THE GREAT ROCK-SYSTEMS. 9 
secondary epoch, that of Pine Forests and Reptiles; the 
jourth, or tertiary epoch, that of Foliaceous Forests and of 
Mammals; finally, the fifth, or quaternary epoch, the era 
of Man and his Civilization. The divisions or perdods 
which we distinguish in each of the five long eras 
(p. 14) are determined by the different systems of strata 
into which each of the five great rock-groups is divided 
(p. 15). We shall now take a cursory glance at the series of 
these systems, and at the same time at the populations of 
the five great epochs. 
The first and longest division of the organic history of the 
earth.is formed by the primordial epoch, or the era of the 
Tangle Forests. It comprises the immense period from the 
first spontaneous generation, from the origin of the first ter- 
restrial organism, to the end of the Silurian system of 
deposits. During this immeasurable space of time, which in 
all probability was much longer than all the other four 
epochs taken together, the three most extensive of all the 
neptunic systems of strata were deposited, namely, the 
Laurentian, upon that the Cambrian, and upon that the 
Silurian system. The approximate thickness or size of these 
three systems together amounts to 70,000 feet. Of these 
about 30,000 belong to the Laurentian, 18,000 to the Cam- 
brian, and 22,000 to the Silurian system. The average 
thickness of all the four other rock groups, the primary, 
secondary, tertiary, and quaternary, taken together, may 
amount at most to 60,000 feet; and from this fact alone, 
apart from many other reasons, it is evident that the 
duration of the primordial period was probably much longer 
than the duration of all the subsequent periods down to the 
present day. Many thousands of millions of years were re- 
