SINO-MONGOLIAN FRONTIER 
As the Clark Expedition subsequently traversed 
a good portion of our route, and as I have already 
published notes on the geological formations along 
that portion, there is no need to go over it again 
here. 
From Yen-an Fu to Ching-pien Hsien we 
travelled up the valley of the Yen-shui, and noted 
that the sandstone strata, usual in North Shensi, 
occurred all the way, without fault and practically 
horizontal. As far as I can make out this sand- 
stone belongs, or at least corresponds, to the Red 
Beds, which form the upper strata of the Shansi 
formation. What corroborates this supposition 
is the fact that no coal seams appear along the 
ravines, as is the case in the coal-producing districts 
of Shansi, yet coal is extensively mined through- 
out North Shensi, vertical shafts being sunk to 
reach the seams. As the average altitude of the 
upper surface of the sandstone strata does not 
exceed 38,000 feet, it follows that the Shansi coal 
measures are very much higher than those of 
Shensi. Further, there seems to be no reason to 
suppose that they are two distinct systems, so 
that one can only suppose that the strata of North 
Shensi must lie in an extensive and shallow synclinal 
fold, or that they have become depressed below 
those of Shansi owing to faults somewhere along 
the course of the Yellow River. We know that 
there is a series of faults with downthrow on the 
west, running roughly parallel with the Yellow ' 
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