
398 STRUCTURAL AND FIELD GEOLOGY 
or even, in many places, quite overturned and lying on their 
sides (see Fig. 140). This complex flexing and folding is 
usually, as already indicated, accompanied by great disloca- 
tions and displacements of the strata, and not infrequently 
by the appearance of veins, dykes, and irregular masses of 
formerly molten matter, which may traverse the disturbed 
strata in all directions. 
When the present external form or configuration of folded 
mountains is compared with their internal or geological 
structure, the two are seldom found to coincidee ine 
longitudinal ranges and intermediate longitudinal valleys of 
a mountain chain do not correspond save in a very general 
way with flexures or folds of the strata. Mountains do not 
necessarily or even often coincide with saddle-backed or 

FIG. 140.—ALPINE TYPES OF UNSYMMETRICAL FOLDs. 
arched structures—nor do valleys invariably or even frequently 
correspond with trough-shaped arrangements of the strata. 
Possibly, when a mountain chain came into existence, the 
external form of the region may have been more or less 
clearly an expression of the underground structure. That 
is to say, the mountain ranges may have coincided with 
anticlines, or saddle-backs, and the intervening hollows may 
have corresponded with geological synclines or troughs. 
But so long a time has elapsed since even the youngest 
of our mountain chains was upheaved, that the whole surface 
has been greatly modified. Everywhere we see evidence of 
enormous erosion and denudation. Vast masses of rock 
have been gradually worn down and removed—swept away 
as sediment from the heights, and distributed over the 
adjacent low grounds or upon the floor of the sea. Thus, 
in many places, the original configuration of a chain has 
been obliterated—saddle-backed mountains have been replaced 

