QUATERNARY CLIMATIC CHANGES OF EASTERN PERSIA. 275 
mountains, on the other hand, terraces seem to be the result of a changing climate, 
that is, the maximum effect, of deposition at least, is produced during the change 
from moist to drj' conditions. A large number of the terraces of Persia, it will be 
remembered, are cut in stream-laid gravel which lies with a slight unconformity upon 
finer deposits of silt. It may be that these can be explained as the product of 
epochs of increased cold alternating with epochs of relative wannth, but data are as 
yet insufficient. It is also possible to explain them as the product of epochs of 
increased precipitation alternating with epochs of relative aridity, but this theory 
does not necessarily exclude the other. 
In Persia, as has frequently come to our notice, the rock slopes of mature 
mountains are ungraded because of the aridity of the climate. If the climate were 
to become moister the process of grading the slopes and covering them with 
vegetation would at once begin. So long as solid rock lay near the surface the 
increased moisture and the increased number of decaying plants would accelerate 
the process of weathering. Whether this increased weathering would increase the 
load of the streams depends on whether the increased vegetation is able to hold 
back the larger amount of waste which is now supplied by the decaying rocks. 
However this may be, it is certain that the load of the streams would become finer 
as the process of grading the slopes went on, and ultimately the flood-plains would 
be covered w'ith fine material, usually silt, no matter whether the streams were 
aggrading or degrading their beds. 
When the climate once more becomes arid the graded character of the mountain 
slopes will soon disappear and the old conditions will reign once more. The process 
of grading the slopes must of necessity be slow and lag long after the change of 
climate which gives rise to it, for nmch time is required to convert solid rock into 
soil. The process of ungrading the slopes, on the contrary', is rapid, and scarcely 
lags behind the change of climate which causes it As soon as vegetation begins 
to drj- up because of decreased rainfall, the streams will begin to carry off the soil 
and weathered fragments which cloak the mountain sides. The carrying power 
of the streams will remain approximatelj- the same, but their load will be so greatly 
increased that they will be obliged to deposit the coarser portions upon the silts of 
the valley flood-plains. This process of building up deposits can not last indefinitely, 
however, for the supply of weathered material is limited, and when once it is 
exhausted the weakened forces of weathering can ftimish new supplies ver}- slowh-. 
Therefore it will continue while the climate is changing. When the change is fin- 
ished and the climate has become arid, the streams will no longer act as heavily loaded 
agents of deposition, but will be insufficiently loaded and will act as agents of erosion. 
Other more striking examples of terraces exist in the semiarid regions of North 
America, but little attention has been paid to them. It must not be understood that 
these few examples of terraces in Turkey and North America are supposed to lead 
to any definite conclusion. They are presented merely with the purpose of showing 
that if our conclusion as to the climatic origin of the terraces of Central Asia is cor- 
rect, these features in other lands are what we should expect. Prolonged study is 
necessary before correlating facts so widely separated. The glacial period was a 
world-wide phenomenon, and to understand it fully demands a world-wide view. 
