Ch. XXVI.] AGE OF IIYPOGENE ROCKS. 377 



modified by heat, but which contains casts of shells, and often 

 displays unequivocal marks of being an aggregate of fragments 

 of pre-existing rocks. 



Those geologists who shrink from the theory, that all the 

 hypogene strata, so beautifully compact and crystalline as they 

 are, have once been in the state of the ordinary mud, clay, marl, 

 sand, gravel, limestone, and other deposits now forming be- 

 neath the waters, resort, in their desire to escape from such 

 conclusions, to the hypothesis, that chemical causes once acted 

 with intense energy, and that by their influence more crystalline 

 strata were precipitated ; but this theory appears to us to be as 

 mysterious and unphilosophical as the doctrine of a ( plastic 

 virtue,' introduced by the earlier writers to explain the origin 

 of fossil-shells and bones. 



Relative age of the visible hypogene rocks. We shall now 

 return to the subject already in part alluded to at the close of 

 the last chapter the relative age of the hypogene rocks as 

 compared to the secondary. How far are they entitled in 

 general to the appellation of ' primary,' in the sense of being 

 anterior in age to the period of the carboniferous strata, in 

 which last we include the greywacke and many of the rocks 

 commonly called transition ? It is undoubtedly true that we 

 can rarely point out metamorphic or plutonic rocks which can 

 be proved to have been formed in any secondary or tertiary 

 period. We can, in some instances, demonstrate, as we have 

 already shown, that there are granites of posterior origin to 

 certain secondary strata, and that secondary strata have some- 

 times been converted into the metamorphic. But examples of 

 such phenomena are rare, and their rarity is quite consistent 

 with the theory, that the hypogene formations, both stratified 

 and unstratified, have been always generated in equal quan- 

 tities during periods of equal duration. 



We conceive that the granite and gneiss, formed at periods 

 more recent than the carboniferous era, are still for the most 

 part concealed, and those portions which are visible can rarely 

 be shown, by geological evidence, to have originated during 



