242 ELEMENTARY GEOLOGY 
The present surface merely represents the present 
stage in the history; and this surface, the product of 
change, is changing now and will continue to do so 
just as long as the land stands above the sea in expos- 
ure to the air. Denudation is one of the most potent 
sets of geological forces; for it tears down the rocks of 
the land, and builds new ones out of their fragments. 
Being of slow action, it does not attract general atten- 
tion. Fora long time it escaped even the students of 
nature; but now that we recognize it, we see in it 
one of the most powerful causes for the changes in the 
land. 
