THE GENERAL FEATURES OF THE EARTH 19 
sea. Associated with these are volcanoes, which are 
conical peaks, often attaining great height, and sending 
forth steam and molten rock. 
Besides the plains, plateaus, mountains, and volca- 
noes, there are minor irregularities of hills and valleys ; 
and so, by all these features, the spherical surface of the 
earth is roughened. The geologist finds that these 
are still being made: the hills are either growing 
higher or are being lowered, the valleys are becoming 
* 10,000 FT. CONTINENTAL 
PLATEAU 
SEALEVEL 
ete ae OCEANIC PLATEAU 
= 10,000 FT, erat ere 
20,000 FT, 
+ 30,000 FT. 
BiG... 4: 
Diagram to show the amount of surface at various elevations above and below 
sea-level. 
deeper or more shallow, the volcanoes are building 
their cones or are losing in elevation, and many 
mountains are still increasing or suffering loss, while 
even the continents and oceans are changing their rela- 
tive positions. This is a world of change, and even 
that most enduring of features, the rock of the moun- 
tain peak, is slowly wearing and wasting away. 
Forces Modifying the Surface. Many of these changes 
are made possible by reason of energy which comes to 
