
STRUCTURE AND SURFACE FEATURES 399 
‘by valleys and depressions, while trough-shaped strata have 
been carved into mountain heights (see Fig. 141). 

FIG, 141.—APPALACHIAN RIDGES OF PENNSYLVANIA. 
a, a, anticlines; s, s, synclines. 
The forms which folded mountains ultimately assume, under the 
action of denudation, are determined essentially by the character of the 
rocks and the mode of their arrangement. Certain kinds of rock and 
particular types of structure are more readily reduced than others. It 
is the more durable rocks and the stronger structures, therefore, that: 
tend in the long-run to constitute the mountain ranges ofa chain. In 
the younger mountain chains of the globe the remodelling of the surface 
is only partially accomplished. Consequently, amongst these the con- 
figuration is still in many places an expression of the underground 
structure. Individual ranges and intervening depressions continue to 
coincide more or less closely with the folds and displacements of the 
strata. But with increasing age such coincidence becomes less and less 
marked, until in the oldest mountain chains it ceases to appear. Amongst 
these ancient mountains, all weakly constructed heights, such as anti- 
clinal ridges, have been reduced, while the more enduring rock arrange- 
ments, such as synclines or troughs, no longer form depressions, but 
have most frequently been converted into elevations by the removal 
of the weaker structures which formerly dominated them. The contrast 
is illustrated by Figures 139 and 141, the former representing the structure 
of a portion of the Swiss Jura—a relatively young chain, while the latter 
shows the Appalachian Ridges of Pennsylvania—mountains of vastly 
greater antiquity. 
In both cases the strata, it will be observed, are arranged in sym- 
metrical folds, but, as already stated, the structure of many mountain 
chains is much more complicated—the folds being closely compressed 
and pushed over, so as to lie ontheir sides. The configuration ultimately 
acquired by mountains of the Alpine type naturally differs from that 
assumed by an elevated region of symmetrically folded strata. Generally, 
we find that strata of variable character which have been compressed 
into a succession of steeply-inclined folds tend in time to assume the 
aspect of escarpment-mountains. The crests of the anticlinal ridges are 
removed, but the synclinal troughs are not developed into mountains, as 
in the case of symmetrically folded strata. It is the outcrops of the 
more durable rocks which determine the position of the heights—which, 
as a glance at Fig. 142 will show, are escarpments. The illustration, it 
need hardly be said, is only a diagram. In point of fact, the structure of 
such folded mountains is infinitely more complex, the folding being 
usually complicated by dislocations and displacements of all dimensions, 


