380 
RE L AWE AGE OF 
[Ch. XXVI. 
but ill order to see them, we must travel to lands near the 
equator. 
In like manner, if the hypogene rocks can only originate at 
great depths in the regions of subterranean heat, and if it 
requires many geological epochs to raise them to the surface, 
they must be very ancient before they make their appearance 
in the superficial parts of the earth's crust. They may still be 
forming in every century, and they may have been produced in 
equal quantities during each successive geological period of equal 
duration ; but in order to see them in a nascent state, slowly 
consolidating from a state of fusion, or semi-fusion, Ave must 
descend into the 1 fuelled entrails ' of the earth, into the regions 
described by the poets, where for ages the land has 
ever buru'd 
With solid, as the lake with liquid fire. 
As the progress of decay and reproduction by aqueous agency 
is incessant on the surface of the continents, and in the bed of 
the ocean, while the hypogene rocks are generated below, or 
are rising gradually from the volcanic foci, thus there must 
ever be a remodelling of the earth's surface in the time 
intermediate between the origin of each set of plutonic and 
metamorphic rocks, and the protrusion of the same into the 
atmosphere or the ocean. Suppose the principal source of the 
Etnean lavas to lie at the depth of ten miles, we may easily 
conceive that before they can be uplifted to the day several 
distinct series of earthquakes must occur, and between each of 
these there might usually be one or more periods of tranquillity. 
The time required for so great a development of subterranean 
elevatory movements might well be protracted until the depo- 
sition of a series of sedimentary rocks, equal in extent to all 
our secondary and tertiary formations, had taken place. We 
conceive, therefore, that the relative age of the visible plutonic 
and metamorphic rocks, as compared to the unaltered sedimen- 
tary strata, must always be determined by the relations of two 
forces,— the power which uplifts the hypogene rocks, and that 
aqueous agency which degrades and renovates the earth's 
