274 THE BASIN OF EASTERN PERSIA AND SISTAN. 
greater precipitation. And in the same way a comparison of winter and summer 
conditions affords no assistance, for the colder winters are also times of relatively large 
precipitation. Nor does it help lis if we compare different years, the colder with 
the wanner ; for we have no statistics as to the growth of vegetation, the length of 
rivers, and the other factors which affect the problem. If we suppose, however, that 
the precipitation remains constant and the degree of cold increases, the amount of 
evaporation must decrease and the size of ri\'ers and lakes correspondingly increase. 
It would probably require a great decrease in temperature to bring the lakes to the 
dimensions of even the more recent lacustral epochs. Another effect of increased 
cold would be an increase in vegetation and perhaps in the rate of weathering of the 
rocks by reason of the greater amount of moisture which would remain in the 
gfround. Here again we have no means of measuring the effect of any possible 
increase in cold, and so can not feel any assurance as to the adequacy of this cause 
to produce the observed effects upon the erosion and deposition of streams. 
The influence of an increase of rainfall upon the ungraded slopes of mountains 
such as those of Turkestan and Persia can be more easily estimated. In the case of 
young mountains with slopes so steep that they can not possibly become graded, the 
whole effect of increased precipitation would be to accelerate weathering and thus to 
increase the load of the streams. In the case of streams which were approaching 
grade, but were not yet graded, the result of the increased load would be that it would 
no longer be possible for the streams to cut downward, because they would be so 
heavily loaded as to cover their beds with debris. Hence they would cut laterally 
and form flood-pains covered with gravel. Upon the advent of a drier time the load 
of the streams would decrease, but their carr}-ing power would remain almost the 
same ; for the carn,-ing power of a stream depends upon its maximum size, and the 
maximum of flood size of streams in arid regions is almost as great as in moister 
regions. With lessened loads and unlessened capacity the streams would begin to 
cut downward once more, and terraces would be formed which would show a cap- 
ping of gravel with solid rock beneath, as is common among the lofty and arid young 
mountains of the southern border of the Tian Shan range on the northern side of the 
Kashgar basin. 
Among young mountains, where the main streams are graded, the result of 
increased rainfall and increased load would probably be that streams would build 
up their flood-plains and the valley bottoms would become filled with alluvium, 
most of which would be gravel. A drier epoch would allow this to be dissected 
and terraces composed wholly of gravel would be fonned, like those found in the 
moister parts of the Tian Shan Mountains, and to a certain extent in Persia. 
Thus among young mountains o.scillations between an arid and a moister climate 
would apparently produce two kinds of terraces ; first, ungraded valleys would 
contain terraces cut partly in rock and partly in stream-laid gravel ; second, graded 
valleys would contain terraces cut wholly in stream-laid gravel. 
In these two cases the terraces of young mountains are the result of a changed 
climate, that is, the maximum effects of deposition and of erosion are produced under 
the extreme conditions of moisture or of drought. Among mature but ungraded 
