Ch. XXVI.] 
AGE OF HYPOGENE 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 c 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 f 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 
