PERSIA AS AN EXAMPLE OF AN ARID COUNTRY. 247 
MATDRITY. 
As youth advances toward maturity the difference between the physiographic 
fonns of a moist country and of a dry countr>- increases apace. At the beginning 
of maturity in a laud of sufficient rainfall the lakes have for the most part been drained, 
and the topographic fonns are almost universally due to erosion guided by the struc- 
ture and texture of the rocks. In Persia, the t>'pical drj- countrj-, on the other liand, 
almost none of the lakes have been drained, and the proportion of the surface where 
topographic fonns produced by erosion prevail, has reached and passed a maxinuim. 
Indeed, the process of decreasing the area subject to erosion goes on from earl}- jouth 
until old age, and is perhaps the most prominent characteristic of the activities con- 
trolled by an arid climate. The streams which come from the mountains laden with 
detritus are compelled to deposit much of their load on reaching the foot of the moun- 
tains and changing from a steep to a gentle grade. Even a small stream can flow a 
long distance in a very arid region, provided it is confined to a small rock channel 
where there is little opportunity for evaporation. As soon, however, as a region of 
deposition is reached, the stream begins to spread into many channels, which reduces 
the already diminished velocity and causes further deposition. Moreover, the stream 
itself quickly comes to an end, for much water is lost by reason of the larger area 
exposed to evaporation in the many channels, and even more sinks into the 
thirsty gravel. For this reason the small running streams of Persia are almost 
entirely confined to the higher mountains and are continually growing shorter 
Each new addition of gravel to the fan of a stream represents a decrease in the 
height of the mountains which in the course of ages is sufficient to cause a decrease 
in rainfall. It also raises the height of the fan itself, and compels the stream to 
divide and to lose itself in the gravel at a higher elevation than fonnerly. Both 
these processes tend to shorten the streams and cause them to deposit their loads 
higher and higher, building up the fans indefinitely. Everjthing tends to increase 
the areas of deposition until finally basin coalesces with basin ; the lower hills are 
buried out of sight ; those of greater height rise as islands in vast expanses of 
gravel ; and even the highest nioimtains are half-buried in great fans of the same 
material. Thus in full maturity only the mountains present fonns due to erosion, 
and even of the mountains the lower portions are buried by constantly increasing 
products of deposition. 
This is the condition which has been reached in Eastern Persia south of Binalud 
Kuh. One of its prominent characteristics is the isolation of the mountains, which 
is well seen in the basin of Nemeksar. One of the most striking examples is in 
the playa of Kulberenj, south of the main playa of Nemeksar. From the ver)' 
floor of the playa rise several small, dark islands, whose roots seem to descend 
beneath the plain as though these were the pointed tops of what once were high 
hills or mountains. 
The erosion 0/ the mouiitaiiis. — Another prominent characteristic of the mature 
mountains of Persia is their nakedness, roughness, and sterility. In a young 
country it is to be expected that there shall be large areas of naked rock, but in a 
mature country-, if the rainfall is abundant, most of the surface, except the imme- 
