O THE HISTORY OF CEEATTON, 



of the earth alternate with one another in the course of 

 millions of years, first this and then that part of the earth's 

 surface is above or below the level of the sea. I have 

 already given examples of this in the preceding chapter 

 (vol. i. p. 361). Hence, in all probability, there is no part of 

 the outer crust of the earth which has not been repeatedly 

 above and also below the level of the sea. This repeated 

 change explains the variety and the different composition of 

 the numerous neptunic strata of rocks, which in most places 

 have been deposited one above another in considerable 

 thickness. In the different periods of the earth's history 

 during which these deposits took place there lived various 

 and different populations of animals and plants. When their 

 dead bodies sank to the bottom of the waters, the forms of 

 the bodies impressed themselves upon the soft mud, and 

 imperishable parts, such as hard bones, teeth, shells, etc., 

 became enclosed in it uninjured. These were preserved in 

 the mud, which condensed them into neptunic rock, and as 

 petrifactions they now serve to characterize the respective 

 strata. By a careful comparison of the different strata lying- 

 one above another, and the petrifactions preserved in them, 

 it has become possible to decide the relative age of the 

 strata and groups of sti'ata, and to establish, by direct 

 observation, the principal eras of phylogeny, that is to say, 

 the stages in history of the development of animal and 

 vegetable tribes. 



The different strata of neptunic rocks deposited one above 

 another, which are composed in very various ways of lime- 

 stone, clay, and sand, geologists have grouped together into 

 an ideal System or Series, which corresponds with the whole 

 course of the organic history of the earth, or with that portion 



