564 
ELEMENTS OF GEOLOGY. 
CHAPTER XXXII. 
OIS" THE DIFFERENT AGES OF THE PLUTONIC ROCKS. 
Difficulty in ascertaining the precise Age of a Plutonic Rock.—Test of Age 
by Relative Position.—Test by Intrusion and Alteration.—Test by Mineral 
Composition.—Test by included Fragments.—Recent and Pliocene Plu¬ 
tonic Rocks, why invisible.—Miocene Syenite of the Isle of Skye.—Eocene 
Plutonic Rocks in the Andes.—Granite altering Cretaceous Rocks.—Gran¬ 
ite altering Lias in the Alps and in Skye.—Granite of Dartmoor altering 
Carboniferous Strata.—Granite of the Old Red Sandstone Period.—Syenite 
altering Silurian Strata in Norway.—Blending of the same with Gneiss.— 
Most ancient Plutonic Rocks.—Granite protruded in a solid Form. 
When we adopt the igneous theory of granite, as explained 
in the last chapter, and believe that different plutonic rocks 
have originated at successive periods beneath the surface of 
the planet, we must be prepared to encounter greater diffi¬ 
culty in ascertaining the precise age of such rocks than in 
the case of volcanic and fossiliferous formations. We must 
bear in mind that the evidence of the age of each contempo¬ 
raneous volcanic rock was derived either from lavas poured 
out upon the ancient surface, whether in the sea or in the 
atmosphere, or from tuffs and conglomerates, also deposited 
at the surface, and either containing organic remains them¬ 
selves or intercalated between strata containing fossils. But 
the same tests entirely fail, or are only applicable in a modi¬ 
fied, degree, when we endeavor to fix the chronology of a 
rock which has crystallized from a state of fusion in the 
bowels of the earth. In that case we are reduced to the 
tests of relative position, intrusion, alteration of the rocks in 
contact, included fragments, and mineral character; but all 
these may yield at best a somewhat ambiguous result. 
Test of Age by Eelative Position. —Unaltered fossiliferous 
strata of every age are met with reposing immediately on 
plutonic rocks; as at Christiania, in Norway, where the 
Post-pliocene deposits rest on granite; in Auvergne, where 
the fresh-water Miocene strata, and at Heidelberg, on the 
Rhine, where the New Red sandstone occupy a similar place. 
In all these, and similar instances, inferiority in position is 
connected with the superior antiquity of granite. The crys¬ 
talline rock was solid before the sedimentary beds were su¬ 
perimposed, and the latter usually contain in them rounded 
pebbles of the subjacent granite. 
