158 LYELL'S ELEMENTS OF GEOLOGY. 



Ages of Rocks. 



schists," restricting the name of primary to the older fossilifer- 

 ous or transition strata. 



As there are secondary fossiliferous strata, so we shall find 

 that there are plutonic, volcanic, and metamorphic rocks of con- 

 temporaneous origin, which I shall also term secondary. 



In the next chapter it will be shown that the strata above the 

 chalk have been called tertiary. If, therefore, we discover any 

 volcanic, plutonic, or metamorphic rocks, which have originated 

 since the deposition of the chalk, these also will rank as tertiary 

 formations. 



It may perhaps be suggested that some metamorphic strata, 

 and some granites, may be anterior in date to the oldest of the 

 primary fossiliferous rocks. The opinion is certainly not impro- 

 bable, and will be discussed in future chapters ; but I may here 

 observe, that when we arrange the four classes of rocks in four 

 parallel columns in one table of chronology, it is by no means 

 assumed that these columns are all of equal length ; one may 

 begin at an earlier period than the rest, and another may come 

 down to a later point of time. In the small part of the globe 

 hitherto examined, it is hardly to be expected that we should 

 have discovered either the oldest or the newest of all the four 

 classes of rocks. Thus, if there be primary, secondary, and 

 tertiary rocks of the fossiliferous class, and in like manner pri- 

 mary, secondary, and tertiary plutonic formations, we may not 

 be yet acquainted with the most ancient of the primary fossili- 

 ferous beds, or with the newest of the plutonic, and so of the 

 rest. 



