302 GENERAL ZOOLOGY 



have a good working knowledge of geology in order to arrive at a 

 correct estimate of the age of any given fossil-bearing geological 

 stratum. Fossils are found in what is known as sedimentary rock, 

 produced by the solidification of fine particles eroded from the 

 earth's surface and eventually washed into lakes, streams, and 

 oceans. It is possible to measure the amount of erosion at the 

 present time over a given surface of land, from which the time 

 required to form a layer of a certain thickness can be estimated. 

 Thus the thickness of a geological formation gives an indication 

 of the approximate time required for its formation. If all of the 

 different kinds of strata occurring in all parts of the world could 

 be piled on top of one another, the total thickness of the super- 

 imposed layers would be about 100 miles. Knowing the approxi- 

 mate rate of formation of the series, the total time for its forma- 

 tion may be computed. All of the components of the geological 

 series are not found in one place because more recent rocks are 

 formed in valleys from material eroded from materials of more 

 ancient layers, which at the time constituted the hills and moun- 

 tains. When all of the strata of sedimentary rocks are arranged 

 in order of their formation, the age of contained fossil remains 

 must correspond to the age of the strata in which they are found. 

 Another method of determining the age of the earth depends 

 upon measuring the rate of the decomposition of radioactive 

 ores contained in igneous rocks, from which the amount of time 

 required to form the decomposition products occurring in a given 

 sample can be estimated. Igneous rocks have been formed at 

 various times in the history of the earth, but since they contain 

 no fossils, their age in terms of geological eras and periods must 

 be determined largely from their relations to fossiliferous strata. 

 The total elapsed time since the beginning of the formation of 

 sedimentary rocks, based on a combination of purely geological 

 data and measurements of the decomposition of radioactive 

 ores, is about 1,800 million years. 



A plan of the distribution of life in time is shown in Table 1, 

 in which the latest time is at the top and the earliest at 

 the bottom. "Eras" are intervals separated from one another 

 by marked changes in the earth's plant and animal life. "Peri- 

 ods" are distinguished by geological changes principally. 

 "Epochs," which are subdivisions of periods, are not shown. 

 The figure opposite each era is the duration of time for the era 



