Study ou Rocks* 
29 
pute is tliat the interior of the earth is intensely hot. 
In either case the central mass continues to cool and 
therefore to contract. One result of this is, that the 
crust is thrown into folds, or vast masses of the crust 
may break off and sink below the rest. 
We are accustomed to speak of the ocean as a type 
of instability, and to refer to the eternal hills as 
typical of the unchanging. If there is one conclusion 
more than another that geologists are agreed upon it is 
that the crust of the earth is not stable and unchanging. 
On the contrary, thehills are worn down by rain, winds, 
and frost ; streams and rivers cut for themselves 
channels, the eroded materials are transported as sedi- 
ments to lakes and seas, and there spread out in strata. 
Vast quantities of this eroded material are sometimes 
spread over alluvial plains by flood waters. 
The sea itself is encroaching on the land, in many 
places at an alarming rate. The breakers are sawing 
into the cliffs around the dry land. The overhanging 
rocks are undermined and topple over, in the course of 
time. These, hurled back by the waves against the 
cliff, help in the work of destruction, so that, as some- 
one has remarked, the cliffs of the world supply an 
artillery to destroy themselves. 
Then there are the known movements of the 
earth’s crust, some portions being elevated, and some 
portions being slowly submerged. Many of these earth 
movements are so gradual as to be hardly appreciable 
in a century. On the other hand, sudden and violent 
