CHAPTER XV. 



DISTRIBUTION IN TIME. 



ALL the facts which concern the existence of living beings 

 in past periods of the earth's history come under the general 

 head of " Distribution in Time." 



The laws of distribution in time are, however, from the 

 nature of the case, less perfectly known than are the laws 

 of lateral or vertical distribution, since these latter concern 

 beings which we are able to examine directly. The follow- 

 ing are the chief facts which it is necessary for the student 

 to bear in mind : 



1. The rocks which compose the crust of the earth have 

 been formed at successive periods, and may be roughly 

 divided into aqueous or sedimentary rocks, and igneous 

 rocks. 



2. The igneous rocks are produced by the agency of heat, 

 are mostly unstratified (i. e., are not deposited in distinct 

 layers or strata], and, with few exceptions, are destitute of 

 any traces of past life. 



3. The sedimentary or aqueous rocks owe their origin to 

 the action of water, are stratified (i. <?., consist of separate 

 layers or strata), and mostly exhibit " fossils " that is to 

 say, the remains or traces of animals or plants which were 

 in existence at the time when the rocks were deposited. 



4. The series of aqueous rocks is capable of being divided 



