the gravel bluff; second, the level of the old outlet ; third, the level of the upper terrace, although this may be the saine as the old outlet ; and fourth, the level of the lower terrace and the beach which may be reached to-day in time of flood.
At a little distance from the lake we find evidence of still other changes in the behavior of the water-courses. If the level top of the gravel bluff south of the lake be followed northwestward, it will be found that it merges smoothly into the plain of Kalagak. In the plain the streams have incised themselves to a depth of from 4 to ro feet, in response apparently to the changes in lake level indicated by the little terraces. On the edges of the plain two terraces of quite a different kind present strong escarpments heavily capped with gravel. These correspond to the uppermost of the terraces of the Heri Rud, as appears by following them to the south, where they are well displayed. Along the Jam River seven terraces can be seen in certain places, but as two of them, near the bottom of the series, do not seem to be permanent, we shall consider them as adventitious and leave them out of account. The third terrace, counting the upper and oldest as the first, corresponds to the Kalagak plain and to the top of the bluff south of the lake. The fourth terrace, that is, the one next to the bottom, if it be traced toward the lake, is found to coincide with the bottom of the old outlet. A little tributary of the Jam is now gnawing back into the soft gravel in which the former outlet is trenched, and will in time cut through the bluff and drain the lake. The fourth and fifth terraces along the Jam seem to correspond to the low terraces northwest of the lake and to the slight channeling of the Kalagak plain. The terraces along the Jam are strong while the others are weak, because the main river was able to continually deepen its channel, while the lake furnished a but slightly changeable base-level and prevented its tributaries from cutting deeply.
Turning now from a mere statement of facts to a consideration of causes, we must first sum up the history of the lake of Kogneh and the neighboring rivers. Originally an uninterrupted stream must have flowed from Kalagak to Daniduo, where, after passing what is now the site of Kogneh Lake, it joined the Jam River, and the combined streams emptied into the Heri Rud. For some reason this whole river system was subjected to certain systematic changes by which the streams were at first induced to deposit abundant gravel and to wander widely from side to side. Then other conditions ensued under which the streams acted in exactly the opposite fashion and cut deeply into their beds, carrying away much of the gravel, cutting even into the underlying rock and forming high terraces. Just how many such alternations took place we are unable to say, but there were at least two before the formation of Kogneh Lake. During the third time of gravel deposition and river wandering, the large Jam River deposited its load so rapidly across the mouth of the Kalagak stream that the latter could not keep an outlet clear. Thus a bar was formed across the mouth of the Kalagak Valley, and behind this the brook spread out into the lake of Kogneh, finding an outlet to the main stream as best it could among the gravels of its bigger neighbors. Up to this point the history of the region is explicable either on the tectonic or the climatic theory ; from this time onward only the climatic theory seems competent to account for all the facts.