618 General and Applied Biology 



Fossils of certain characteristics are included in certain strata of the 

 earth and give clues as to the geologic age of these strata. This is im- 

 portant in knowing when those particular sediments which formed these 

 strata were laid down. Hence, certain plant and animal fossils are 

 known as index fossils because through them it is possible to determine 

 particular geologic eras and periods. Fossils also demonstrate that life 

 has not existed without changes in the past because of the revelations of 

 the records of past plants and animals. 



V. GEOLOGIC TIME CHART 



Because of the extensive studies of the strata of the earth, geologists 

 have divided the earth's history into eras (Figs. 320 to 322). Each era 

 has been divided into periods and the periods subdivided into epochs. 

 Each of these eras, periods, and epochs has specific characteristics and 

 definite ages and durations, as well as certain types of life which were 

 dominant during that particular time. The most recent fossils are found 

 in the upper strata, and the more ancient are successively arranged be- 

 low, with the most ancient at the bottom. We find the fossil remains of 

 plant and animal organisms distributed in this order in the strata of the 

 earth. 



The reader is probably wondering how the relative and actual lengths 

 of the eras and periods have been calculated. This can be accomplished 

 in two ways. The age can be approximated by the thickness of the sedi- 

 mentary rocks formed during each period. It is known that a definite 

 time is required to form a certain thickness of sedimentary rock of a cer- 

 tain type. From these data it can be estimated how long it would re- 

 quire a certain thickness to be formed. Another method of ascertaining 

 the age of various strata is by the radioactive disintegration method. 

 The radioactive elements, uranium and thorium, disintegrate sponta- 

 neously at constant, determined rates with the formation of lead. The 

 age of a uranium mineral thus can be calculated from the proportions 

 of uranium and the lead it contains. Determinations of the uranium- 

 lead content of the oldest rocks suggest that the age of the earth is ap- 

 proximately 2,000,000,000 years. An accurate analysis of minerals re- 

 veals the fact that the Paleozoic era began over 500,000,000 years ago. 

 Figures secured by this method correspond with similar figures secured 

 by estimating the amount of time required for such sedimentary rocks 

 to be formed. The characteristics of the various periods of the Cenozoic, 

 Mesozoic, Paleozoic, Proterozoic, and Archeozoic eras, as well as domi- 

 nant organisms of each era, are shown in Figs. 320 to 322. 



