SINO-MONGOLIAN FRONTIER 
as marbles. All the granite up this way is of a 
pale buff colour. 
The hills south of the lake are of basalt, as also 
are those on the west as far as the pass beyond 
Ning-yiian. Here an extensive outcrop of granite, 
a continuation of that occurring north of the lake, 
extends as far as Yang-p’o-yao-tzi. From this 
point to the edge of the K’uei-hua-ch’éng plain 
basalt occurs, showing fine columnar structure. 
The basalt and granite are frequently superim- 
posed by loess, but not to any very great depth or 
extent. 
No outcrops of any kind occur on the plain, 
which is mainly alluvial, with patches of wind- 
deposited sand. 
The T’ai Hai (lake) lies in an immense basin some 
twenty miles long and ten miles wide. This basin 
is bounded on three sides by low hills of volcanic 
formation, and on the fourth (the north) by high 
mountains of granite. It is difficult to explain 
the existence of this hollow in which the lake lies. 
The latter is said to be very deep in the centre. 
At first one might suppose that it occupies the 
crater of an extinct volcano, but this is by no 
means certain. Further investigation is needed 
before such a theory can be confidently advanced. 
It would, however, be interesting to know the 
origin of all the basalt, so prevalent in this 
country. 
Referring to the mountains north-west of K’uei- 
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