290 THE BASIN OF EASTERN PERSIA AND SISTAN. 
or fluviatile deposits which would merge imperceptibly into those of the lake. 
Rapid movements would involve unconfonnities between the lacustrine and fluviatile 
deposits, but slow movements would involve just such transitions as actually exist. 
The structure and texture of the clays agree with the demands of the theory of 
warping. 
The weakness of this theor>' is that it does not explain the difference between 
the red color of the fluviatile strata and the green of the lacustrine layers. The 
transition from red to green must mean that the grains of waste at the bottom of a 
red layer were exposed to different conditions of weathering from those of the green 
grains a few inches lower. An important and widespread change must have taken 
place in a short time. It is quite impossible that such a change should take place 
merely because the border of the lake has been shifted a few miles. Whether the 
lake shore is a mile or two this side or that of a given point, the deposits must have 
come from approximately the same mountains and must have been subjected to the 
.same journey under precisely the .same conditions, except for the last mile or two. 
If the pink grains have had time to become highly oxidized, it is inconceivable that 
the green grains, brought by the same stream, have scarceh' had time to become 
weathered at all. This might indeed happen in the case of two individual grains, 
one of which was brought from the mountains in a single year by a flood, while 
the other spent many years upon the way, but it could not possibly happen with an 
infinite number of grains. The color of the clays seems to be an insunnountable 
obstacle to the acceptance of the theory' of rhjthmic war])ing. 
(d) Fluvial or Laciistral Theory. — The theory which explains the phenomena 
of Sistan by a succession of fluvial and interfluvial ejx)chs is an expansion of the 
principles which have become so well established in the study of the glacial phenom- 
ena of Europe and North America. During fluvial or lacustral epochs the increased 
rainfall or decreased evaporation would cause a large lake in the basin of Sistan ; the 
streams from the surrounding mountains would become fuller and more perennial, 
vegetation would become more abundant, and the mountain slopes would tend to 
become graded. As a result of all this the load of the stream would be fine in texture 
and would be carried quickly to the lake, where it would be deposited without having 
an opportunit)' to become highl}- weathered. The lake bottom would be covered with 
unoxidized clays of fine texture and light color. On the advent of an interfluvial 
epoch, the lake would decrease in size, and marshes would encroach upon its edges ; 
the rivers would dwindle and become intermittent, and at the same time would 
become subject to fiercer floods ; vegetation would ever}-where decrease ; and the 
slopes would become ungraded. These changes would allow coarser materials, such 
as sand and even gravel, to be washed in over the exposed portions of the old lake 
bed. The total amount of material might be greater than during the moister period, 
for the flood torrents would be loaded to the utmost ; but the jotiniey of a given 
particle would be much slower, for the laden floods would quickly spread into a 
sheet and deposit their loads, and many short journeys separated by long periods of 
exposure woidd be required to bring the waste of the mountains to its final resting 
place. During this protracted journey the redness which characterizes the fluviatile 
