CHAPTER XXI 



THE GEOLOGICAL RECORD 



Assuming that the earth began its development as a central 

 core which grew by the accretion of matter from space — and 

 millions of meteors still reach our atmosphere every day — the 

 force of gravity would be sufficient to hold an atmosphere when 

 the sphere reached a diameter of slightly more than three thou- 

 sand miles. As soon as gases and water had collected around the 

 earth, winds and rains would be present, and the erosion of the 

 surface would begin. 



With an increase in diameter there would be an increase of 

 pressure at the center, and heat would be generated, probably 

 sufficient to transform the core into the state of a ''solid gas". 

 If then there were any reduction of pressure, these minerals 

 would become liquid. The crust of the earth is reasonably firm 

 and rigid, but fissures and weak lines develop which permit the 

 potentially molten interior to push upward toward the surface. 

 It follows that volcanic or igneous rocks would then be formed 

 as part of the crust, due to the cooling and crystallization of the 

 laval outcrops. Following the laws of crystallization, the more 

 rapid the cooling process the smaller the crystals, and igneous 

 rocks can be found which form a progressive series from those 

 which resemble glass, to others composed of huge crystals. All 

 volcanic flows do not reach the surface and erupt at a volcanic 

 peak. Some are columnar masses, others are vertical planes which 

 open and fill a crack; but the majority of such flows intrude be- 

 tween layers of rock already formed, and are left as great hori- 

 zontal sheets of igneous rocks. It follows that all types of 

 volcanic intrusion will vary in size from minute lines to great 

 sheets which cover miles of territory. 



The sedimentary rocks are the ones which interest the evolu- 

 tionist. The meteoric rocks were the original mass, and they 



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