Ch. XXV.] 
TRAP ROCKS. 
359 
newer than a secondary formation containing belemnites, but 
we can form no conjecture when it originated, not even whether 
it be of secondary or tertiary date. It is, indeed, very 
necessary to be on our guard against the inference that a granite 
is usually of about the same age as the group of strata into 
which it has intruded itself, for in that case we shall be inclined 
to assume rashly that the granites found penetrating a more 
modem secondary rock, such as the lias for example, are much 
newer than those found invading strata older than the car- 
boniferous series. The contrary may often be true, for the 
plutonic rock which was last in a melted state, may not have 
been forced up anywhere so near the surface as to enter into 
the newer groups of strata, and it may have been injected 
into a part of the earth's crust formed exclusively of the older 
sedimentary formations. 
'In a deep series of strata,' says Dr. Macculloch, ' the superior 
or distant portions may have been but slightly disturbed, or 
have entirely escaped disturbance, by a granite which has not 
emitted its veins far beyond its immediate boundary. How- 
ever certain, therefore, it may be, that any mass of granite is 
posterior to the gneiss, the micaceous schist, or the argillaceous 
schists, which it traverses, or into which it intrudes, we are 
unable to prove that it is not also posterior to the secondary 
strata that lie above them *.* 
There can be no doubt, however, that some granites are 
more ancient than any of our regular series which we identify 
by organic remains,because there are rounded pebbles of granite, 
as well as gneiss, in the conglomerates of the oldest fossiliferous 
groups. 
Distinction between volcanic and plutonic rocks — Trap. — 
The next point to consider is the distinction between the plu- 
tonic and volcanic rocks. When geologists first began to 
examine attentively the structure of the northern parts of 
Europe, they were almost entirely ignorant of the phenomena 
of existing volcanos, and when they met with basalt and other 
* Syst. of Geo!., vol. i. p*136. 
