222 ROCKS AND SOIL 



National Park Service 



A fossil tree of the Arizona desert. These trees are found by the hundreds in some 

 parts of the west, in some cases having been changed to agate or other semi- 

 precious stones. This one is now held up by a concrete base. 



figuring the amount of time it took to carry salt to the 

 oceans to give them their present saltiness. This estimate 

 is about 500,000,000 years. Another and newer estimate 

 has just been completed by a group of scientists appointed 

 by the National Research Council in Washington, and 

 they, basing their calculations on radio activity of certain 

 rocks, have estimated the age of the earth at the incredible 

 figure of over 2,000,000,000 years. 



Of course life did not exist on the earth at first, and 

 nobody knows how the first life came. But we do get this 

 much evidence from the fossils. The very oldest igneous 

 rocks, which we have learned were formed when the earth 

 was very young, do not contain any fossils. The earliest 

 evidences of life come from bacteria, and following them we 

 find tiny plants and animals, all of which lived in water. 



What Fossils Tell. The character of the fossil tells 

 whether it was deposited in salt sea water or fresh lake 



