HYPOTHETICAL STAGES LEADING UP TO THE KNOWN ERAS. 119 



embraces the Proterozoic, Paleozoic, Mesozoic, and Cenozoic eras, the 

 chief portion of known geologic time. 



Synoptical View of the Earth's History. 



If we take a sweeping view of the stages of the earth's history from 

 first to last, they fall into two great groups, with a transition period 

 between them: the stages of growth and the stages of maturity, with an 

 intervening transitional stage. In tabular form, and numbered in 

 chronologic order, these stages appear as follows: 



III. The Gradational 

 Eon (Relative ma 

 turity) 



II. The Extrusive 

 Eon (Transi- 

 tional) 



I. The Formative 

 Eon (Birth and 

 adolescence) 



11. The Cenozoic era 



10. The Mesozoic era 



9. The Paleozoic era 



8. The Proterozoic 



era 



7. The Archeozoic era 



(Under planetesimal 

 hypothesis) 



6. The initial life 

 stage 



5. The initial hydro- 

 spheric stage 



4. The initial atmos- 

 pheric stage 



3. The initial volcanic 

 stage 



2. The atmosphere- 

 less stage 



1. The nuclear or 

 nebular stage. 



C. The better 

 known eras. 



B. The partially 

 known era. 



(Under gaseous 

 hypothesis) 



The initial life ' 

 stage 



The congelation 

 stage 



The molten 

 stage 



The gaseous or 

 nebular stage 



A. The hypothe- 

 tical eras. 



SUPPLEMENTARY CONSIDERATIONS. 



I. The Differentiation of the Rocks during the Stages of Growth. 

 The general nature of the process by which the surface rocks of the conti- 

 nental areas were, by hypothesis, rendered more acidic and less heavy, has been 

 set forth. To show more fully the bearing of the hypothesis on the general 

 characteristics of the crystalline rocks, and on some of the outstanding problems 

 presented by them, there is need to enter somewhat further into details, though 

 the subject cannot be fully elaborated here. 



The average original material. — If meteoritic material represents, in a general 

 way, the average of the heavier and more refractory material of the siderial 

 system, as seems probable from the fact that it comes from all quarters of the 

 heavens and seems to represent almost as many different parent bodies as there 

 are distinct meteoritic falls, it may be regarded as the type of what was the average 

 earth-material in its atmosphereless state, if it had such state. If there be added 

 to this a certain unknown proportion of the lighter and less refractory material 



