98 
MOUNTAINS AND THEIR ORIGIN. 
What has been done thus far is chiefly to clas- 
sify the inequalities of the earth’s surface, and to 
detect the different causes which have produced 
them. Foldings of the earth’s crust, low hills, 
extensive plains, mountain-chains and narrow 
valleys, broad table-lands and wide valleys, local 
chimneys or volcanoes, river-beds, lake-basins, in- 
land seas, — such are some of the phenomena 
which, disconnected as they seem at first glance, 
have nevertheless been brought under certain 
principles, and explained according to definite 
physical laws. 
Formerly men looked upon the earth as a unit 
in time, as the result of one creative act, with all 
its outlines established from the beginning. It 
has been the work of modern science to show 
that its inequalities are not contemporaneous or 
simultaneous, but successive, including a law of 
growth, — that heat and cold, and the consequent 
expansion and contraction of its crust, have pro- 
duced wrinkles and folds upon the surface, while 
constant oscillations, changes of level which are 
even' now going on, have modified its conforma- 
tion, and moulded its general outline through 
successive ages. 
In thinking of the formation of the globe, we 
must at once free ourselves from the erroneous 
impression that the crust of the earth is a solid, 
steadfast foundation. So far from being immov* 
