FOSSIL ANIMALS 329 



the bottom of the overriding portion, an older stratum is above a younger 

 one. Often this disturbance is readily recognized, but not always. 



Considerable help in recognizing disturbed strata is given by the 

 fossils they contain. While the earth's crust was changing, plants and 

 animals were also evolving. Animals of one period were distinctly 

 different from those of another. So characteristic of a given period 

 are certain kinds of animals that the fossils are known as index fossils. 

 Good index fossils must be abundant and widely distributed over the 

 earth, and large enough not to be overlooked. Occurrence of an index 

 fossil in a stratum at one place is not, however, a complete guarantee 

 that any other stratum containing such fossils was contemporaneous 

 with the first. These animals had to have a certain type of environ- 

 ment, and there are reasons to believe that similar environments occurred 

 in different areas at different times in the earth's history. For example, 

 the "red beds," made red by the oxidation of iron under certain climatic 

 conditions, occur in the Conemaugh formation in Pennsylvania and 

 West Virginia, and in the Wichita formation of mid-continental United 

 States; but according to other evidence the Wichita is much younger 

 than the Conemaugh. 



While there are other ways of correlating rock strata of different 

 regions, the changes in types of animals occurring simultaneously 

 with changes in the earth are among the most reliable of the means 

 of identification. 



Divisions of Geological History. — Geologists use a classification of 

 the earth's history which serves much the same purpose as does taxonomy 

 for zoologists. The classification is known as the geological time scale. 

 Major revolutions of the earth's crust caused elevation of great mountain 

 systems, erosion on a grand and extremely rapid scale, and redeposit of 

 the eroded material elseAvhere. As a result of these great changes, layers 

 of the earth's crust having very different characteristics and containing 

 very different fossils lie next to one another. These contrasts, known 

 as unconformities, are used to divide geological time into five great eras. 

 Within each of these eras the land of continents sank in large areas 

 so that the sea invaded the land, then rose again to push the oceans back. 

 On the basis of such changes, each era is divided into periods. Minor 

 and local changes of the same general type are used to divide the periods 

 into epochs. 



All the rocks belonging to a period constitute a system, those of an 

 epoch make a series, while smaller divisions than the epochs have their 

 rock formations. These terms are not generally used in this book, but 

 are constantly met in geological works. 



The accompanying table gives the geological time scale as far as the 

 terms are needed in an elementary study of biology. 



