22 ELEMENTARY GEOLOGY 
mountain formation. Where fissures reach down into 
this heated mass of the mterior, volcanoes may be 
formed. ; 
Thus we have two sets of forces acting on the earth: 
changes originating outside of the earth itself and tend- 
ing to Jessen the irregularities, and those from within 
the earth, which are buildmg them up. These are 
in opposition, and the form of the land as we know it, 
is a resultant of their interaction. So far the elevat- 
ing forces from within have exceeded the destructive 
agents ; otherwise the land would have been worn down 
nearly to sea-level, while the waste would have been 
strewn over the bottom of the sea. 
