Geological researches in China, Mongolia, and Japan, during the years 1862-1865.

CHINA, MONGOLIA, AND JAPAN. 9
wvere near enough to the limestone to be cemented into a hard mass by the waters
flowing from it.
The bed of the Yangtse must have been cut to about its present depth, when a
diminution of its average fall took place, permitting the formation of these terrace
deposits. Subsequently another change, by increasing the fall, caused the river to
scour out, again, the greater part of the valley. As with the river so with the
Tungting lake; this large sheet of water, which then occupied all the plain of
Hupeh and Hunan, must have been filled up with the terrace deposit, the remains
of which now form its shores. With the returning increase of fall, the lake was
scoured out by the rivers Yangtse, Han, Siang, and Yuen. Since this erosion, it
would seem probable that the velocity of the current has slightly diminished, as
the material brought down by these rivers has converted nearly nine-tenths of the
former lake into dry land. A large part of this lake-plain is said, by ancient
Chinese writers, to have been an immense marsh where it is now cultivated land.
We have, at present, no observations to show whether the oscillations of Central
China, which are thus recorded in the Yangtse Valley, were contemporaneous with
the raising of the western edge of the delta-plain; but whether they were or not,
the cause which was exerted across the whole breadth of China, must be looked for
in a vertical movement, either in the Tibetan highland or along the eastern coast.
A remarkable instance of the formation of a deposit of fine material, in the
swiftest part of the river, is observable in the first rapids, just above the Ichang
gorge. Granite rocks rising to the surface, near the shore, form an obstruction to
the current, which is here from fifteen to eighteen miles an hour, causing eddies in
their lee, in which a constant precipitation of sand takes place. Banks of quicksands are thus formed, their tops almost even with the surface of the river. Their
sides, too steep to remain at rest, are constantly being washed away, and as constantly replaced by the freshly precipitated material. At lowv water these banks
line the shores, and, during the high water season of 1863, I noticed one more than
half a mile long, and twenty-five or thirty feet above the river; the result of some
previous very high freshet.
2 April, 1866.