Geological researches in China, Mongolia, and Japan, during the years 1862-1865.

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 where this formation predominates are easily distinguished from those consisting 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.