GEOLOGY 



2433 



GEOLOGY 



Vegetation Of Carboniferous Period 



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V^JEOLOGY. jeol'oji, the story of the 

 earth from its beginning through the millions 

 of years or ages that no man can number, 

 down to the present time. 



The Story of a Shell. One spring day Tom 

 brought his teacher a piece of rock in which 

 there was a shell almost exactly like shells that 

 he had found on the seashore when he was in 

 Florida. The shell had turned to stone but its 

 form had not been changed; even the smallest 

 markings on it could be distinctly seen. Tom 

 lived in a state far from the sea; moreover, he 

 had broken this fragment of rock from a ledge 

 on a high hill, and he said that the ledge was 

 full of these shells. The first question he 

 naturally asked was, "How did these shells get 

 where I found them?" 



Since all the pupils were interested in the 

 question, the teacher told them he would help 

 them to answer it, by showing them how to 

 read the oldest, the largest and most peculiar 

 book that man knows anything about, the 

 book of the earth. 



The Old Book. We usually think of the 

 Bible as the oldest book, and so it is if we 

 consider only books with paper leaves on which 

 words have been printed; but the book which 



this wise teacher taught his pupils to read so 

 they could answer Tom's question was made 

 long before there were any men to read it. 

 It is The Earth, truly a huge and heavy vol- 

 ume. The rocks are its leaves, and the fossils 

 of the past ages, together with the plants and 

 animals of the present time, are its letters and 

 figures. The leaves are sometimes misplaced 

 and now and then we find that one has been 

 torn out, so in some places the record is not 

 complete. But it is a wonderful story. 



This book contains the history of the earth 

 from its beginning to the present time. It 

 tells us something about how the earth was 

 formed, of the different changes it has under- 

 gone, the different kinds of rocks in its crust 

 and of the many strange plants and animals 

 that lived in the long, long ago. The story 

 this book tells the scientists call Geology, for 

 Gea was the old mythological name for the 

 earth, but Tom's teacher called it The Story 

 of the Earth; both mean the same thing to 

 us. We shall read this story more easily if 

 we begin at the present time; from observing 

 the forces now at work changing the surface 

 of the earth we can gain an understanding of 

 how these same forces worked in the past ages. 



The Three Spheres 



Earth, Air and Water. The ancients be- 

 lieved that there were four substances from 

 which all things were created ; these were earth, 

 air, fire and water. We have long since learned 

 that fire is not a substance, but we know that 

 the other three form the earth. The geolo- 

 gists consider these three substances as three 

 spheres the atmosphere (air sphere), the 

 hydrosphere (water sphere), and the litho- 

 sphere (rock sphere). 



The Atmosphere. We live at the bottom of 

 a great ocean of air which extends upward to 

 an unknown height possibly 200 miles. At 

 sea level it presses down upon us with the 

 force of almost fifteen pounds to the square 

 inch, and were this pressure materially changed 

 we should become very uncomfortable. The 

 pressure decreases as one rises to heights above 

 153 



sea level ; the density is so much less even four 

 miles upward that few people can survive an 

 ascent of that distance. The air is composed 

 of numerous substances mixed together; the 

 most important of these are oxygen, nitrogen, 

 carbonic acid gas and water vapor (see AIR). 

 The atmosphere is and always has been an 

 important agent in changing the surface of the 

 solid parts of the earth. It does this chiefly 

 through weathering, winds and rainfall. 



Weathering. The atmosphere covers the 

 earth like a mantle. By absorbing the heat of 

 the sun's rays and reducing their intensity it 

 shields the land and water from the intense 

 heat which would otherwise fall upon them. 

 It also prevents the absorbed heat from escap- 

 ing into space when the sun is not shining. 

 Thus the atmosphere is the great equalizer of 



