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 

 modern 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 Geol.,vol.i.p.;i36, 



