INTRODUCTION 
Westward from the point where it strikes the 
Yellow River near He-k’ou this boundary is the 
natural one, being marked, first by a section of 
the river, then by the sandy edges of the Ordos 
Desert, next by a second stretch of the Yellow 
River, and finally by the southern border of the 
Gobi Desert. 
Eastward from He-k’ou it is more difficult to 
trace a natural boundary, as the whole country is 
broken up by irregular ranges of hills and moun- 
tains interspersed with wide river valleys, which 
in some instances assume the dimensions of plains. 
On the whole the course of the outer loop of the 
Great Wall may be said to define most satisfac- 
torily the natural boundary. Especially is this 
the case immediately north and north-west of 
Kalgan, where a line of high cliffs of volcanic 
origin marks the edge of what is generally known 
as the Mongolian Plateau. 
From Kalgan the Wall runs first east and then, 
after making a big loop to enclose the valley of 
Pei Ho, turns south-east, following the crests of 
the mountain ranges, finally ending abruptly on 
the sea coast at Shan-hai-kuan (lit. Mountain-sea 
pass). 
The line of demarcation between the land 
actually occupied by the Mongols onthe one hand 
and the Chinese on the other follows the political 
boundary, but, so far as the writer has been able 
to observe, the floral and faunistic, as well as the 
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