XXll 
CONTENTS. 
CHAPTER XXVIII. 
VOLCANIC ROCKS. 
External Form, Structure, and Origin of Volcanic Mountains.—Cones and 
Craters.—Hypothesis of “Elevation Craters” considered.—Trap Rocks.— 
Name whence derived.—Minerals most abundant in Volcanic Rocks.— 
Table of the Analysis of Minerals in the Volcanic and Hypogene Rocks.— 
Similar Minerals in Meteorites.—Theory of Isomorphism.—Basaltic Rocks. 
—Trachytic Rocks.—Special Forms of Structure.—The columnar and 
globular Forms.—Trap Dikes and Veins.—Alteration of Rocks by vol¬ 
canic Dikes.—Conversion of Chalk into Marble.—Intrusion of Trap be¬ 
tween Strata.—Relation of trappean Rocks to the Products of active 
Volcanoes.Page 494 
CHAPTER XXIX. 
ON THE AGES OF VOLCANIC ROCKS. 
Tests of relative Age of Volcanic 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.—Vesuvius, Auvergne, Puy de Come, and Puy de 
Pariou.—New^er Pliocene volcanic Rocks.—Cyclopean Isles, Etna, Dikes 
ofPalagonia, Madeira,—Older Pliocene volcanic Rocks.—Italy.—Pliocene 
Volcanoes of the Eifel.—Trass. 520 
CHAPTER XXX. 
AGE OF VOLCANIC ROCKS—CONTINUED. 
Volcanic Rocks of the Upper Miocene Period.—Madeira.—Grand Canary.— 
Azores.—^Lower Miocene Volcanic Rocks.—Isle of Mull.—Staffa and 
Antrim.—The Eifel.—Upper and Lower Miocene Volcanic Rocks of 
Auvergne.—Hill of Gergovia.^—Eocene Volcanic Rocks of Monte Bolca. 
—Trap of Cretaceous Period.—Oolitic Period.—Triassic Period.—Permi¬ 
an Period.—Carboniferous Period.—Erect Trees buried in Volcanic Ash 
in the Island of Arran.—Old Red Sandstone Period.—Silurian Period.— 
Cambrian Period.—Laurentian Volcanic Rocks. 53G 
CHAPTER XXXI. 
PLUTONIC ROCKS. 
General Aspect of Plutonic Rocks.—Granite and its Varieties.—Decompos¬ 
ing into Spherical Masses.—Rude columnar Structure.—Graphic Gran¬ 
ite.—Mutual Penetration of Crystals of Quartz and Feldspar.—Glass Cav¬ 
ities in Quartz of Granite.—Porphyritic, talcose, and syenitic Granite.— 
Schorlrock and Eurite.—Syenite.—Connection’ of the Granites and Sy¬ 
enites with the Volcanic Rocks.—Analogy in Composition of Trachyte and 
Granite.—Granite Veins in Glen Tilt, Cape of Good Hope, and Corn¬ 
wall.—Metalliferous Veins in Strata near their Junction with Granite.— 
Quartz Veins.—Exposure of Plutonic Rocks at the Surface due to De¬ 
nudation . 551 
