AMERICA THE OLD WORLD. 
13 
stirred on tlie surface of the earth ; the Secondary 
or middle period, when animals and plants were 
introduced and the land began to assume conti- 
nental proportions ; and the Tertiary period, or 
comparatively modern geological times, when the 
aspect of the earth as well as its inhabitants was 
approaching more nearly to the present condition 
of things. But as their investigations proceeded, 
they found that every one of these great ages of 
the world’s history was divided into numerous 
lesser epochs, each of which had been character- 
ized by a peculiar set of animals and plants, and 
had been closed by some great physical convul- 
sion, disturbing and displacing the materials ac- 
cumulated during such a period of rest. 
The further study of these subordinate periods 
showed that what had been called Primary for- 
mations, the volcanic or Plutonic rocks, formerly 
believed to be confined to the first geological 
ages, belonged to all the periods, successive erup- 
tions having taken place at all times, pouring 
up through the accumulated deposits, penetrat- 
ing and injecting their cracks, fissures, and in- 
equalities, as well as throwing out large masses 
on the surface. Up to our own day there has 
never been a period when such eruptions have 
not taken place, though they have been con- 
stantly diminishing in frequency and extent. In 
consequence of this discovery, that rocks of ig- 
