The Principles of Petrology

N this book the task of summarising modern petrology I from the genetic standpoint has been attempted. The scale of the work is small as compared with the magni­ tude of its subject, but it is nevertheless believed that the field has been reasonably covered. In conformity with the genetic viewpoint petrology, as contrasted with petrography, has been emphasised throughout; and purely descriptive mineralogical and petrographical detail has been omitted. Every petrologist who reads this book will recognise the author's indebtedness to Dr. A. Harker and Dr. A. Holmes, among British workers; to Prof. R. A. Daly, Dr. H. S. Washington, and Dr. N. L. Bowen, among American petrologists; and to Prof. J. H. L. Vogt, Prof. V. M. Goldschmidt, Prof. A. Lacroix, and Prof. P. Niggli. among European investigators. The emphasis laid on modern views, and the relative poverty of references to the works of the older generation of petrologists, does not imply any disrespect of the latter. It is due to recognition of the desirability of affording the petrological student a newer and wider range of reading references than is usually supplied in this class of work; for refer­ ences tend to become stereotyped as well as text and illustrations. Furthermore it is believed that all that is good and living in the older work has been incorporated, consciously or unconsciously, in the newer.

I Introduction.- The Science of Petrology.- The Earth Zones.- The Barysphere.- Composition of the Earth Shells.- Chemical Composition of the Crust.- Rocks and their Composition.- The Rock-forming Minerals.- The Classification of Rocks.- I The Igneous Rocks.- II Forms and Structures of Igneous Rocks.- Forms of Igneous Rocks.- Lava Flows.- Intrusions and Their Relations to Geological Structure.- Forms in Unfolded Regions.- Sills.- Laccoliths.- Lopoliths.- Dykes.- Ring-dykes and Cone-sheets.- Volcanic Necks.- Forms in Folded Regions.- Phacoliths.- Chonoliths.- Batholiths.- Multiple Intrusions.- Composite Intrusions.- Differentiated Intrusions.- Structures of Igneous Rocks.- Structure and Texture.- Vesicular and Amygdaloidal Structure.- Block Lava and Ropy Lava.- Pillow Structure.- Flow Structure.- Jointing, Sheet, and Platy Structures.- Columnar and Prismatic Structures.- Rift and Grain.- III Composition and Constitution of Magmas.- The Magma.- Composition of Magmas.- The Pyrogenetic Minerals.- The Physico-chemical Constitution of Magmas.- Primary Magmas.- IV The Formation of Igneous Rocks.- Glass and Crystals.- Crystallisation of a Unicomponent Magma.- Grain of Igneous Rocks.- Formation of Glass.- Crystallisation of Binary Magmas.- Eutectics—Mixed Crystals.- Crystallisation of Ternary Magmas.- Crystallisation of some Ternary systems.- The Reaction Relation.- V Textures and Microstructures.- Definition and Description.- Crystallinity.- The Beginnings of Crystallisation. Crystallites and Microlites.- Devitrification.- Granularity.- Shapes of Crystals.- Mutual Relations of Crystals.- Equigranular Textures.- Inequigranular Textures.- Porphyritic Texture.- Poikilitic Texture.- Directive Textures.- Intergrowth Textures.- Reaction Structures.- Xenolithic Structures.- Orbicular Structure.- Spherulitic Structure.- Fracture Forms.- VI Classification of Igneous Rocks.- Bases of Classification.- The Factor of Chemical Composition.- The Factor of Mineral Composition.- The Factor of Geological Occurrence and Texture.- Tabular Classification.- Nomenclature.- Granite, Granodiorite, and Diorite.- Syenite, Nepheline-syenite, and Related Alkaline Rocks.- Gabbro, Anorthosite, and Peridotite.- Dolerite and Lamprophyre.- Rhyolite and Dacite.- Trachyte and Phonolite.