GEANITIC EOCKS. 
563 
tonic rock, it is only in proportion as that rock begins to ac- 
quire a trappean aspect. 
It has been already hinted that the heat which in every 
active volcano extends downward to indefinite depths must 
produce simultaneously very difierent effects near the sur¬ 
face and far below it; and we can not suppose that rocks re¬ 
sulting from the crystallizing of fused matter under a press¬ 
ure of several thousand feet, much less several miles, of the 
earth’s crust can exactly resemble those formed at or near 
the surface. Hence the production at great depths of a class 
of rocks analogous to the volcanic, and yet differing in many 
particulars, might have been predicted, even had we no plu- 
tonic formations to account for. How well these agree, both 
in their positive and negative characters, with the theory of 
their deep subterranean origin, the student will be able to 
judge by considering the descriptions already given. 
It has, however, been objected, that if the granitic and vol¬ 
canic rocks were simply different parts of one great series, 
we ought to find in mountain chains volcanic dikes passing 
upward into lava and downward into granite. But we may 
answer that our vertical sections are usually of small extent; 
and if we find in certain places a transition from trap to po¬ 
rous lava, and in others a passage from granite to trap, it is 
as much as could be expected of this evidence. 
The prodigious extent of denudation which has been al¬ 
ready demonstrated to have occurred at former periods, will 
reconcile the student to the belief that crystalline rocks of 
high antiquity, although deep in the earth’s crust when orig¬ 
inally formed, may have become uncovered and exposed at 
the surface. Their actual elevation above the sea may be 
referred to the same causes to which we have attributed the 
upheaval of marine strata, even to the summits of some 
mountain chains. 
