THE ORIGIN OF LIFE 



491 



MilLiOWS 



OF 



TIME 



UNITS 



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2 6- 



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ASK OF MAN 



OF 

 MAM M ALS 



AGE 



OF 

 REPTIl.ES 



AGE 

 OF 



AMPHIBIANS 



ACE 



OF 



FISHES 



AGE 

 OF 

 INVERTE- 

 BRATES 



u 





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liJ 

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Q. 



QUATgRNAWV 



TERTIARY 



U PPE R 

 CRETACEOUS 



LOWER 



CRETACEOUS 



(comancmean) 



JURASSIC 



TRIASSIC 



PERMIAN 



PEN NSYLVAN- 



I AN 



(u Pf^ R 



carboniferous) 



m ississippian 



(lower 

 carboniferous) 



DEVONIAN 



SILURIAN 



OROOVICIAN 



CAMBRIAN 



SED 



ROCKS GENERALLY METAMORPHOSED: IGNEOUS PREDOMINANT; 



MENTARY SECONDARY. LIMESTONE. IRON ORE. AND GRAPHITE 

 INDIRECT EVIDENCE OF FORMER LIFE. FOSSILS SCARCE. 



PRECAMBRIAN," 30.000.000 Tl M E UNITS 



Fig. 259. — Geologic time-scale, eras, and succession of animal life. 



Time-units in geology can be stated relatively with a fair degree of certainty, but not 

 absolutely. Thus it is possible to say that the Quaternary, or the Tertiary, or the Cambrian 

 represents a certain fraction of the whole; but for the absolute length of time represented 

 by the 60,000,000 units shown, it can only be said that the entire period represents a vast 

 lapse of time, as long as 1,500,000,000 years according to some geologists. It will be 

 noted that the time-units before the Cambrian are as great as those shown by the scale. 

 The absence of many fossils in the pre-Cambrian rocks is probably due to the metamor- 

 phosis of sedimentary deposits that once had fossils, and perhaps to the restriction of abun- 

 dant life to hmited areas. (After Osborn, from the time-scale of Pirsson and Schuchert.) 



