188 FIRST YEAR COURSE IN GENERAL SCIENCE 



region must be ascer- 

 tained by knowledge of 

 the rock material and 

 the agencies at work. 

 The Appalachian Moun- 

 tain system, about one 

 hundred miles wide, was 

 formed by the folding of 

 strata of f ragmental rock 

 many thousands of feet 

 thick. The accumula- 

 tion of this material 

 under the water is esti- 

 mated to have taken at 

 least thirty-six millions 

 of years. Since that 

 was done, it is thought 

 that one third as much 

 time has elapsed. Scien- 

 tists conclude, from this 

 estimate alone, that the 

 earth must be many 

 millions of years old. 



Another method of 

 judging the age of the 

 earth is by the study of 

 fossils. A fossil is the 

 remains or impression of 

 a plant or animal that 

 was buried in mud or 

 sand. The mud or sand 

 afterward became rock 

 and thus preserved the 

 hard parts of the body. 

 The soft parts decomposed and passed off as gases or liq- 



FIG. 97. FOSSIL FORE LIMB OF AN 

 ANCIENT REPTILE 



A "fossil hunter" has been putting 

 plaster of Paris around the bones and in 

 the cracks of this valuable specimen so that 

 it can be safely shipped to some museum. 

 1. Compare the size of the man's hand and 

 the foot. 2. Compare the length of his 

 arm from wrist to shoulder with the fossil 

 bones between the foot and the upper end. 



