364 
AGE OF PLUTONIC ROCKS. 
[Cli. XXV. 
contrary, that it has been heated, and still continues for cen- 
turies, or thousands of years, at a red heat, the vein may 
acquire a highly crystalline texture. 
The great pressure of a superincumbent mass, and exclusion 
from contact with air or water, are probably the usual con- 
ditions necessary to produce the granitic texture; but the 
same may sometimes be superinduced at a slighter distance 
from the surface by slow refrigeration, when additional sup- 
plies of heat check, from time to time, the cooling process and 
cause it to be indefinitely protracted. 
If, for the reasons above alluded to, we conceive it probable 
that plutonic rocks have originated in the nether parts of the 
earth's crust, as often as the volcanic have been generated at 
the surface, we may imagine that no small quantity of the 
former class has been forming in the recent epoch, since we 
suppose that about 2000 volcanic eruptions may occur in the 
course of every century, either above the waters of the sea or 
beneath them *. 
We may also infer, that during each preceding period, 
whether tertiary or secondary, there have been granites and 
granitiform rocks generated, because we have already dis- 
covered the monuments of ancient volcanic eruptions at almost 
every period. 
In the next chapter we shall endeavour to show, that in 
consequence of the great depths at which the plutonic rocks 
usually originate, and the manner in which they are associated 
with the older sedimentary strata of each district, it is rarely 
possible to determine with exactness their relative age. Yet 
there is reason to believe that the greater portion of the plutonic 
formations now visible are of higher antiquity than the oldest 
secondary strata. We shall also endeavour to point out, that 
this opinion is by no means inconsistent with the theory that 
equal quantities of granite may have been produced in succes- 
sion, during equal periods of time, from the earliest to the most 
modern epochs. 
* See vol. i. chap. xxii. 
