CHAPTER IV 

 GEOLOGICAL HISTORY OF THE EARTH 



Plants and animals have developed from lower forms. 



The generally accepted theory is that at one time the 

 earth's surface was of solid rock, but the action of wind, 

 water, glaciers, earthquakes, heat, cold and other forces, 

 during many ages, has broken up this surface into 

 smaller particles which form the larger part of our soil 

 as we see it today. 



Some of the lower forms of plant life which live 

 largely from the elements of the air, at first grew upon 

 the rocky surfaces and on the pulverized surface ma- 

 terials, and gradually deposited small amounts of organic 

 matter which, together with the disintegrated soil par- 

 ticles, formed rich soils on which higher plants and 

 animals could grow. As the ages passed a large growth 

 of plants developed on the surface, and the organized 

 matter of decaying plants, mixed with fine particles of 

 earth, helped make a productive soil. 



In many places the rains washed these plant and 

 animal remains into streams, and, together with par- 

 ticles of soil, they were deposited along the river beds 

 or deltas in layers. Where earthquakes, the washing 

 of water or other influences have operated to expose the 

 edges of these layers, the geologist has studied and 

 unraveled the history of the layers of earth. Sur- 

 face layers which have been formed in compar- 

 atively recent times are found to contain the 

 fossil forms of the plants and animals now living on the 

 earth. As he goes deeper among the older deposits, the 

 geologist finds many lower forms of animals and plants, 

 of some of which no living representatives have ever 



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