CHAPTER III 

 ORIGIN AND PRE-GEOLOGIC HISTORY OF THE EARTH 



If we define geology as the study of the history of the earth 

 and its inhabitants as revealed in the rocks, it is evident that the 

 problems of the origin and very early development of the earth are 

 strictly astronomic rather than geologic. It is generally agreed 

 that geological history did not begin till the ordinary earth proc- 

 esses, such as weathering and erosion, transportation and depo- 

 sition of sediments, etc., began to operate. Since, however, the 

 pre-geologic condition of the earth must have gradually given way 

 to its geologic condition, it is a matter of interest for the geologist 

 to consider the hypotheses regarding the very early development 

 of the earth. 



The Solar System 



The sun has a diameter of about 866,000 miles, and a volume 

 1,300,000 times that of the earth. Around this central sun eight 

 planets — Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, 

 and Neptune — revolve in nearly circular orbits. Three of these 

 planets — Mercury, Venus, and Mars — are smaller than the earth, 

 while the others are larger, Jupiter being 1,300 times as large. 

 The earth is about 93,000,000 miles from the sun and requires one 

 year for a trip in its orbit around the sun, while Neptune, the most 

 distant planet, is about 2,800,000,000 miles from the sun and re- 

 quires 164 years for a revolution about the sun. Each planet also 

 rotates upon its axis, the earth accomplishing a rotation every 

 twenty-four hours. Most of the planets have smaller bodies called 

 satellites or moons revolving about them, such as Earth with its 

 one moon, Saturn with eight moons, etc. The sun and the eight 

 planets with their satellites, together with a group of many small 

 independently revolving bodies called " planetoids," comprise the 

 solar system. That this solar system constitutes only a very small 

 part of the universe is clearly proved by the fact that the nearest 

 fixed star is several trillions of miles from the earth. 



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