244 ELEMENTARY GEOLOGY 
These ocean rocks are of great interest to the geolo- 
gist, because the land is largely formed of them. 
More than one half of the earth’s crust is made of 
rocks that were laid down in the sea, and have since 
been raised above the surface. They therefore tell us 
much of former changes, and from them we are able 
to learn much about the physical geography of the 
gae See 
Fig. 135. 
Bowlder beach, Cape Ann, Mass. 
past. Moreover, since this action of deposition in 
the ocean has at all times been in operation, and since 
on the land we find such rocks formed in all ages of 
the past, and enclosing remains of animals then living, 
they constitute a veritable tomb in which is preserved 
a record of the development of life on the earth. 
Hence their study is full of interest. 
