THE INFLUENCE OF CHANGES OF CLIMATE. 253 
SUMMARY. 
Ill earl}- \oiitli the main forms of Persia probably differed but little from those 
of a moist coiiiitn.-. There was more nakedness, roughness, and sharpness, but this 
was confined to the minor details. From }-outh onward, however, through maturity 
the land forms of Persia increasingly diverge from the fonns of moister regions. 
Those of the latter are shaped by erosion ; those of Persia largely by deposition. 
In the one case basins are destroyed ; in the other the}- are preserved. The di\-er- 
geuce between the two types is at a maximum during mid-maturity, when, in a 
moist country such as the southern Appalachian region of the United States, inclosed 
basins have wholly disappeared, a subsequent drainage follows implicitly the lines 
of rock structure, and the slopes of the mountains are completely graded ; while in 
an arid countr}-, such as Eastern Persia, inclosed basins are the rule. The drainage is 
largely interfered with by immense areas of deposition which have no connection 
with the underlying rocks, and the slopes of even the lower mountains are rough 
and naked. From mid-maturity onward the main topographic forms of moist and 
of arid lands again approach one another somewhat, until in old age both are reduced 
to peneplains. If the wind is active, however, the arid peneplain will continue to 
develop and ma\- be eroded below sea-level. 
PERSIA AS AN EXAMPLE OF THE INFLUEN-CE OF CHANGES OF CLIMATE. 
Although as a whole the basin deposits of Eastern Persia indicate the long 
prevalence of an arid climate, there are certain phenomena which suggest a depart- 
ure from the present condition during relatively recent times. These consist, in 
the first place, of fine deposits of silt and clay which seem to be of lacustrine or 
playa origin, although they lie in regions which are never inundated under the 
present climatic conditions. Associated with these are shore terraces of the kind 
which are usually characteristic of lakes. These are best explained bv supposing 
either that the rainfall of former times was greater than to-day, or that the climate 
was colder, evaporation was less, and a greater accumulation of water was possible 
in the basins. 
Another class of facts seems to have some connection with the lakes, but is by 
no means so well understood. Numerous valle\s in all parts of Eastern Persia 
contain a series of terraces ranging up to five in number, and closely similar to the 
terraces of Turkestan. Sometimes the terraces are cut partly in rock and partlv in 
stream-laid gravel, sometimes wholh- in stream-laid gravel, and sometimes in stream- 
laid gravel which lies with a slight unconfonnity upon finer deposits of silt. Such 
unconformities are common on the edges of the ancient lakes, and in almost ever}- 
case coarse material lies above and finer material below, while the transition in the 
reverse order from coarse below to fine above seems to be gradual, without au}- 
sudden change. As the terraces and the associated phenomena represent the most 
recent physiographic changes which have taken place in Persia, it is not impossible 
that some of them originated since the advent of man, and they must be carefully 
explained. 
