246 THE BASIN OF EASTERN PERSIA AND SISTAN. 
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. Tlie 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 funda- 
mental 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 deter- 
mined by the forces of erosion acting under the guidance of rock-stnicture and rock- 
texture, while in Persia a large proportion of the main fonns are detennined 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 CYCLE OF EROSION AND DEPOSITION IN PERSI.\. 
The changes through which Persia has passed in the earlier stages of its de\-elo]> 
ment, 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 tjpical 
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 couutr)-. 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 countr}-, 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 com- 
pletely 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 same wa)- 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. 
