EXAMPLES OF TERRACES. 263 
When the third period of degradation and valley deepening set in, the volume 
of the streams, according to the tectonic theor\', 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 al^andoned 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, b}- which the lower terrace was fonned, will have to be 
explained by the same gratuitous assumption that a tributar\- of the Kalagak was 
again diverted, this time toward instead of away from the lake. It can not have 
been the same tributar\- 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 
Heri Rud. It is possible to explain the phenomena of Kogneh Lake on the tectonic 
hypothesis, but it in\olves 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 fonned, 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 fonned a third tenace. 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 tenace, while in the river valleys slight deposits 
of gravel were laid down. Lasth-, 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 theor}- is the true explanation 
of the phenomena of Kogneh Lake, it must apply also to the terrace of the Heri 
Rud, for the two are inextricably connected. 
THE SALT LAKE OF PDL-IKHATCN. 
The other salt lake to which reference has been made lies back upon our line 
of march in Russian tenitory about 7 miles east of the gorge of the Heri Rud at 
Piil-i-Khattm. Shor Kul, or Salt Lake, as it is called, is realh- a plaja, a perfectly 
smooth expanse of salt-covered mud, 3 or 4 miles long and half as wide, and l>ing 
at a height of about 2,000 feet above the sea. It was so dn- in November, 1903, 
