308 EARLY STAGES OF EARTH'S HISTORY 



close of the era, the formation of limestones and iron-carbonate by chemical 

 methods, removing carbonic acid from the air and so commencing its purifi- 

 cation; the accumulation of sediments without immediate crystallization or 

 metamorphism, and thereby the beginning of the earth's supercrust. 

 III. The Archeozoic 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 beginning 

 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." 



Difficulties 



Quite apart from the objections to the Laplacian hypothesis, 

 stated in the last chapter, two serious questions exist relative to the 

 stages sketched above. The one grows out of the failure to find 

 any great formation beneath all others having the distinctive char- 

 acteristics of an original crust; and the other from doubt as to the 

 possibility of the prodigious atmosphere postulated. 



Relative to an original crust. The theory of a molten earth 

 carries the presumption that the liquid substance of the earth was 

 arranged so that the heaviest matter was at the center and the 

 lightest on the outside. As the granitoids are the lightest of the 

 large classes of igneous rocks, the granite-like magmas should have 

 formed the outer zone of the molten earth. The solid crust should 

 have been light (for rock) and homogeneous, and it should have 

 formed a stratum over the whole earth. Except at the very surface, 

 it should have been completely crystallized, for the cooling must 

 have been very slow, a condition favorable for the growth of crystals. 

 No very large amount of fragmental volcanic material can be as- 

 sumed to have covered the original crust if the atmosphere contained 

 all the water of the future hydrosphere, for that would allow no 

 steam in the molten globe to produce abundant volcanic fragments. 

 Pyroclastic material of later times can hardly be supposed to have 

 concealed the original crust permanently, for many thousands of 

 feet of rock have been eroded from the surface of the oldest known 

 areas. It is equally improbable that the original crust has been 

 concealed everywhere beneath sediments derived from itself. 



