THE GREAT EOCK-SYSTEMS. 9 



secondary epoch, that of Pine Forests and Reptiles; the 

 fourth, 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 periods 

 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 

 fii'st 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 ^ t X- 

 three systems together amounts to 70,000 feet. Of these ^"7/" "2.. ^ 

 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- 



