HUMAN HISTORY 107 



^ , f Recent 



Quaternary j pj.igtocene 



Cainozoic \ / Pliocene 



mi- Miocene 



Tertiary Qligocene 



^ Eocene 

 [ Cretaceous 



Secondary \ Jurassic 



( Triassic 

 / Permian 

 Carboniferous 



Primary -^ Devonian 



Silurian 

 ^ Cambrian "*" 



Archaeozoic Precambrian 



It is with the Cainozoic period that we are alone concerned, and 

 chiefly with the Pleistocene and Recent subdivisions of that 

 period. Before we consider the Quaternary epoch, one or two 

 facts with regard to the dating of the other periods may be men- 

 tioned. However long it is supposed that the deposition of the 

 sedimentary deposits has occupied, it is usually held that the 

 Archaeozoic period occupied at least half of the whole length of 

 time. This supposition is rendered necessary by the fact that 

 already in the Cambrian era organisms of a high degree of com- 

 plexity are found ; thus in this era Crustaceans, Brachiopods, 

 and Worms are common ; Echinoderms, Coelenterates, and 

 Sponges are also known. In the Silurian most of the classes of 

 the animal kingdom are represented, the exceptions being am- 

 phibians, reptiles, birds, and mammals. The presence of fish 

 shows that by this time vertebrates had already been evolved. 

 Amphibians first appear in the Carboniferous and reptiles in the 

 Permian. Birds first occur in the Upper Jurassic and mammals 

 towards the close of the Triassic. The estimates as to the time 

 occupied by the deposition of all these strata taken together vary 

 very greatly — the average being about 100,000,000 years, though 

 it should be mentioned that as long a period as 1,000,000,000 

 years has been proposed.^ With regard to the length of time 



* Jolly, Science Progress, 1914, No. 33, p. 41, states that there are four methods 

 of estimating the duration of the whole period, by considering (1) the thickness 

 of the sediment, (2) the mass of the sediment, (3) the sodium contained in the 

 sea, and (4) radioactive transformation. (1) gives about 100 to 134 million years, 

 (2) about 87 million years, (3) about 80 to 90 million years, and (4) a much longer 

 period. 



