136 INTRODUCTION TO EVOLUTION 



fossils; when limestone is subjected to the process described the product is 

 marble. Metamorphism is a most efficient eraser of fossils. 



Of the fossils which escape the destruction just described, how many will 

 be known to geologists a million years or more later? It is evident that de- 

 posits which remain deeply buried under younger strata or under the sea 

 will remain largely unknown. Some exception to this statement is afforded 

 by mines, in the walls of which fossils may be found. Our knowledge of 

 animals contemporaneous with the vegetation which entered into forma- 

 tion of coal depends to considerable extent upon fossils collected in con- 

 nection with the mining of coal. Borings for artesian wells and oil wells re- 

 veal something of fossils in the strata through which they pass. But on the 

 whole locating fossils depends largely upon their being exposed on the sur- 

 face of the earth. Erosion acts on fossil-bearing rocks, wearing away the 

 surrounding rock and exposing the fossil. If the latter happens to fall under 

 the eye of a geologist, well and good, but if not, erosion will eventually de- 

 stroy it along with the enclosing rock. Erosion is continually exposing fos- 

 sils in this way. Most of them are probably never seen by geologists. The 

 earth is a big place, and geologists are few in number and strictly budgeted 

 in time and funds. These same reasons explain why it is not feasible to start 

 digging more or less at random on the chance of uncovering fossils. In some 

 places where fossils are known to be particularly abundant such a process 

 may be practicable, as, for example, in the Rancho La Brea tar pits. But 

 generally, owing to the scattered distribution of fossils, it would be far too 

 costly of time and money. 



While we have by no means enumerated all the reasons why our knowl- 

 edge of past life on the earth is incomplete, we have emphasized some of 

 the main forces operating to deprive us of such knowledge. The wonder is, 

 not that the geologic record is incomplete, but that it is as complete as it is. 



The Geologic Time Scale 



The "chapters" of the geologic record are arranged in chronological se- 

 quence, the result being the geologic time scale (Table 7.1). The time 

 scale is arranged to agree with the fact that, when undisturbed, older 

 strata lie beneath younger ones. Thus we begin to read the chart at the 

 bottom instead of at the top. 



The total span of geologic time is divided into five large divisions, called 

 eras (first column of Table 7.1). The oldest era, Archeozoic, is placed at 

 the bottom of the chart. The boundaries of the successive eras are deter- 



