

400 STRUCTURAL AND FIELD GEOLOGY 
and not infrequently by abundant intrusions of igneous rock. But in 
ancient mountains of this type the forms worked out by erosion and 
denudation are in every case determined by the character of the 
rocks and the mode of their arrangement. And as highly inclined folds 
are the rule in such mountains, the successive ranges carved out by 
epigene action frequently coincide with the outcrops of the more durable 
rocks—they are essentially escarpments—the escarpments and their dip- 
slopes becoming more and more pronounced as the axial planes of the 
folds are increasingly inclined from the vertical. 

FIG. 142,—UNSYMMETRICAL FLEXURES GIVING RISE TO ESCARPMENT 
MOUNTAINS. 
s, Ss, relatively ‘‘ soft” rocks; h, h, relatively ‘‘ hard” rocks. 
If mountains are gradually lowered by denudation, it 
would seem to follow that they must in time be wholly 
reduced. Such, indeed, has been the fate of Wimivsame 
mountain chains of great geological antiquity. Frequently 
we encounter plateaus and plains which, notwithstanding 
their superficial form, have all the structural characteristics of 
folded mountains. Thus, in former ages, an extensive range 
of mountains stretched across what are now the low-lying 
plains of Belgium. The structure of that region is highly 
complicated, the strata being arranged in a succession of 
closely compressed, unsymmetrical folds, so that younger 
rocks are folded underneath older rocks, while here and there 
the crust has yielded to the lateral movement by fracturing, 
and vast masses of strata have been thrust forward over the 
surface of rocks much younger than themselves. Yet the 
gently undulating ground gives no hint as to the presence of 
these buried mountain structures. The old mountains have 
been gradually removed and cast into the sea. Should the 
relative level of land and ocean remain unchanged for a 

