6 THE HISTORY OF CREATION. 
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 strata, 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 
