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. ^Vhat 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 3,000 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 



259 



