

394 STRUCTURAL AND FIELD GEOLOGY 
highly denuded lands the forms assumed by the surface have 
been determined mainly by the character of the rocks and 
the mode of their arrangement. Hence it may be eaigeam 
general terms, that when a region is exposed to denudation, 
its relatively “soft” rocks and weak structures will be reduced 
more rapidly than its relatively “hard” rocks and strong 
structures, the latter coming in time to dominate the former 
in a more and more pronounced degree. 
To sum up, we may conclude that many prominent features 
of the land are directly due to deformation of the crust, and 
that others are the immediate result of denudation and 
accumulation, while, between these two groups of land-forms, 
we recognise an intermediate type in the production of which 
both hypogene and epigene forces have been concerned. A 
glance at the structure and origin of the leading surface 
features of the land may suffice to make this clear. 
1. MOUNTAINS 
There are only two great classes of mountains. One of 
these comprises every height which owes its origin either (a) 
to the heaping-up of materials at the surface of the earth, or 
(6) to subterranean action, which has resulted in the deforma- 
tion of the earth’s crust. These two groups form the great 
class of Original or Tectonic Mountains. ‘The second great 
class includes a vast variety of heights of quite a different 
origin. They have not been built up by accumulation, nor 
upheaved by crustal movements, but are simply the surviving 
remnants of ancient high lands. They now form mountains 
because the land masses by which they were at one time 
encompassed have gradually been worn down and removed. 
Lofty plateaus, for example, have been so deeply excavated 
by superficial action, that they have frequently lost their 
plateau character and acquired quite a mountainous aspect. 
Mountains of this kind we term Swbseguent or Relzct. 
I, ORIGINAL OR TECTONIC MOUNTAINS.—These include 
two groups, namely, Accumulation Mountains and Deforma- 
tion Mountains, of which the latter is by far the more 
important. a 
(2) Accumulation Mountains.—We need not dwell on 

