TEST OF AGE OF PLUTONIC ROCKS. 
565 
Test by Intrusion and Alteration. —But when plutonic rocks 
send veins into strata, and alter them near the point of con¬ 
tact, in the manner before described (p. 559), 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 by Mineral Composition. —Notwithstanding a general 
uniformity in the aspect of plutonic rocks, we have seen in 
the last chapter that there are many varieties, such as sy¬ 
enite, talcose granite, and others. One of these varieties is 
sometimes found exclusively prevailing throughout an ex¬ 
tensive region, where it preserves a homogeneous character; 
so that, having ascertained its relative age in one place, we 
can recognize its identity in others, and thus determine from 
a single section the chronological relations of large mount¬ 
ain 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 a post-Silurian date. 
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, be¬ 
ing more modern than common or micaceous granite. But 
modern investigations have proved these generalizations to 
have been premature. 
Test by Included Fragments. —This criterion can rarely be 
of much importance, because the fragments involved in gran¬ 
ite are usually so much altered that they can not be refer¬ 
red with certainty to the rocks whence they were derived. 
In the White Mountains, in North America, according to 
Professor Hubbard, a granite vein, traversing granite, con¬ 
tains 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 those slaty and trappean formations from which 
the fragments were derived. 
Recent and Pliocene Plutonic Rocks, why invisible. —The ex¬ 
planations already given in the 28th and in the last chapter 
of the probable relation of the plutonic to the volcanic for¬ 
mations, will naturally lead the reader to infer that rocks of 
the one class can never be produced at or near the surface 
without some members of the other being formed below. It 
is not uncommon for lava-streams to require more than ten 
years to cool in the open air; and where they are of great 
* Silliman’s Jour., No. 69, p. 128, 
