PERSIA AS AN EXAMPLE OF AN ARID COUNTRY. 25I 
be transported and spread in a sheet on so gentle a slope. East of Sistan the won- 
der becomes greater. According to Colonel McIVIahon, the drear}- expanse of the 
Dasht-i-Margo extends eastward 150 miles without change and without interniption 
from the top of the bluffs of Sistan to the first mountain, Malik-Dokhand in Balu- 
chistan. In tliis distance the plain rises 2,500 feet — less than 17 feet permile — and 
yet gravel has been smoothly distributed ever\-\vhere. It is noteworth}- that in the 
driest regions the accumulation of gravel is most extensive, provided the relief of 
the neighboring mountains is great. 
Proceeding now from the coarser to the finer deposits, we find that the center 
of each basin usually holds a salt lake or playa, bordered by an area of fine silts. 
Plajas and salt lakes are so abundant and various that the Persians have different 
names for different sorts. The names are used loosely and often overlap, but there 
seems to be some system. Thus " darya " signifies simply a lake or any large body 
of water ; " hamun," which is often translated " swamp," is used for a body of water 
which is partly open and partly filled with reeds ; next comes " nemeksar," a salt 
lake which is dry part of the year, but contains water during the rainy season, and, 
on dr\ing up, deposits salt which can be used commercially. Still drier than the 
nemeksar is the " kavir," a salt playa or swamp which may sometimes be covered 
with water for a brief period, but never fonns a real lake in which salt of economic 
value is deposited. Its deposits are always mixed somewhat with sand and silt. 
The Persian naturally looks upon the utilitarian side of physiography, but his classi- 
fication is exact enough to be of scientific \-alue. In all these fonns of lake, swamp, 
or playa the deposits which are not composed of salt present much the same 
appearance, being usually fine-grained saline clays or silts. 
OLD AGB. 
In the absence of obser\'ational knowledge of any countn,- which has grown old 
under arid conditions, we are obliged to fall back upon deduction in order to dis- 
cover the ultimate fate of Persia if the present conditions of climate and elevation 
remain unchanged. In youth and maturity the elevation of the floors of the basins 
above sea-level is of small importance. In old age it plays an important part. If 
the floor of a basin is below the level that would be occupied by a peneplain at the 
same distance from the sea, it will never be affected by aqueous erosion and, 
unless otherwise influenced, will preserve the fonns due to deposition as long as the 
continent continues to exist. If the center of the basin is considerably above sea- 
level, on the other hand, the basin fonn and the features due to deposition will 
eventually disappear. In ever\- region where there is any aqueous erosion the 
divides between different drainage areas must shift continually until the slope on 
the two sides is equal. In a basin region such a state of equilibrium can never be 
attained so long as the streams on one side flow to the ocean and those on the other 
to a basin ; for the local base-level of the basin rises indefinitely by reason of depo- 
sition, and the slope of the streams flowing inward is continually diminished. The 
ocean base-level, on the contrary-, remains fixed, and the slope of the streams reach- 
ing it is diminished only by the lowering of the divide which affects the streams 
on both sides to an equal extent. Accordingly the ocean streams will always have 
a slightly perhaps imperceptibly, steeper slope than their opponents, and the di\-ide 
