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 volcanic Dikes. — Conversion of Chalk into Marble. — Intrusion of Trap between Strata. — Relation of trappean Rocks to the Products of active Volcanoes. Chapter XXIX—ON THE AGES OF VOLCANIC ROCKS. Tests of relative Age of Volcanic Rocks. — Why ancient and modern Rocks cannot be identical. — Tests by Superposition and intrusion. — Test by Alteration of Rocks in Contact. — Test by Organic Remains. — Test of Age by Mineral Character. — Test by Included Fragments. — Recent and Post-pliocene volcanic Rocks. — Vesuvius, Auvergne, Puy de Come, and Puy de Pariou. — Newer Pliocene volcanic Rocks. — Cyclopean Isles, Etna, Dikes of Palagonia, Madeira. — Older Pliocene volcanic Rocks. — Italy. — Pliocene Volcanoes of the Eifel. — Trass. 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. — Permian 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. Chapter XXXI—PLUTONIC ROCKS. General Aspect of Plutonic Rocks. — Granite and its Varieties. — Decomposing into Spherical Masses. — Rude columnar Structure. — Graphic Granite. — Mutual Penetration of Crystals of Quartz and Feldspar. — Glass Cavities in Quartz of Granite. — Porphyritic, talcose, and syenitic Granite. — Schorlrock and Eurite. — Syenite. — Connection of the Granites and Syenites with the Volcanic Rocks. — Analogy in Composition of Trachyte and Granite. — Granite Veins in Glen Tilt, Cape of Good Hope, and Cornwall. — Metalliferous Veins in Strata near their Junction with Granite. — Quartz Veins. — Exposure of Plutonic Rocks at the surface due to Denudation.