EXAMPLES OF TERRACES. 269 
According to the climatic hj-pothesis this difficulty disappears. During a fluvial 
epoch a decrease in evaporation or, still more, an increase in precipitation, would 
cause ungraded mountain slopes to become graded and covered with vegetation. 
The material washed down from such graded slopes and deposited in the valleys 
and plains would be relatively fine, whether it happened to be deposited in valley 
bottoms, in playas, or in lakes. When an interfluvial epoch ensued, vegetation would 
become scarce, floods would be more frequent and violent, and it would be but a short 
time before the slopes would assume their present ungraded condition. During this 
process the streams would at first be heavily loaded with the products of previous 
weathering, which they would deposit in widespread beds of gravel ; but ere long 
the supply would fail, and the streams would begin to deepen their channels and fonn 
terraces. This process might be repeated a number of times in rapid succession and 
thus a series of terraces would be formed. In parts of its flood-plain where a stream 
happened to be flowing at the time of a change from wet to dry conditions, the transi- 
tion from silt to gravel would be gradual and there would be no unconformity. 
Elsewhere the change would be marked by a sharp unconfonnity. Both of these 
conditions are found, but, as might be expected, unconformability is the rule. 
At the northeastern end of the basin of Bajistan, not far from the city of 
Turbat-i-Haideri, there are again four terraces, which must be due to a highly 
specialized warping of that particular basin, unless they are due to changes of 
I climate. The tops of the red- 
~/^^\k dish silts, previously described, 
/ / /^^^^^^^^9^^^^^^9=aa /fi2332E923S| are beveled by an old grade plain 
y/ / / y/ / / / /^'^ / / / / covered with from 5 to 15 feet of 
/ / / /////////A gravel (fig. 162). Below this are 
Fig. 162. — Terraces at the Northeastern Comer of the Bajistan traces of a SCCOnd grade plain 
forming a second terrace which 
is almost consumed. At the base of the second terrace lies the broad plain of Bajistan, 
in which are cut two more terraces. There are thus four gravel-covered terraces sepa- 
rated into two groups. The same phenomenon is noticeable at Kogneh Lake and 
elsewhere. There seems to have been a long interval between the formation of two 
groups of terraces. The fact that this division is observed in widely separated places 
makes it probable that the cause of the terracing was of such widespread appli- 
cation as to affect enormous areas in precisely the same wa3^ This would be true 
of climate, but not of warping. 
SUMMARY. 
The part of Central Asia touched upon in this report and in the preceding 
report on Turkestan embraces 22 degrees of longitude and 12 degrees of latitude in 
the heart of the arid portion of the continent. Between the extreme limits of 
Kizil Arvat on the west and Issik Kul Lake on the east the distance is i,2CX) miles, 
while from north to south tlie distance is 800 miles. Throughout this large area, 
wherever young or mature mountains have been observed, the valleys contain 
terraces composed in whole or in part of gravel which must have been brought 
into them after they had reached nearly their present condition. 
