Ch. IV.] RELATIVE AGES OF ROCKS. 37 



tion of another in the immediate neighbourhood, and may be 

 observed to include within it fragments of such older rocks. 

 Thus, for example, we may find chalk with flints, and in 

 another part of the same country, a distinct series, consisting of 

 alternations of clay, sand, and pebbles. If some of these peb- 

 bles consist of flints, with silicified fossil shells of the same 

 species as those in the chalk, we may confidently infer, that the 

 chalk is the oldest of the two formations. 



We remarked in the second chapter, that some granite must 

 have existed before the most ancient of our secondary rocks, 

 because some of the latter contain rounded pebbles of granite. 

 But for the existence of such evidence, we might not have felt 

 assured that all the granite which we see had not been pro- 

 truded from below in a state of fusion, subsequently to the 

 origin of the secondary strata. 



Proofs of contemporaneous origin derived from mineral 

 characters. 



When we have established the relative age of two forma- 

 tions in a given place, by direct superposition, or by other 

 evidence, a far more difficult task remains, to trace the conti- 

 nuity of the same formation, or, in other cases, to find means 

 of referring detached groups of rocks to a contemporaneous 

 origin. Such identifications in age are chiefly derivable from 

 two sources mineral character and organic contents; but the 

 utmost skill and caution are required in the application of 

 such tests, for scarcely any general rules can be laid down re- 

 specting either, that do not admit of important exceptions. 



If, at certain periods of the past, rocks of peculiar mineral 

 composition had been precipitated simultaneously upon the 

 floor of an ' universal ocean,' so as to invest the whole earth in a 

 succession of concentric coats, the determination of relative 

 dates in geology might have been a matter of the greatest sim- 

 plicity. To explain, indeed, the phenomenon would have 

 been difficult, or rather impossible, as such appearances would 

 have implied a former state of the globe, without any analogy 