- Andesite and Basalt.- VII The Distribution of Igneous Rocks in Space and Time.- Consanguinity.- The Diagrammatic Representation of Igneous Rock Series.- Kindreds of Igneous Rocks.- Petrographic Provinces and Periods.- Igneous Action and Earth Movements.- Time Sequences in Igneous Rocks.- VIII The Origin of Igneous Rocks.- Variations in Igneous Rocks.- Evidences of Differentiation. Variation within a Single Rock Body.- Theories of Differentiation.- Differentiation by Liquid Immiscibility.- Differentiation by Crystallisation.- Gravitational Differentiation.- Filtration Differentiation.- The Rôle of Volatile Constituents in Differentiation.- Pegmatite and Aplite.- Assimilation and Hybrid Rocks.- Origin of Alkaline Rocks.- II The Secondary Rocks.- IX Introduction.- General.- The Breaking-down of the Rocks.- Decomposition of Rocks.- Disintegration of Rocks.- Transport.- Deposition.- Classification of the Secondary Rocks.- X The Residual Deposits.- Residual Deposits in General.- Terra Rossa.- Clay-with-Flints.- Laterite and Bauxite.- Soils.- XI Sedimentary Rocks: Mineralogical, Textural, and Structural Characters.- Mineral Composition.- Grain Size.- Heavy Minerals in Sands and Sandstones.- Shape and Rounding of Grains.- Cohesion.- Stratification.- Minor Structures and Markings.- XII Sedimentary Rocks. Descriptive.- Classification.- Rudaceous Rocks: Breccias and Conglomerates.- Arenaceous Rocks: Sands and Sandstones.- Silt and Siltstone.- Argillaceous Rocks: Clays and Shales.- XIII Deposits of Chemical Origin.- Chemical Deposits in General.- Concretions.- Secretions.- Colloids.- Siliceous Deposits.- Carbonate Deposits.- Ferruginous Deposits.- Salts.- XIV Deposits of Organic Origin.- Organic Deposits in General.- Organic Rocks of Calcareous Composition: The Limestones.- Phosphatic Deposits of Organic Origin.- Siliceous Deposits of Organic Origin.- Carbonaceous Deposits: Peat and Coal.- III The Metamorphic Rocks.- XV Metamorphism.- General Character of Metamorphism.- Agents of Metamorphism.- Kinds of Metamorphism.- Facies and Grades of Metamorphism.- XVI Metamorphic Minerals, Processes, and Structures.- Influence of Original Composition.- Influence of Heat and Uniform Pressure.- Influence of Directed Pressure.- Textures and Structures in Metamorphic Rocks.- Shapes of Metamorphic Minerals.- Growth and Mutual Relations of Minerals in Metamorphic Rocks.- Structures of Metamorphic Rocks.- Classification and Nomenclature of Metamorphic Rocks.- XVII Cataclastic Metamorphism and Its Products.- Cataclastic Metamorphism in General.- Slates and Slaty Cleavage.- Crush-breccia and Cataclasite.- Flaser Rocks and Mylonite.- XVIII Thermal Metamorphism and Its Products.- Thermal Metamorphism in General.- Thermal Metamorphism of Clay Rocks.- Contact Metamorphism of Clay Rocks.- Pyrometamorphism of Clay Rocks.- Thermal Metamorphism of Limestones.- Thermal Metamorphism of Arenaceous Rocks.- Thermal Metamorphism of Basic Lavas and Tuffs.- XIX Dynamothermal Metamorphism and Its Products.- Dynamothermal Metamorphism in General.- Dynamothermal Metamorphism of Argillaceous Rocks.- Dynamothermal Metamorphism of Quartzo-Felspathic Rocks.- Dynamothermal Metamorphism of Basic Igneous Rocks and Tuffs.- XX Plutonic Metamorphism and Its Products.- Plutonic Metamorphism in General.- Granulite, Leptynite, Leptite.- Pyroxene-gneiss, Pyroxene-granulite, and the Charnockite Series.- Eclogite and Garnet-amphibolite.- XXI Metasomatism and Additive Processes of Metamorphism.- Metasomatism.- Metasomatic Processes.- Pneumatolytic Metamorphism.- Injection Metamorphism and Autometamorphism.- Lit-par-lit Gneiss, Composite Gneiss, Anatexis and Palingenesis.