520 
ELEMENTS OE GEOLOGY. 
CHAPTER XXIX. 
ON THE AGES OF VOLCANIC KOCKS. 
Tests of relative Age of Yoleanic Rocks.—^Why ancient and modern Rocks 
can not be identical.—Tests by Superposition and Intrusion.—Test by Al¬ 
teration of Rocks in Contact.—Test by Organic Remains.—Test of Age by 
Mineral Character.—Test by Included Fragments.—Recent and Post-plio¬ 
cene volcanic Rocks.—Yesuvius, Auvergne, Puy de Come, and Puy de 
Pariou.—Newer Pliocene volcanic Rocks.—Cyclopean Isles, Etna, Dikes 
ofPalagonia, Madeira.—Older Pliocene volcanic Rocks.—Italy.—Pliocene 
Yolcanoes of the Eifel.—Trass. 
Haying in the former part of this work referred the sedi¬ 
mentary strata to a long succession of geological periods, we 
have now to consider how far the volcanic formations can be 
classed in a similar chronological order. The tests of rela¬ 
tive age in this class of rocks are four: 1st, superposition and 
intrusion, with or without alteration of the rocks in contact; 
2d, organic remains; 3d, mineral characters; 4th, included 
fragments of older rocks. 
Besides these four tests it may be said, in a general way, 
that volcanic rocks of Primary or Palaeozoic antiquity differ 
from those of the Secondary or Mesozoic age, and these 
again from the Tertiary and Recent. Not, perhaps, that 
they differed originally in a greater degree than the modern 
volcanic rocks of one region, such as that of the Andes, dif¬ 
fer from those of another, such as Iceland, but because all 
rocks permeated by water, especially if its temperature be 
high, are liable to undergo a slow transmutation, even when 
they do not assume a new crystalline form like that of the 
hypogene rocks. 
Although subaerial and submarine denudation,,as before 
stated, remove, in the. course of ages, large portions of the 
upper or more superficial products of volcanoes, yet these 
are sometimes preserved by subsidence, becoming covered 
by the sea or by superimposed marine deposits. In this way 
they may be protected for ages from the waves of the sea, 
or the destroying action of rivers, while, at the same time, 
they may not sink so deep as to be exposed to that plutonic 
action (to be spoken of in Chapter XXXI.) which would 
convert them into crystalline rocks. But even in this case 
they will not remain unaltered, because they will be perco¬ 
lated by water often of high temperature, and charged with 
