Ch. XXXIY.] TESTS OF AGE OF PLUTONIC ROCKS. 71/7 



student to the belief that crystalline rocks of high antiquity, al- 

 though deep in the earth's crust when originally formed, may have 

 become uncovered and exposed at the surface. Their actual ele- 

 vation above the sea may be referred to the same causes to which 

 we have attributed the upheaval of marine strata, even to the sum- 

 mits of some mountain chains. But to these and other topics I 

 shall revert when speaking, in the next chapter, of the relative ages 

 of different masses of granite. 



CHAPTER XXXIY. 



ON THE DIFFERENT AOES 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 plutonic rocks, why invisible — Ter- 

 tiary plutonic rocks in the Andes — Granite altering Cretaceous rocks — Granite 

 altering Lias in the Alps and in Skye — Granite of Dartmoor altering Carbonifer- 

 ous strata — Granite of the Old Red Sandstone period — Syenite altering Silurian 

 strata in Xorway — Blending of the same with gneiss — Most ancient plutonic 

 rocks — Granite protruded in a solid form — On the probable age of the granites 

 of Arran, in Scotland. 



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 eucounter greater difficulty in ascertaining the precise 

 age of such rocks, than in the case of volcanic and fossiliferous for- 

 mations. "We must bear in mind, that the evideuce of the age of 

 each contemporaneous volcanic rock was derived, either from lavas 

 poured out upon the ancient surface, whether iu the sea or in the 

 atmosphere, or from tuffs and conglomerates, also deposited at the 

 surface, and either containing organic remains themselves, or inter- 

 calated between strata containing fossils. But all these tests fail 

 when we endeavor to fix the chronology of a rock which has crystal- 

 lized from a state of fusion in the bowels of the earth. In that case, 

 we are reduced to the following tests : 1st, relative position ; 2dly, 

 intrusion, and alteration of the rocks in contact ; 3 dry, mineral char- 

 acters ; 4thly, included fragments. 



Test of Age hy Relative Position. — Unaltered fossiliferous strata of 

 every age are met with reposing immediately on plutonic rocks ; as 

 at Christiania, in Norway, where the Post-pliocene deposit rests on 

 granite ; in Auvergne, where the freshwater Miocene strata, and at 



