278 THE HISTORY OF 



as now appear no longer possible upon our earth. The 

 ocean was probably the birth-place of these organisms, 

 and their forms as yet very simple. The dead organisms 

 falling to the bottom of the sea, were buried, and either 

 wholly or in their more solid parts (shells and bones) pre- 

 served at least their external form, even though the 

 organic substance was in great part decomposed and, 

 frequently, replaced by inorganic matters which penetrated, 

 causing what is called petrefaction. From what has 

 already been told of the history of the formation of the 

 rocks, it follows that such petrefaction can only occur in 

 the stratified rocks. In subsequent periods, organisms 

 also originated upon the dry land, and the remains of 

 these passed as petrefactions into the rocks, and this in 

 two ways either the bodies were carried into the ocean 

 by floods and the larger streams, or the whole surface of 

 earth on which they lived, sank down in the way above 

 described, beneath the surface of the sea, and thus they 

 became buried in vast masses beneath the deposits from 

 the water. 



The careful study of the rock-systems and masses, and 

 petrefactions (fossils), has enabled us now to divide the 

 gradual formation of the earth into determinate periods ; 

 not, indeed, limited according to time, but according to 

 their products ; and these products are called rock-forma- 

 tions, which, arranged in definite series, have such relation 

 to one another, that no formation belonging lower down in 

 the series is ever found deposited upon one higher up in 

 the list, so that we can assume with safety, that they have 

 been formed one after another in this order. Several of 

 these formations are then collected together, so as to form 

 greater periods of formation, as it were stages of the 



