314 HISTORICAL GEOLOGY 



present day. The overpowering force of gravity tends to 

 compress the material of the earth into an ever denser form, 

 and this compression generates heat. As the planet grew 

 larger, its gravity increased in strength. The interior was 

 therefore more compressed and became constantly hotter. 

 All rocks, whether such as belong to the interior of the earth, 

 or such as come down as meteorites, contain a variety of gases 

 or material out of which gases may be formed by heat. The 

 growing pressure and heat of the earth's interior are therefore 

 supposed gradually to have driven out some of these gases, 

 and thus furnished the material for an atmosphere. The 

 gases issuing from volcanoes to-day illustrate this process. 

 So, also, particles of gas are supposed to have been parts of the 

 nebula and to have been gathered in from the outside. It 

 was therefore only necessary for the earth to acquire suffi- 

 cient force of gravity to hold these gases upon its surface, to 

 enable it to accumulate an atmosphere. At first this prob- 

 ably consisted of such gases as carbon dioxide, nitrogen, and 

 water vapor, as these are the ones chiefly given off by rocks 

 and meteorites. Carbon dioxide, however, is chemically 

 active and must have been partly disposed of by uniting with 

 the rocks, while nitrogen is little disposed to enter into 

 combination and must have constantly accumulated. At 

 the same time certain agencies that decompose water vapor 

 probably gave rise to oxygen. 



As the water vapor accumulated it eventually condensed 

 into rain, and with the rain came the beginning of other 

 geological processes. At first, the water circulating through 

 the porous outer portion of the earth dissolved out minerals 

 in certain places and deposited them elsewhere in the form of 

 cementing material and vein fillings. Later, as the ground 

 water level rose, ponds and lakes appeared in the lowest de- 

 pressions of the earth's surface, for the haphazard infall of the 

 solid planetesimals probably left many irregularities, and in 

 addition there were numerous volcanic craters. As time 

 went on these lakes grew and coalesced into great seas and 



