HISTORY OF THE EARTH 305 



found. For example, fossil reptiles were abundant in the 

 rocks which represent a certain part of the geologic column 

 (Mesozoic era), but above that section they are rare, while on 

 the other hand the mammals appear in profusion. This point 

 of change made a convenient place to separate one division 

 from another. At first such changes in the fossils were made 

 the sole basis for subdividing the column, but now we are 

 coming to realize that these very transformations among the 

 animals and plants are brought about by corresponding 

 changes in the conditions under which they lived ; or, in other 

 words, by changes in the climate, the topography, the relations 

 of land and sea, etc. So, we now try to divide the geologic 

 record at the points where these revolutionary physical changes 

 are indicated, and to make corresponding divisions of geologic 

 time itself. Thus there are two kinds of divisions ; one for the 

 rocks themselves, and the other for the time represented by 

 the rocks: 



Time Divisions Rock Divisions 



Era Group 



Period System 



Epoch Series 



There is some difference of usage among geologists as to 

 how the geologic column should be divided, particularly as 

 to the rank of certain of the divisions. The classification 

 here adopted, although not to be regarded as permanent, 

 agrees well with the facts now known. As the names of the 

 divisions will be constantly used in later pages, the following 

 table should be learned thoroughly. 1 



1 The names of these time divisions have grown up in a somewhat haphaz- 

 ard way in the course of a century or more of progress in the study of 

 geology. At an early time it was supposed that certain periods were dis- 

 tinguished by the formation of particular rocks, and so we have such names 

 as "Cretaceous" for the period when the chalk (Latin creta) of England was 

 produced. Later it has become customary to name periods after regions 

 in which the rocks of that age are well known ; thus Devonian is named 

 for the county of Devon in England. 



