a mountain range according to a scientific definition, nor is it strictly a plateau. It is a region of mountainous structure, and once of truly mountainous form, but it long ago reached old age, and has since been uplifted to its present height with relatively little renewed folding of the strata. In structure it is still mountainous, but its present form and altitude are due to an uplift of the uniform kind which is usually associated with the formation of plateaus. To-day it may best be described as a plateau ; to-morrow, geologically speaking, when all the remnants of the uplifted peneplain surface and the last of the post-Paleozoic strata have been removed and dissection has gone far enough to produce strong relief, it will again become a typical mountain region of highly folded limestones. The general structure is shown in the accompanying section (fig. 124), which is about 200 miles long and extends south-southwest from the mouth of the Juuka Su, 25 miles west of the east end of Issik Kul, to the Kashgar desert at Sugun, 3o miles west of Shor Kul. The section represents the general character of the plateau in its least dissected portion. Farther east and farther west the surface is more deeply trenched by the main streams. Along the section the profile is essentially a very broad anticline of the Uinta type, as defined by Powell, where the sides are monoclines and the top is flat. The fact that the component strata were already highly folded does not alter the character of the last uplift, although it makes it less evident in the cross-section. If the line representing the surface is looked at alone, the true nature of the deformation is evident. The anticline is not strictly flat on top, but undulating. The troughs form broad basins at an altitude of from 10,000 to 12,000 feet, while the crests form broad ridges which reach a height of from 13,000 to 15,000 feet.
On the steep northern slope of the broad anticline the valleys are fairly open at the base where they reach the Tertiary strata of the Issik Kul basin, but they are for the most part narrow canyons with inaccessible walls of naked rocks