Cn. XXXIV.] TESTS OF AGE OF PLUTONIC ROCKS. 57B 



CHAPTER XXXIV. 



ON 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 composi- 

 tion — Test by included fragments — Recent and Pliocene pkunic rocks, why 

 invisible — Tertiary plutonic rocks in the Andes — Granite altering Cretaceous 

 rocks — Granite 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 — 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 pre- 

 pared to encounter greater difficulty 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 themselves, or intercalated between strata containing 

 fossils. But all these tests fail when we endeavour to fix the chrono- 

 logy 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 following tests ; 1st, 

 relative position ; 2dly, intrusion, and alteration of the rocks in contact ; 

 3dly, mineral characters ; 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 Newer Pliocene deposits rest on gra- 

 nite ; in Auvergne, where the fresh-water Eocene strata, and at Heidel- 

 berg, on the Rhine, where the New Red sandstone, occupy a similar 

 place. In all these, and similar instances, inferiority in position is con- 

 nected with the superior antiquity of granite. The crystalline rock was 

 solid before the sedimentary beds were superimposed, and the latter 

 usually contain in them rounded pebbles of the subjacent granite. 



Test hy intrusion and alteration. — But when plutonic rocks send 

 veins into strata, and alter .them near the point of contact, in.the manner 

 before described (p. bQl), it is clear that, like intrusive traps, they are 

 newer than the strata which they invade and alter. Examples of the 

 application of this test will be given in the sequel. 



Test hy mineral composition. — Notwithstanding a general uniformity 

 in the aspect of plutonic rocks, we have seen in the last chapter that 



