PIIYSIOGRAPIIY OF EASTERN PERSIA. 225
running water or in playas when the lake floor was almost free from permanent water, and hence during epochs of aridity. The green clays on the other hand are so fine-grained and uniform in texture and so free from changes of structure that they appear to be lacustrine deposits, laid down at a time when the lake was full of water, and hence during epochs of more abundant moisture. The entire formation of alternating reddish and green strata is most satisfactorily explained on the theory that it is the product of a series of climatic oscillations during which the lake was first dry and then full. The history of the region after the volcanic eruptions is recorded in gravel deposits which overlie the strata just described and alternate with fine gravel and in terraces which dissect all the strata. The gravels and terraces appear to indicate a continuation of the climatic oscillation down to very recent times. The total number of oscillations amounts to fourteen or fifteen, and may have been more.
As one ascends from the bottom to the top of the deposits, the greenish layers increase in frequency and to a less extent in thickness up to a certain point, while the red layers become correspondingly thinner. After the green beds have reached their maximum development there is again a decrease in thickness which can be traced only through a few stages because the clays soon give place to gravels. The thickness of the layers is probably proportional to the length of time consumed in their accumulation. Therefore where the red layers are thin, epochs of desiccation must have been short, and epochs of lake expansion must have prevailed for relatively long periods. Where the red layers are thick, on the contrary, the epochs of desiccation must have been longer and more important, and those of lake expansion must have been short. The meaning of the clays, the overlying gravels, and the terraces seems to be that the Quaternary era in Persia consisted of a long series of increasingly strong climatic oscillations, followed by a nearly equal series of 'decreasingly strong oscillations. The latter appear to correspond to the series of oscillations which we know as the glacial period in more northern countries. Furthermore, there is evidence, based on physiographic, archeological, and historical observations, which indicates that the last of the climatic oscillations may have been in progress during historical times.
THE PHYSIOGRAPHY OF EASTERN PERSIA.
Eastern Persia is a land of gravel and nakedness, of huge desert basins and desolate, interminable slopes, of tantalizing mirages and bare mountains. Springs and fountains are things to dream of, except directly among the mountains, and the traveler and his tired animals must be content with the brackish water of rare wells or the poor brine of an ever-diminishing salt stream. Day after day one sees the same sad monotony of parched plains and lifeless mountains. At long intervals nomads pitch their black tents beside wretched wells and feed their sheep and camels on the sparse brown grass which springs up for a brief month at the end of winter. The mountains are naked masses of rough, jagged rock, rising as islands in the midst of their own waste. Drought and aridity are everywhere written large in the dearth of vegetation and in the very forms that the earth itself