24: GEOLOGICAL RESEARCHES IN 



Here is the most important gate of the Great Wall through which pass all the 

 caravans to Russia, and nearly all those that trade with Western Asia. 



The mountains here consist of the tufaceous rocks of the Kalgan porphyry, which 

 are traversed by dykes, and contain beds, of the parent rock. The portions of the 

 range Avhere this formation predominates are easily distinguished from those con- 

 sisting of the usual granite and metamorphic schists, the latter forming pyramidal 

 hills, while the former have the castellated appearance that is given by cliffs and 

 dykes. The white and red tufas form low hills west of Kalgan, and in the wall of 

 the gorge, in the Barrier range, beds of these rocks trending E. W., and dipping 

 about 45° to N., seem to extend under the porphyry, Fig. 8. 



