STAGES OF THE EARTH'S HISTORY 427 



2. The Oceanic Era: Commencing with the waters condensed into an 

 . . ocean over the earth, or in an oceanic depression, with finally 

 some emerging lands, the temperature perhaps about 500 F., 

 if the atmospheric pressure was still 50 atmospheres. The first of 

 tides and the beginning of the retardation of the earth's rotation. 

 Oceanic waves and currents and embryo rivers begin work about 

 the emerged and emerging lands; the large excess of carbonic acid 

 and oxygen in the air and water a source of rock-destruction; 

 before the close of the era, the formation of limestones and iron- 

 carbonate by chemical methods, removing carbonic acid from 

 the air and so commencing its purification; the accumulation 

 of sediments without immediate crystallization or metamorphism, 

 and thereby the beginning of the earth's supercrust. 

 III. The Archceozoic aeon. Life in its lowest forms in existence. 



1. The Era of the First Plants: Algae, and later of aquatic Fungi 



(Bacteria), commencing with the mean temperature of the ocean 

 at possibly 150 F., since plants now live in waters up to and even 

 above 180 F. Limestones formed from vegetable secretions, 

 and silica deposits from silica secretions; iron-carbonate, and 

 perhaps iron-oxides formed through the aid of the carbonic acid 

 of the atmosphere and water; large sedimentary accumulation, 

 where conditions favored, thickening the supercrust. 



2. The Era of the First Animal Life: Mean temperature at the begin- 



ning probably about 115 F., and at the end 90 F., or lower; 

 limestones and silica deposits formed from animal secretions; 

 deposits of iron-carbonate and iron-oxides continued; large 

 sedimentary accumulations. 



Quite apart from any doubt as to the mode of genesis, two 

 serious questions relative to the processes outlined in this sketch 

 have arisen from recent investigations, the one growing out of the 

 failure to find any great basal formation having the distinctive 

 characteristics of an original crust; and the other from doubt as 

 to the possibility of so prodigious an atmosphere as that postulated. 



1. Relative to an Original Crust 



The theory of a molten earth carries the presumption that the 

 liquid mass arranged itself concentrically, with the heaviest matter 

 at the center and the lightest on the outside. As the granitoids 

 are the lightest of the large classes of igneous rocks, the granitoid 

 magmas should have formed the outer zone of the molten earth, 

 and should have been spread out homogeneously according to their 



