SURVEYING THE DESERT. 253
extended 12 km. farther to the north than it does now, and implies that its area was then at least twice as great as it is now. On the whole the table shows that the lake has shrunk at a pretty regular rate, the only interval that points to a quicker rate than the average is that between the fourth elevation and the fifth. It is of course probable that oscillations took place during the preceding periods, but any traces there may have been of temporary expansions have become obliterated. The hydrographical changes which year after year take place in and around the Kara-koschun form a pretty complicated tangle, and the observations made on one occasion at one place often appear to be contradicted by those made on another occasion at some other place. When I say, for instance, that the lake-basin is being filled up with solid material, which is causing its water-level to rise, then the northern shore ought not to advance south at the same regular rate as the shore elevations indicate; and if the water area of the lake is diminishing to the extent implied, so that there are no less than six shore-lines within a distance of 12 km. from its present northern shore, it is astonishing that the seventh, the nearest and latest shore-line, only two day's journey to the south-west, has been pierced by the masses of water which in the course of only two years have thrust back the shore not 12, but 20, km. towards the north. Yet of this apparent contradiction a satisfactory explanation can be found. Many different factors are co-operating to produce the changes which are taking place. Hence it is obvious, that the changes in question cannot proceed parallel to one another, for in one quarter one factor is predominant and in another quarter another factor. Here mud and sand are being deposited, there corrasion is performing its excavating labour, and it is in this way that the depression of the Desert of Lop is diversified, as we plainly see from the changeable positions and varying distribution of the water. With the object of gaining a general idea of the physical forces that co-operate in the re-shaping of the earth's crust in this part of the world, I will proceed to consider them in combination. Before I do so, however, there are one or two other questions which for a moment or two demand attention.
IIIFカラー高解像度
