STAGES OF THE EARTH'S HISTORY 431 



primitive atmosphere the accessions of the great volcanic period, 

 and these together would probably have given a gaseous envelope 

 not altogether unlike that of the preceding view, though less 

 excessive. 



The modified hypothesis furnishes a somewhat better basis 

 than the older view for the volcanic activity of later periods, and 

 for a supply of gases to the atmosphere to offset the loss due to 

 chemical combination of its constituents with the surface rocks; 

 but it is not clear that it is adequate. 



Stages under the Modified Hypothesis 



The stages of evolution under this view may be summarized as 

 follows : 



1. The astral eon. The separation of the material of the earth 

 from the parent nebula and its aggregation into a rotating gaseous 

 spheroid. 



2. The molten eon. The condensation of the rock matter of 

 the gaseous spheroid into a molten spheroid surrounded by a hot 

 vaporous atmosphere, the molten spheroid retaining occluded within 

 itself some notable part of the water of the present hydrosphere, 

 as well as much of the carbon dioxide represented by the present 

 carbonates and carbonaceous deposits. 



3. The lithic eon. The solidification of the molten spheroid, 

 beginning perhaps at the center, on account of pressure. 



4. The primitive volcanic eon. Prodigious volcanic action, 

 closely following the solidification of the crust, during which great 

 beds of lava were poured out on the surface, followed by great in- 

 trusions of other lavas into and through them. Contemporaneous 

 with this volcanic action, the atmosphere and hydrosphere gave 

 rise to some sedimentary deposits, which were interbedded with 

 the volcanic products, but were greatly inferior to them in volume. 

 This period would correspond to the Archeozoic eon. 



5. The sedimentary eon. The remaining time up to the pres- 

 ent has been characterized by the dominance of the atmospheric 

 and the hydrospheric activities over the volcanic, and the result is 

 recorded in the common sorts of sedimentary rocks. This eon 



