272 ELEMENTARY GEOLOGY 
was deposited in shallow water. To account for these 
two apparently contradictory facts, we can only con- 
clude that at the time of their formation the ocean 
bottom there was slowly sinking, and that the rate 
of subsidence was never so great as to produce really 
deep water, but always maintained a relatively shallow 
sea. 
Of these great and. slow changes in the level of 
the land and the sea-bottom, there are other evi- 
dences (see next two chapters); but this alone would 
seem sufficient. For some reason, as yet unascertained, 
the bottom of the sea has in some localities slowly 
subsided for a long time, and as it settled, sediment 
has naturally accumulated, forming a series of layers 
which attain great depths, and are often composed of 
rocks of very different kinds. Then the reverse 
process has come about, and the sediments thus accu- 
mulated have been raised to form part of the dry land. 
In various parts of the earth, there are places where 
these two opposite movements are even now in 
progress (Chapter XVI.). The fact that ocean formed 
sediments are so abundant even on the highest land, 
is proof of great and widespread uplifts of the sea- 
bottom. 
