Ch. fv.] 
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 
