42 
HAROLD’S DISCUSSIONS. 
This was the age when invertebrates had every¬ 
thing their own way; it extended through a long 
period of time, perhaps millions of years, but the end 
came. The crust shifted ; more land was wrested 
from the gras]) of the 
sea, in broad belts 
about the existing isl¬ 
ands, particularly at 
their southern limits. 
The sea settled down 
in smaller and per- 
Fig. 32.—Green and brown 
alga?. 
Fig. 23.—One of the earliest fishes, 
found in old red sandstone 
stratum. 
haps deeper hollows to begin the problems of an¬ 
other age. 
THE DEVONIAN PERIOD. 
On top of the Silurian rocks we find other strata 
which crop out in many places, and hence may be 
