200 



ROCKS AND SOILS 



metamorphic rocks are also banded, tending to split 

 into layers; in this quality they resemble sedimentary 

 rocks. The table on page 199 names a few meta- 

 morphic rocks and the rocks from which they were 

 formed. Try to find specimens of each and examine 

 them. 



What are fossils? One of the most interesting 

 things about the earth is the life upon it. So far as 

 we know there is no other heavenly body that pos- 

 sesses living inhabitants. It is true that Venus and 



Courtesy American Muscttm of Natural History 

 FIG. 323. A FOSSIL FERN 



Mars seem in some ways possible abodes of some 

 forms of life, but whether they actually do have any 

 life upon them is a matter of speculation. 



Today our earth teems with life. Hundreds of thou- 

 sands of different plants and animals inhabit the air, 

 land, and water. When, where, and how did life get 

 started on our earth? These are questions which 

 scientists are not yet able to answer entirely. 



Scientists believe that the earth is at least two 

 billion years old. They also believe that there was a 

 time when the earth was without life and that the 

 first forms of life that appeared on the earth were 

 simple, lower forms. What is the foundation for these 

 beliefs? Fortunately many of the ancient forms of life 



have left records for us to read. They have done this 

 in several ways. 



In rocks we often find evidence of life of the kind 

 that existed when the rock was being formed. We find 

 imprints or the remains of plants and animals. Some- 

 times we find a part of an animal ; for example, shells 

 and bones. Limestone rock often shows traces of 

 shells of which it is chiefly composed. Sometimes the 

 plant or animal was buried in sediment and there 

 slowly dissolved. Little by little, mineral matter 

 would take the place of the flesh and bone until finally 

 there would seem to be an animal or plant made of 

 stone. Remains of plant life in the form of impres- 

 sions of leaves, stems, or petrified tree trunks are 

 found. It is not unusual to find the imprint of an an- 

 cient fern whose shape shows on a piece of coal that 

 it helped to form. All these remains of ancient plants 

 and animals, including petrified forms and imprints, 

 are called fossils. 



In some parts of the world where it is always cold, 

 especially in Siberia, the frozen bodies of ancient ani- 

 mals have been found. Chief among these is the mam- 

 moth, which was somewhat like an elephant, but more 

 hairy. Being covered with snow and ice, it was pro- 

 tected from flesh-eating animals, and the temperature 

 was low enough to keep it from decaying. 



Many small animals such as insects have been found 

 in amber. Amber is a hardened resin which came from 

 certain trees. Some of the animals found preserved 

 in resin are nowhere on the earth today. 



Near Los Angeles are tar pits, places where a 

 heavy, sticky liquid appears at the surface of the earth. 

 From these pits have been recovered bones of ani- 

 mals that must have lost their lives a long time ago, 



Courtesy American Museum of Natural History 



FIG. 324. A FOSSIL SHELL" 



