PROBLEM A What Can We Learn from Fossils about 



Prehistoric Living Things? 



Reading the history of the earth. If you 



laid tliis book down with its front cover 

 against the table, the first chapter would 

 be at the bottom. The last chapter would 

 be on top. If each chapter had been 

 printed on paper of a different color you 

 would see many distinct layers, some 

 thick, some thin. To geologists the rocks 

 are like an enormous book in which the 

 history of the earth can be read. At the 

 bottom are the rocks laid down first, the 

 oldest rocks. The most recent rocks lie 

 on top. In the rocks, too, there are layers 

 or strata, "chapters" of different colors. 

 In these strata are fossils of animals and 

 plants; and since the lowest rocks are the 

 oldest, they contain fossils of the plants 

 and animals that lived in the earliest days; 

 just above them lie the remains of slightly 

 more recent organisms. In the uppermost 

 strata lie the remains of the most recent 

 plants and animals. Thus there is a suc- 

 cession of fossils in the rocks just as there 

 is a succession of ideas in a book. See 

 Exercise i. 



To be sure, the rock strata are harder 

 to read than the pages of a printed book 

 because in places they are torn and 

 twisted so that pieces of the lowest, or 

 oldest, strata now lie at the surface of the 

 earth, or on edge, or on top of younger 

 rocks. The book has been mutilated to 

 such an extent that at times it is difficult 

 to know which are the earliest chapters. 

 How^ever, through long study of the 

 rock and the fossils, geologists have 

 learned to read the book quite accurately. 

 They have worked out the sequence of 

 fossils and have learned much about the 

 different eras and periods of the earth's 

 history. Let us examine the geologists' 

 book. We shall find the following chap- 

 ters listed in the "table of contents of the 

 Book of the Rocks." But we must remem- 

 ber to read from the bottom up. 



Would you be interested in a rapid 

 glance at the earth as we think it must 

 have looked in these various eras? As 

 you read you might illustrate what you 

 read, as suggested in Exercise 2. 



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