国立情報学研究所 - ディジタル・シルクロード・プロジェクト
『東洋文庫所蔵』貴重書デジタルアーカイブ

under the increasing load; and on the other hand the mountains are continually
rising to maintain their height. The strain established in the rigid crust, between
the sinking zone of deposition and the rising mountain range, finds relief in the
development of fractures along the range, as well as others which permit a differ-
ential uplifting of great block-masses—a fact which had before us been observed
by Muschketof. The evidence of this compensatory maintenance of hydrostatic
equilibrium is strikingly recorded both in the Kopet range and in the zone of
deposition. All along the range the lines of fracturing are visible on a large scale
in well-developed faultings; and the border of the alluvial plain is bent sharply
upward, having been dragged up by the rising mountains. Deep longitudinal
valleys are carved by erosion along the lines of weakness offered by these fault-
planes. On the mountainward side of the valley rise the older rocks of the range,
while on the other is a steep wall, formed often by the basset edges of beds of con-
glomerate which are the up-bent representatives near the mountains of the alluvial
strata of the plains. On the other hand, the sinking of the zone of deposition is
proved in the deep artesian well southeast of Askhabad. This boring remained to
a depth of 2,195 feet in a pure delta-formation and was still in this when boring
was stopped. The plain at Askhabad is 820 feet above the Caspian; the bottom
of this trough is, therefore, more than 1,400 feet below the level of the ocean.

Two factors were necessary to the growth of the deltas—precipitation in
the mountain region to supply the water to bring down the products of disintegra-
tion, and a rising movement of the highland to maintain a grade necessary for the
transportation of these products by water.

The delta is broadly divided into three zones of deposition—that of quickly
dropped coarse detritus at the apex; the main body of the delta, the rapidly
descending broad surface of which receives sediments from the overflow during the
floods; and the outer, more or less flat, border, which receives both the finest
material and all of the materials that are carried or rolled down through the channel
or channels through which the water that does not overflow the delta finds its way,
to be lost beyond on the bordering plain.

This bordering zone belongs not only to the delta but to the desert as well;
and it is here that is waged the eternal struggle between the desert with its breath
of fire and its overwhelming sea of sand on the one hand, and the life-bringing
waters on the other. The sands from the desert encircle the whole delta with a
wall of great wave-like dunes. That they do not bury it when they are not them-
selves arrested by sparse grass on their surface is due wholly to the ability of the
waters to prevent the formation of barkhans and dunes on its surface; for the
water, spreading in flood-time over the even surface of the fan, distributes and
assimilates annually a great part of the sand that lies in ripples on the surface;
while another part, finding no irregularities to form the starting points of dunes,
is blown to join the sandhills on the opposite side.

The delta streams maintain channels through the dunes bordering the foot
of the delta; and through this the excess waters of the floods find their way to
spread out among dune-locked depressions, where on evaporating they leave their