Ch, XXVI.] 
HYPOGENE ROCKS. 
379 
which it penetrated ; but there would rarely be any date to 
show that it might not have been injected at the close of the 
Liassic period, or at some much later era. 
The metamorphic rocks must be the oldest, that is to say, 
they must lie at the bottom of each series of superimposed 
strata, because the influence of the volcanic heat proceeds from 
below upwards ; but the hypogene strata of one country may 
be, and frequently are, of a very different age from those of 
another. The greater part, however, of the visible hypogene 
rocks are, we believe, more ancient than the carboniferous for- 
mations. In the latter, we frequently discover pebbles of hypo- 
gene rocks, namely, granite, gneiss, mica-schist, and clay-slate ; 
and the carboniferous rocks often rest unchanged upon the 
hypogene. According to our views of the operations of earth- 
quakes, we ought not to expect pi utonic and metamorphic rocks 
of the more modern eras to have reached the surface generally, 
for we must imagine many geological periods to elapse before a 
mass which has put on its particular form far below the level of 
the sea, can have been upraised and laid open to view above 
that level. Beds containing marine shells sometimes appear at 
the height of two or three miles in the principal mountain-chains, 
but they always belong to formations of considerable antiquity ; 
still more should we be prepared to find the hypogene rocks 
now in sight to be of high relative antiquity, since, in order to 
be brought up to view, they must probably have risen from a 
position far inferior to the bottom of the ocean. 
We shall endeavour to elucidate the cause of the great age 
of the plutonic and metamorphic rocks, now in sight, by a 
familiar illustration. Suppose two months to be the usual 
time required for passing from some tropical country to our 
island, and that an annual importation takes place of a certain 
tropical species of insect, the ordinary term of whose life is two 
months, and which can only be reared in the climate of that 
equatorial country. It is evident that no living individuals 
could ever be seen in England except in extreme old age. The 
young may come annually into the world in great numbers, 
