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

CHINA, MONGOLIA, AND JAPAN. 73
which is mentioned in a previous chapter as occurring along the southern edge of
the plateau, in the erosion of the lava region. In all these instances the depressions
are entirely in the solid rock, and vary in size from a few yards to several thousand
feet across. They have the appearance of being produced by erosion and not by
sinking. In the instance before us this conformation is often assisted by cross
dykes of greenstone. But the occurrence generally would seem to arise from inequalities in the texture of the rock. Whatever the cause of these depressions may
be, their manner of formation is probably closely connected with the origin of a
large class of desert lake beds.
For many miles the surface of the rock was entirely bare of soil, excepting in the
bottoms of the depressions just mentioned, where ponds are probably formed in wet
years.
From this hilly region we came gradually into another of those broad plains,
which form, in the aggregate, the true plateau. These plains, the steppes of the
Russians, and tala of the Mongols, are like those of our own deserts in the Rocky
mountains. They are great valleys, often from twenty to sixty miles broad, filled
with marine deposits that have retained their horizontal position and remained often
intact from erosion. Their surface is not, strictly speaking, horizontal, but slopes
from both sides to the centre.
The deposit forming the substructure of this plain, seems to be the same sandstone and conglomerate that we have seen on the Tamchintala, judging from some
blocks of these rocks seen near a Mongol dwelling.
Crossing this plain we came, near its northern edge, to a line of basaltic cones
from 100 to 150 feet high, isolated from the low flat hills to the north, and apparently resting on clay slate. They seemed thus to belong to a bed or stream rather
than to a dyke. Whether the flat hills near by are a continuation of the same
volcanic rock I could not determine.
The rock is a brownish-black, minutely crystalline basalt. On the surface of the
plain, near these hills, I found large numbers of fragments of black and red cellular
lava, and abundant angular pieces of chalcedony, and red and green jasper, etc.
Dec. 2. During this day we crossed two broad valley depressions, the same calcareous sandstone and conglomerate already mentioned, forming apparently the substructure both of the long valley slopes and of the higher land intervening between
these. A few fragments of blue limestone and white quartz, derived probably from
the formation we crossed yesterday, were found in the surface gravel; but a large
percentage of this gravel consisted of chalcedony, cornelian, and agate.
From the highest ground the flat outline of the plateau was visible in every
direction, excepting to the south, where we could see the hills of the past two or
three days rising to the height of perhaps 1000 feet above th6 neighboring plateau.
Dec. 3 We travelled the past night and this day on the continuation of the steppes
of the last two days. During the afternoon the plain descended gradually to the
north till it ceased abruptly against a granite ridge from 50 to 100 feet high. Beyond
this ridge, for a few miles, the country though somewhat lower than the plain of
the morning, is bare of the steppe deposit, and presents a rough, granite surface.
Dec. 4. Detained one day by a bouzran or snow-storm of great violence.
10 May, 1866.