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HISTORICAL GEOLOGY 



with other elements to form the rocks. When the crust finally cooled 

 to such an extent that water could remain on its surface, the oceans 

 were formed and the atmosphere gradually lost its gases until its pres- 

 ent composition and character were attained. 



Planetesimal Hypothesis. — The planetesimal hypothesis has 

 been offered as a substitute for the nebular hypothesis. Omitting 

 the astronomical considerations, this theory assumes that the earth 

 was never in a molten condition, but grew gradually by the ingather- 

 ing of small particles (Fig. 365) called planetesimals (little planets). 



Fig. 365. — Spiral nebula. (Yerkes Observatory.) 



In its early stages, if this hypothesis is true, the earth had no atmos- 

 phere, since, on account of its small mass, the attraction of gravity 

 was insufficient to hold the gases which were lost in space because of 

 their activity. With its present mass the earth's attraction is suffi- 

 cient to prevent the escape of most of the gases, but such gases as 

 hydrogen and helion are still superior to its attraction. The moon 

 appears to be devoid of an atmosphere, the gravity of its mass being 

 insufficient to hold the gases to it. When the earth was as small as 

 the moon is now, it, too, probably had no atmosphere, but as it grew 

 by the- addition of meteoric matter, gravitational attraction increased, 

 permitting it to bind to itself more and more gases until the present 

 condition was reached. The gases first held by the attraction of the 

 earth were the heavier ones, such as carbon dioxide, nitrogen, and 

 water vapor. Of these, carbon dioxide, being chemically active 



