814 ELEMENTARY GEOLOGY 
Thus there are many complexities of rock structure 
and mountain form; but the two main characteristics 
which are almost always present are: (1) disturbed 
rocks which have been more or less complexly moved 
from their original positions ; and (2) the effect of de- 
nudation in planing down mountain tops, etching out 
the softer strata, and leaving the harder to stand above 
them, either as ridges.or peaks. 
Denudation is very active among mountains, for 
there is plenty of slope down which the water may 
run, and many mountain tops rise above the zone of 
the protection of vegetation, into a region where frost 
and winds readily attack the exposed rocks. Because 
the strata are of such different kinds, and so readily 
etched, the first effect of denudation is to produce an 
extremely uneven topography. It cannot be too 
plainly stated that folding is but one of several causes 
for the ruggedness of mountains. 
Position of Mountains.— Mountains are present in 
nearly all parts of the world. So far as we know, they 
are more commonly found on the continents than in 
the oceans, but there are many mountain chains in the 
sea, and it is not unlikely that their apparent greater 
abundance on the land may be due, at least in part, to 
our imperfect knowledge of the ocean beds. Of these 
oceanic chains the Hawaiian Islands furnish a good 
illustration (Fig. 189). These extend approximately 
