288 THE BASIN OF EASTERN PERSIA AND SISTAN. 
the year of his stay in Sistan the lake was always clear and deposition proceeded 
verj' slowly. Even during the great flood of 1903 the water was muddy only off 
the river mouths, although everywhere covered with the wreckage of reeds and 
tamarisks. The transition from sul^niergence to exposure must also have been 
comparati\ely slow, for the change from pink to green deposits is frequently gradual, 
although always distinct. The .sandy layers mixed with the upper and lower 
portions of the lacustrine clay seem to indicate shore conditions, and the layers of 
purple and yellow clay, with the included fossil plants, probably point to the existence 
of marshes during the disappearance of the lakes. In brief, an examination of the 
sections exposed in the bluffs of Sistan suggests that in \-ery recent geological times 
the basin of Sistan has been subjected to a long series of slow and gradual changes 
by which large portions of the floor of the basin became alternately areas of sub- 
aerial and of lacustrine deposition. In other words, the predecessor of the present 
lake of Sistan has either again and again passed from conditions of extreme expan- 
sion to those of great diminution or else has repeatedly and systematically shifted 
its position. 
THE CAUSE OF THE ALTERNATIONS OF THE SISTAN STRATA. 
When we attempt to explain these variations in the lake, four plausible theories 
present themselves, (a) The lake may have had an outlet which was repeatedly 
dammed by volcanic eruptions or otherwise, and as frequenth' swept clear. {/>) Some 
large tributary may have been diverted alternately to and from the lake, first filling 
it and then leaving it diminished, (c) The basin of Sistan niaj- ha\e been sub- 
jected to rhythmic earth-movements by which the lake was poured alternately from 
one side to the other, (d) The fluvial period may have consisted of a greater 
number of epochs than has been supposed, and each epoch may have caused an 
expansion of the lake. 
(a) Outlet Theory. — If the lake had an outlet as recently as the time of deposition 
of the clays, traces of it must still remain. The Shila can not be considered in this 
connection, because the Hamun-i-vSistan and the God-i-Zirrah are on nearly the same 
level and form parts of a single lake so far as the present problem is concerned. 
The entire border of the basin of Sistan has not been explored, but it is certain that 
even the lower passes stand at least 1,500 or 2,000 feet above the lake of Sistan. 
Any outlet which may have existed in fonner geological times must have ceased to 
do duty ages before the formation of the modern clays which now concern us. 
{b) Diversion Theory. — In the case of the lakes of Sistan and Zirrah we have 
seen that the diversion of the Helmund caused great changes in the area subject to 
inundation. If a similar change could have taken place repeatedh- in earlier times 
it might have caused the deposition of alternating lacustrine and subaerial strata. 
The diversion, if such there was, must have taken place much nearer the mountains. 
This theory can not be absolutely excluded, because our knowledge of tlie ri\ers in 
Afghanistan is exceedingly limited. The extreme improbability that a river .should 
be diverted back and forth eight or ten times at regular inter\als is a strong a priori 
argument against it. Moreover, if such diversions did take place, it is still necessary 
to explain win-. Such behavior on the part of a ri\-er could only be due to some 
systematic cause, such as repeated warping of the crust or climatic changes. 
