CONTENTS. 
XXlll 
CHAPTEE XXXIL 
ON THE DIFFERENT AGES OF THE PLUTONIC ROCKS. 
Difficulty in ascertaining the precise Age of a Plutonic Eock.—Test of Age 
by Eelative Position.—Test by Intrusion and Alteration.—Test by Mineral 
Composition.—Test by included Eragments.—Eecent and Pliocene Plu¬ 
tonic Eocks, why invisible.—Miocene Syenite of the Isle of Skye.—Eocene 
Plutonic Eocks in the Andes.—Granite altering Cretaceous Eocks.—Granite 
altering Lias in the Alps and in Skye.—Granite of Dartmoor altering Carbon¬ 
iferous Strata.—Granite of the Old Eed Sandstone Period.—Syenite altering 
Silurian Strata in Norway.—Blending of the same with Gneiss.—Most an¬ 
cient Plutonic Eocks.—Granite protruded in a solid Eorm.Page 564 
CHAPTEE XXXIII. 
METAMORPHIC ROCKS. 
General Character of Metamorphic Eocks.—Gneiss.—Hornblende-schist.— 
Serpentine.—Mica - schist.—Clay - slate.—Quartzite.—Chlorite - schist.— 
Metamorphic Limestone.— Origin of the metamorphic Strata.— Their 
Stratification.—Eossiliferous Strata near intrusive Masses of Granite con¬ 
verted into Eocks identical with difierent Members of the metamorphic 
Series.—Arguments hence derived as to the Nature of Plutonic Action.— 
Hydrothermal Action, or the Influence of Steam and Gases in producing 
Metamorphism.—Objections to the metamorphic Theory considered... 576 
CHAPTEE XXXIV. 
METAMORPHIC ROCKS—CONTINUED. 
Definition of slaty Cleavage and Joints.—Supposed Causes of these Struc¬ 
tures.—Crystalline Theory of Cleavage.—Mechanical Theory of Cleavage. 
—Condensation and Elongation of slate Eocks by lateral Pressure.—Lam¬ 
ination of some volcanic Eocks due to Motion.—Whether the Foliation 
of the crystalline Schists be usually parallel with the original Planes of 
Stratification.—Examples in Norway and Scotland.—Causes of Irregular¬ 
ity in the Planes of Foliation. 588 
CHAPTEE XXXY. 
ON THE DIFFERENT AGES OF THE METAMORPHIC ROCKS. 
Difficulty of ascertaining the Age of metamorphic Strata.—Metamorphic 
Strata of Eocene date in the Alps of Switzerland and Savoy.—Lime¬ 
stone and Shale of Carrara.—Metamorphic Strata of older date than the 
Silurian and Cambrian Eocks.—Order of Succession in metamorphic 
Eocks.—Uniformity of mineral Character.—Supposed Azoic Period.—Con¬ 
nection between the Absence of Organic Eemains and the Scarcity of cal¬ 
careous Matter in metamorphic Eocks. 597 
CHAPTEE XXXVI. 
MINERAL VEINS. 
Different Kinds of mineral Veins.—Ordinary metalliferous Veins or Lodes. 
—Their frequent Coincidence with Faults.—Proofs that they originated 
