264 TIIE BASIN OF EASTERN PERSIA AND SISTAN.
that in riding almost across the middle of it my horse's hoofs sank in only about an inch. The whole expanse is covered with a beautifully crystalline deposit of salt, thinner on the edges and thicker toward the middle, where it crackles like snow under the horse's hoofs. The smooth playa floor is divided into broad concentric bands which grow successively whiter and more thickly covered with salt toward the center. They appear to mark stages to which the water had risen during the last season, and have the irregular boundaries characteristic of water which stands on an almost level surface. To the eye the surface of the playa seems perfectly level, but there is a slight slope, as is shown by a beach marking the high-water level of spring. At the south end this can hardly be distinguished from the floor of the playa, but a mile or two farther north it lies distinctly 8 or io feet above the floor. No lacustrine terraces could be detected, although the bluffs, which are cut in soft sandstone, dipping gently northeastward, and which sometimes reach a height of a hundred feet, seem to have been undercut during a former higher stand of the water. The tributary valleys, however, show terraces to the number of three, cut partly in rock and partly in a filling of gravel. Away from the lake these gradually die out in the course of a few miles. They might easily be due to an intermittent warping by which the basin was deepened, although that would demand a rather complex system of movements by which a minor basin warping should be superposed upon the main warping demanded for the Heri Rud. A climatic origin is simpler, but the terraces are too poorly developed to be of great importance.
NEMEKSAR OR THE PLAYA OF KHAF.
Beyond the great northward bend of the Heri Rud, in the southward continuation of the Afghan depression, the playa of Khaf—the Nemeksar par excellence—and its basin continue the terrace phenomena of the more northern districts, though the maximum number of terraces is less. At the northeasten corner of the basin we passed three valleys, all of which show two strong terraces of the usual type, with heavy gravel beds covering the horizontal portions and with the vertical portions cut partly in the gravel and partly in the underlying rock. Besides these there were two minor terraces, scarcely worth mentioning, one between the two strong ones and one below.
In the higher valleys of the pass near Chani Well, 8 or io miles north of the northeast corner of the playa, a phenomenon was noticed which is rather common in the mountains of this part of the world. The steeper valleys are filled with coarse gravel firmly consolidated by a calcareous cement, and now dissected into rude terraces. The deposit is closely analogous to the gravel of the ordinary terraces, but differs in being found in very steep ungraded valleys and in being well consolidated. Apparently these two features belong together, since only a well-consolidated deposit could retain its position under present conditions on so steep a slope. The gravel and terraces appear to be due to changes of climate, since they are found in ungraded valleys which would be wholly uninfluenced by any movement of uplift or warping which did not directly affect their own grade. It is highly improbable that earth movements, determined as they must be by broadly acting and largely subterranean causes, should so adjust themselves as to accelerate all the