3 8 EVOLUTION OF THE EARTH 



rest upon the surface. For a while the balance swung, as one 

 section or another of the crust was broken through and lavas 

 would pour out abundantly. Rapidly, however, from the 

 geologic standpoint, as the surface cooled, the atmosphere of 

 water-vapor condensed in a never ceasing deluge until an 

 ocean, probably universal in its extent, had gathered to a mean 

 depth of several thousand feet. The remaining atmosphere 

 was comparatively rare and cold. Carbon dioxide became the 

 dominant gas, and water-vapor subordinate. Solar heat began 

 to play the principal part in warming the equatorial zone. A 

 system of planetary winds developed in accordance with the 

 new order of nature. The cloud canopy became thin and 

 broken, resolving itself into climatic belts. Sunlight for the 

 first time began to pierce the lower atmosphere and illumine 

 from without the surface of the earth. 



During the earlier time, when the water could exist only as 

 gas in the atmosphere, the great pressure of this envelope had 

 kept much, perhaps most, of the gases in the molten rocks. 

 With the great fall in atmospheric pressure which accompanied 

 the gathering of the ocean, magmas which broke through the 

 higher levels of the crust into the regions of this decreased 

 pressure were able to give off great volumes of gases which in 

 the earlier stage had been repressed. These gases, freed for 

 the first time, are termed juvenile and from this time forward 

 juvenile waters were added to the ocean. In the first ages 

 following the solidification of the earth the additions were 

 large in volume, but igneous, action continues to bring new 

 magmas to the surface recurrently from age to age. These 

 give off the gases which have been repressed in them since 

 the origin of the earth. Thus, in intermittent and lessened 

 rate, the surface waters have increased through geologic time. 

 As Suess has said, the body of the earth has given forth its 

 oceans. 



