
408 STRUCTURAL AND FIELD GEOLOGY 
consists of the younger, and its basal portion of the older, 
rocks, the line separating the two series representing the old 
plain of erosion. 
The forms assumed by Tectonic (Folded) mountains, during the stage 
of early youth, are a more or less direct expression of their internal 
structure. The ranges coincide to some extent with upward folds or 
anticlines, and the intervening parallel hollows with downward folds or 
synclines (see Fig. 139, p. 397). But with increasing age this approxti- 
mate correspondence between configuration and structure gradually 
disappears, until eventually every coincidence of the kind vanishes. 
Under the long-continued operation of the agents of erosion, the 
mountains are completely remodelled (see Fig. 141, p. 399). When a 
mountain chain has passed the age of maturity, the distribution and 
shapes of its component heights are determined directly by the character 
of the rocks and their geological structure. In this respect, therefore, 
highly denuded Tectonic mountains do not differ from Relict mountains 
which have been carved out of an ancient plateau of erosion. In both 
cases it is the character or nature of the rocks and the mode of their 
arrangement which determine the position of the heights and their 
general configuration. Nevertheless, we must distinguish between the 
two kinds of mountains. A Tectonic mountain chain remains original 
throughout all stages of its existence ; it is a true Deformation mountain 
chain until it is at last swept away, and replaced by a plain of erosion. 
A Relict mountain has not been built up, nor is it the direct result of 
crustal deformation. It owes its existence to erosion ;—it is a mountain 
of circumdenudation. But the very causes which have determined its 
existence must eventually work out its destruction. 
We have referred to the forms assumed by mountains which have 
been carved out of plateaus of accumulation—pyramids, and truncated 
pyramids being the typical shapes of such mountains. Under favourable 
conditions mountains of this kind often ascend in a series of abrupt 
terraces, or corbel steps. But much depends on the nature of the rocks. 
If the strata be more or less homogeneous in character, the step-like 
outlines are not likely to be pronounced. Instead of abrupt pyramidal 
heights, we may have smooth, rounded hills. The character of the 
climate has also a powerful influence—an arid climate fostering the 
formation of more or less abrupt pyramidal mountains; while under 
moist conditions the configuration of the heights tends to be smoother 
and less abrupt. Nevertheless, whether the horizontally bedded rocks 
be of one kind or another, or show alternations of many different kinds, 
and whether the climate be dry or humid, equable or the reverse— 
tropical, temperate, or arctic—the mountains and hills sculptured by the 
action of the epigene agents are of the same type. 
Relict mountains derived from the erosion of folded and contorted 
rocks have, as already shown, the general aspect of highly denuded 
Tectonic mountains. Hence sometimes we find them extending in the 
direction of the outcrops of the more durable rock-masses, and then 

