Southern Tibet : vol.7 / 728 ページ（カラー画像）

0728	Southern Tibet : vol.7
南チベット : vol.7

There is much that appears in favour of this theory, much more than for the
alternatives of an outflow by the Indus or the Satlej, especially as the canyons of
the Satlej may easily have been eroded by this river itself, without the assistance
of the Tsangpo. I have crossed the Photu-la or Kore-la twice, and standing on its
saddle I said: We have mounted only 315 feet from the river to the Kore-la, where
the height is 15,292 feet. And from the pass there is a headlong descent to the
Kali Gandak, an affluent of the Ganges. By means of a canal cut through the
Kore-la the uppermost Brahmaputra might be turned into the Ganges .... But the
changes here indicated will some time come to pass without artificial aid, for the
tentacles of the Kali Gandak are eating back northwards into the mountains much
more quickly than the Tsangpo is eroding its valley. Some time or other, perhaps
in a hundred thousand years, the Ganges system will have extended its tentacles
to the bank of the Tsangpo, and then will be formed a bifurcation which, in the
course of time, will bring about a total revolution in the proportions of the two
rivers and their drainage areas.¹

However, if the theories thus set forth by Burrard be admissible we would
get a river much larger than the one I have presumed, taking its origin in the
Tang-la mountains and arising to the Shayok and Indus by way of the Panggong
lakes. And admitting differential movements of the crust, there would be nothing
surprising in the fact that the Bogtsang-tsangpo now flows eastward while the Addan-
tso and Chargut-tso also drain to the east.

There is, however, an alternative way of assuming the outflow from this long
valley. The high degree of maturity of the Bogtsang-tsangpo points to great age.
Even nowadays the Selling-tso is a recipient of rivers both from the east and west.
It is perhaps a more plausible theory that only the western section of the great
latitudinal valley drained to the Indus from a water-parting in the region of the
Shakangsham, and that the eastern section found an escape to the Nak-chu. The Sachu-
tsangpo would then, together with its tributary from the Shakangsham, have turned
in a very sharp bend from the region of the present Selling-tso eastwards via the lakes
Pongok-tso, Burben-tso and Ngamdo-tso-nak, from where it must have turned south-
ward to the Nak-chu. This second alternative has been adopted on the little map
of mountain ranges and ancient rivers at the end of this volume.

Whether the one or the other of these two theories is more likely to be correct
is not as important as is the existence of a depression running through the whole
of interior Tibet and having every appearance of a gigantic river-bed, disfigured, it
is true, by differential movements, by denudation and weathering and by the filling
up of its trough with immeasurable quantities of subaerial and aquatic sediments.

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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

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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