When the third period of degradation and valley deepening set in, the volume of the streams, according to the tectonic theory, was unaffected, and Kogneh Lake must have remained full. Supposing this to be the case, the lake continued to overflow to the Jam River throughout' the third period of degradation and the fourth period of aggradation. At the beginning of the fourth period of degradation the flow of the Kalagak stream was somehow so diminished that the lake no longer flowed, and the abandoned outlet was left at the level of the fourth terrace. The cause of the sudden desiccation of the lake, no marked change of climate being admitted, can only have been a diversion of some of the tributaries of the Kalagak brook. I saw no sign of any such diversion and it is not likely that it took place, but without detailed study of the region this can not be asserted positively. The last change in the lake, by which the lower terrace was formed, will have to be explained by the saine gratuitous assumption that a tributary of the Kalagak was again diverted, this time toward instead of away from the lake. It can not have been the same tributary as on the earlier occasion, because it only sufficed to half fill the lake. Lastly, this second tributary must have been again diverted in order to bring the lake to its present condition. These changes must have taken place at the same time that the lower terraces were being formed along the Jam and the Here Rud. It is possible to explain the phenomena of Kogneh Lake on the tectonic hypothesis, but it involves several assumptions for which there is no basis in facts of observation.
The climatic hypothesis is simpler and involves no assumption beyond the facts of observation. At the end of the third period of aggradation, after the lake had been formed, it is supposed that an interfluvial epoch ensued. The lake was desiccated to such a degree that it no longer overflowed ; the neighboring rivers eroded their channels and formed a third terrace. Another change of climate with an increase in the size of the streams filled the lake to overflowing and caused the rivers to aggrade. When this came to an end the outlet had been cut to a depth which corresponded with the fourth terrace of the Jam, at which level the river was then flowing. Another interfluvial epoch left the lake empty and allowed the cutting of the fourth terrace. During the last fluvial epoch the increase in the volume of the streams was so moderate that the lake was not filled to overflowing, but merely to the level of the lower terrace, while in the river valleys slight deposits of gravel were laid down. Lastly, the present dry epoch leaves the lake almost empty and has allowed the cutting of the lowest terrace along the streams. Theory and fact seem to agree perfectly. If the climatic theory is the true explanation of the phenomena of Kogneh Lake, it must apply also to the terrace of the Hai Rud, for the two are inextricably connected.
THE SALT LAKE Or PUL-I•ICHATUN.
The other salt lake to which reference has been made lies back upon our line of march in Russian territory about 7 miles east of the gorge of the Heri Rud at Pul-i-Khatun. Shor Kul, or Salt Lake, as it is called, is really a playa, a perfectly smooth expanse of salt-covered mud, 3 or 4 miles long and half as wide, and lying at a height of about 2,000 feet above the sea. It was so dry in November, 1903,