PERSIA AS A TYPICAL EXAMPLE OF AN ARID COUNTRY.
Eastern Persia is in the stage of physiographic development where the influence of aridity is most prominent. The climate of the world as a whole is such that soil is abundant, lakes have outlets, rivers discharge into the sea, and agriculture can be carried on without irrigation. Such conditions are so common that it is hard to realize that they are merely the effects of one special variety of climate. In Persia, however, where the whole aspect of nature is different from that to which we are accustomed, it becomes easy to appreciate the influence of climate. The fundamental difference between the topography of Persia and that of a well-watered country like the eastern United States is that in the latter the main forms are determined by the forces of erosion acting under the guidance of rock-structure and rock-texture, while in Persia a large proportion of the main fonns are determined by deposition, which tends to conceal and nullify the influence of rock structure and texture. This can best be illustrated by considering the life history of Persia.
The changes through which Persia has passed in the earlier stages of its development, and also those of the future, must in part be inferred, for, so far as I am aware, no part of the country is in the stages of extreme youth or old age, and no typical examples of these stages have been described elsewhere. The present cycle of erosion in Persia was introduced by the formation of inclosed basins, the most striking topographic feature of the country. As we have already seen, the basins are not due to any peculiar form of warping, but rather to the arid climate which has long prevailed. This is well exemplified in the three basins of Zorabad, Jam, and Meshed, in the northeastern corner of the country, which receive an abundant supply of water from high mountains, and hence are provided with outlets and are fast being transfonned into valleys of erosion. During the youth of the country these three basins, like their neighbors, such as Pul-i-Khatun, to be described later, Nemeksar, Bajistan, and others, were completely closed and in one case at least contained a lake. During early youth it is probable that all the basins were coínpletely closed. At first their development must have proceeded in much the same way as that of the lake-filled basins of moister regions. The mountains also appear to have developed in the saine way as in lands of greater rainfall. Kopet Dagh is the best Persian example which I have seen of young mountains developed under conditions of aridity. Here, however, much of the topography is mature, and the altitude of the mountains has increased the rainfall so that the erosion is not greatly different from that of America and Europe. A better example of young mountains in an arid region is furnished by the southern border of the Tian Shan plateau on the edge of the Kashgar basin. As there exemplified, the chief characteristic of such mountains is extreme sharpness of form and utter nakedness.