282 



LYELL'S ELEMENTS OF GEOLOGY. 



Tests of Relative Age of Plutonic Rocks. 



whether in the sea or in the atmosphere, or from tuffs and con- 

 glomerates, also deposited at the surface, and either containing 

 organic remains themselves, or intercalated between strata con- 

 taining fossils. But all these tests fail when we endeavour 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 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 plu- 

 tonic rocks, as at Christiania, in Norway, where the Newer 

 Pliocene deposits rest on granite ; in Auvergne, where the fresh- 

 water Eocene 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 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. 123.), it is clear that, like in- 

 trusive 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 

 9th chapter that there are many varieties, such as Syenite, .Tal- 

 cose granite, and others. One of these varieties is sometimes 

 found exclusively prevailing throughout an extensive region, 

 where it preserves a homogeneous character; so that having 

 ascertained its relative age in one place, we can easily recognize 

 its identity in others, and thus determine from a single section 

 the chronological relations of large mountain masses. Having 

 observed, for example, that the syenitic granite of Norway, in 

 which the mineral called zircon abounds, has altered the Silurian 

 strata wherever it is in contact, we do not hesitate to refer all 

 masses of the same zircon-syenite in the south of Norway to the 

 same era. (See p. 143.) 



Some have imagined that the age of different granites might, 

 to a great extent, be determined by their mineral characters 

 alone ; syenite, for instance, or granite with hornblende, being 

 more modern than common or micaceous granite. But modern 

 investigations have proved these generalizations to have been 



