Fossils as Guides to the Age of Rockj 29 



The difficulty of explaining the presence of sea shells and 

 remains of crabs and other marine forms among the rocks 

 of a mountain he met with the simple assumption that these 

 rocks were at one time covered by water. That in fact is 

 our understanding today. 



There is abundant evidence that land levels are gradually 

 being raised, even out of the sea. There is abundant evidence 

 that areas now covered with water were at one time high and 

 dry. Fossils are found in sedimentary rocks for common- 

 sense reasons. Rocks that have been molten must have had 

 destroyed in them every vestige of living remains they may 

 have contained. Rocks that have been formed by the grind- 

 ing away of older rocks must have had destroyed in them 

 the remains of living bodies they may have contained. It is 

 only the intact strata of former sea bottoms or lake bottoms 

 that are likely to contain any considerable amount of plant 

 or animal remains. That is what we find to be the fact. 



Fossils as Guides to the Age of Rocks 



From William Smith's observations geologists obtained 

 a clue to the relative age of rocks, when comparing forma- 

 tions in different regions with one another. Rocks contain- 

 ing large proportions of Trilobites (Fig. 3), for example, 

 were older than rocks containing remains of fishes. Coal 

 strata, containing remains of giant club mosses and tree ferns, 

 were older than strata containing fossils of the giant reptiles, 

 and so on. This idea makes possible the useful comparison 

 of series of strata in different parts of the world, and has 

 found its practical application in the search for coal, oil, 

 and other minerals. It cannot, however, be applied without 

 reservation to all regions as a means of determining the ap- 

 proximate age of a given formation. It is conceivable, for 

 example, that although the order in which different types 

 of fossils appeared is the same in all parts of the world, the 

 absolute time at which a given series started was not the 

 same. Thus, coal beds in England, in Indiana and in Ger- 



