CHAPTER XLI. 



GEOLOGY ASTRONOMICAL . 



GEOLOGY is a general science which has the material world for 

 its subject. It attempts to give a history of the earth, to trace 

 its development from a mass of burning gases to a whirling 

 sphere clothed with verdure and teeming with life. From the 

 study of a manufactured article we may learn how it was made 

 and gain some idea of its history. The geologist studies the 

 earth as a planet in its relation to celestial bodies, he studies the 

 rocky pages of the earth's crust, and the data gathered from 

 these sources interpreted according to the principles of physics, 

 and chemistry, botany and zoology enable him to work out a 

 somewhat complete history of the development of the earth. 



The Earth is a member of the solar system which consists of 

 the Sun, Planets, Planetoids or Asteroids, Moons or Satellites, 

 Comets and Meteors. 



The Sun is the central and controlling member of the solar 

 system. It seems to consist of a central nucleus of molten mat- 

 ter bounded by the photosphere, outside of which is an atmos- 

 phere composed largely of the vapors of iron, nickel, manganese, 

 carbon, calcium, sodium, silicon, aluminium, potassium and sev- 

 eral other substances known upon the earth. This envelope is 

 called the chromosphere. Above this layer of vapors, rise flames 

 and prominences composed largely of hydrogen. And beyond 

 these is the solar corona. At times dark spots maybe seen upon 

 the sun that appear to be deep pits in the photosphere ; they vary 

 in size and in duration. The time of the minimum number of 

 spots occurs every eleven years and the time of the maximum 

 number occurs some four or five years afterwards. We know 

 little about the cause of the spots but they seem in some way to 

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