718 KECENT AND PLIOCENE [Oh. XXXIV. 



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 super- 

 imposed, and the latter usually contain in them rounded pebbles of 

 the subjacent granite. 



Test by 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. 709), it is clear that, like intrusive traps, 

 they are newer than the strata which they invade and alter. Exam- 

 ples of the application of this test will be given in the sequel. 



Test by Mineral Composition. — Notwithstanding a general uniform- 

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

 that there are many varieties, such as Syenite, Talcose granite, and 

 others. One of these varieties is sometimes found exclusively pre- 

 vailing throughout an extensive region, where it preserves a homoge- 

 neous character ; so that, having ascertained its relative age in one 

 place, we can easily recognize its identity in others, and thus deter- 

 mine from a single section the chronological relations of large moun- 

 tain 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 other masses of the same zircon-syenite in the south of Norway 

 to the same era. 



Some have imagined that the age of different granites might, to a 

 great extent, be determined by their mineral characters alone ; syen- 

 ite, for instance, or granite with hornblende, being more modern than 

 common or micaceous granite. But modern investigations have 

 proved these generalizations to have been premature. The syenitic 

 granite of Norway already alluded to may be of the same age as the 

 Silurian strata, which it traverses and alters, or may belong to the 

 Old Red Sandstone period ; whereas the granite of Dartmoor, al- 

 though consisting of mica, quartz, and felspar, is newer than the coal. 

 (See p. 725.) 



Test by Included Fragments. — This criterion can rarely be of much 

 importance, because the fragments involved in granite are usually so 

 much altered, that they cannot be referred with certainty to the rocks 

 whence they were derived. In the White Mountains, in North Amer- 

 ica, according to Professor Hubbard, a granite vein, traversing gran- 

 ite, contains fragments of slate and trap which must have fallen into 

 the fissure when the fused materials of the vein were injected from 

 below,* and thus the granite is shown to be newer than certain super- 

 ficial slaty and trappean formations. 



Recent and Pliocene Plutonic PocJcs, why Invisible. — The explana- 

 tions already given in the 29th and in the last chapter of the probable 



* Silliman's Journ., No. 69, p: 123. 



