DESCENDING FROM THE HIGH PLATEAU. 253
ground, sometimes at a considerable distance apart, sometimes with intervals of only one or two meters. They evidently indicate old beach-lines of the disappearing lake. In some places a species of strand-terrace can also be observed, pointing to the same conclusion as the beach-lines, but indicating a more permanent stage in the lake's shrinkage. The gypsum deposit at the bottom of the lake is almost entirely exposed, and is not overlain by any other deposits or by any material washed down off the adjacent mountains. It such material is at times carried down by the rivers after rain, it does not at all events remain; but during the next ensuing dry season it is blown away again, and in this way the gypsum deposit always remains exposed.
The bedding of the gypsum is exceptionally distinct, though not always horizontal. The irregular slope can be especially studied in the small free-standing tabular elevations; as a rule the departure from horizontality is very slight, but sometimes it amounts to i o° to 20°, and the inclination is now in one direction, now in another. This is of course wholly a secondary phenomenon; it has been produced either by a slip of the next underlying bed, a space having been hollowed out underneath by the excavating agency of water, or by erosion at the base of the particular local elevation. The sides of these free-standing elevations afford an excellent opportunity for observing how the different beds of gypsum vary in hardness. Some form indentations or notches, others projecting ledges. Owing to the regularity of their shape and their inflexible parallelism they frequently give the impression almost of having been modelled by human hands. Generally several of these tabular elevations stand in a group together, being separated by furrows or passage-ways more or less broad. Originally this mass of gypsum, which has in the course of time become deposited on the bottom of the lake, was perfectly
Fig. 149. GYPSUM ELEVATIONS.
IIIFカラー高解像度
