HISTORY OF THE EARTH 303 



brief, we may learn from the fossils alone something about 

 the geography and the climate of the remote past. 



The succession of faunas. Fossils also enable us to tell 

 the relative age of rocks in different parts of the world ; and 

 herein lies perhaps their greatest value to geology. Since 

 each successive bed of sediment is laid down upon some other 

 which was there before it, it is obvious that, in any exposure 

 of undisturbed rocks, the beds which lie below are older than 

 those above. This rule sometimes fails to hold, as where 

 rocks have been so highly folded that they have been actually 

 bent back upon themselves, but such exceptions may with 

 care be detected in the field. It is evident, therefore, that if 

 we could find in the sides of some deep valley, like that of the 

 Colorado River, a complete pile of the sedimentary strata 

 which have been laid down upon the earth since the dawn of 

 geologic history, we should have a complete stone record of 

 sedimentation. But unfortunately no section even approach- 

 ing this in completeness is known ; we have only fragmentary 

 exposures more or less concealed by soil, forests, and bodies of 

 water. It is here that fossils come to our aid. 



Figure 309 is drawn to represent two bare hills cut through 

 to show the layers of which they are composed. In the hill 

 on the left we 

 find the remains 

 of a certain fauna 

 or society of 

 animals A in the 



lowest stratum a FIG. 309. Section of strata containing fossils of 

 .. , ,. different ages. 



slightly different 



fauna B in the next higher stratum, and so on for each 

 stratum. In studying a number of other outcrops in the 

 same vicinity, we find that the same sequence of faunas pre- 

 vails in all. We thus have a standard section for beds A-E. 

 Now suppose that in the right-hand hill, we find, at the base, 

 rocks which contain fauna D, and above that fauna E, 

 while the higher layers afford new faunas unlike any seen in 



