Southern Tibet : vol.7 / 685 ページ（白黒高解像度画像）

0685	Southern Tibet : vol.7
南チベット : vol.7

499

mighty mountain ranges, and it therefore occupies ⅛ of its drainage basin. On
the other hand, my »Large lake» of 1901 which is situated on the flat and open
plateau-land and which is surrounded by relatively low mountains at a considerable
distance, occupies only ¹⁄₁₀ of its drainage area.

A careful examination of Colonel Byström's map in 1:1000000, which is the
most detailed and complete general map of Tibet in existence at the present date,
will teach us that the lakes of the plateau-land are arranged in a certain degree of
order, more especially in the northern and southern parts of the region. In the north
we find two lines of lakes from west to east, viz., the Kwen-lun lakes, including my
chain of lakes No. V—No. XX and the lakes of Wellby with their western continuation.
This regular arrangement of the lakes is of great assistance to us in our attempts
to trace the situation and stretching of the principal mountain systems. For the
lakes occupy the lowest parts of their basins, and a long series of lakes situated
upon the same line, indicates a very extended depression. It is therefore natural,
though not necessary, that a chain of lakes should be placed between two parallel
mountain systems. In the same way Wellby's lakes let us suggest that their extended
depression is bounded both to the north and south by parallel ranges.

In the south there are also two chains of lakes in depressions stretching through
the whole plateau-land. In the northern one of these, to which Panggong-tso, Dagtse-tso
and Selling-tso belong, Nain Sing, Littledale and I have travelled. The southern
one is marked by Nganglaring-tso in the west and by Tengri-nor in the east.

The most central part of the plateau-land situated between Wellby's lakes in
the north and the Panggong-tso—Selling-tso Lakes in the south is not sufficiently
well known to allow us to trace such long lacustrine depressions as those just
mentioned. This innermost part of Tibet has been crossed only on five lines by
Bower, de Rhins, Littledale and myself. Our routes proceed from lake to lake,
and it is probable that these lakes are also in reality placed upon more or less regular
lines indicating long depressions between mountain ranges.

In my work Scientific Results, Vol. IV, p. 589 et seq. I have examined the
absolute altitude of some 95 lakes discovered by Nain Sing, Bonvalot, Bower,
de Rhins, Littledale, Deasy, Rawling, Wellby and myself. As a mean altitude
of 58 lakes in Eastern Tibet I got 4811 m., and as the mean altitude of 37 lakes
in western Tibet 4837 m., and I therefore arrived at the conclusion that the mean
altitude of the general plane of the plateau-land was somewhat higher in the west
than in the east. This is probably correct and most likely is due to the fact that
the mountain ranges are more closely pressed together in the west than in the east.

It is more interesting, however, to examine the mean altitude of the lakes
which belong to the same latitudinal valley. Taking, to begin with, the 12 Kwen-lun
lakes of our list, their mean altitude amounts to 4891 m. The 6 lakes of Wellby

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