302 HISTORICAL GEOLOGY 



FOSSILS AND THEIR USES 



Preservation of fossils. The traces of animals and plants 

 preserved in the rocks, and known as fossils, are made in a 

 variety of ways. In rare cases the entire animal is preserved, 

 as when insects are inclosed in resin which afterwards 

 becomes amber. More frequently only the bones or shells 

 are left. Often the shells have been dissolved out and other 

 mineral matter substituted, giving us natural casts of the 

 original. In still other instances we have only the markings 

 made by the living animals, as the burrows of aquatic worms 

 in the sand or the trails of clams in the mud. Whatever the 

 nature of these vestiges of life they are all fossils. 



Manifestly not all animals or plants are allowed to become 

 fossils. The great majority are devoured, while others decay 

 in the open air and disappear. Only those which are pro- 

 tected from the atmosphere are preserved. Complete decay 

 is often prevented if the animal or plant becomes lodged in a 

 bog or lake or beneath the sea. Since the sea is far more 

 extensive than lakes or marshes, it is not surprising that the 

 majority of fossils which have been found are those of 

 marine animals. Correspondingly there is a scarcity of 

 fossil remains of the animals and plants which lived upon the 

 dry land. 



Evidence of past conditions. Since fossils tell us of the 

 living things of bygone ages, obviously they are of interest to 

 the biologist. In geology, however, they have additional uses 

 apart from the study of life itself. For example, we may 

 find in Iowa a bed of limestone which contains abundant fossil 

 corals and other animals of the sea. From this we infer a 

 variety of things about the conditions in the central part of the 

 United States in that remote period when the limestone was 

 formed. Evidently the sea then extended far into the interior 

 of the continent. That its waters were shallow and rela- 

 tively warm is shown by the presence of the kinds of corals 

 which inhabit only the shallow portions of tropical seas. In 



