CHAPTER VII.. The Origin and Descent of Rocks.

THE ORIGIN AND DESCENT OF ROCKS.

+Composition of Igneous Rocks+ 395 Leading elements, 396. Union of elements, 397. Formation of minerals, 397. Sources of complexity, 398. The leading minerals of igneous rocks, 399. The feldspathic minerals, 400. The ferromagnesian minerals, 400. Summary of salient facts, 401.

+The Nature of Molten Magmas+ 401 Time required in crystallization, 402. Successive stages of crystallization, 403.

+The Fragmental Products of Sudden Cooling+ 404 Pyroclastic rocks, 404.

+The Glassy Rocks+ 406 The solid glasses. 406. The first stages of crystallization, 407. The obsidians, 407.

+Special Structures+ 410 Flow structure, 410. Amygdaloids, 411.

+The Porphyritic Rocks+ 411

+The Phanerocrystalline Rocks+ 412 The phanerites, 412. The granites, 413. The syenites, 415. The diorites, 416. The gabbros, 416. The peridotites, 416. The basalts, 417. The dolerites, 417. General names, 418.

+Derivation of Secondary Rocks+ 420 Regolith, 422. Disrupted products: arkose and wacke, 422. Disintegrated products, 422. Classes of Sedimentary Rocks 422 Shales, sandstones, and conglomerates, 422. Limestones and dolomites, 424. Precipitates, 424. Iron-ore beds, 425. Silicious deposits, 425. Organic rocks, 426.

+Internal Alterations of Rocks+ 426 Oxidation and deoxidation, 427. Solution and deposition, 427. Hydration and dehydration, 428. Carbonation and decarbonation, 429. Molecular rearrangements, 431. The Salient Features of Rock Descent 431

+The Reascensional Process+ 432 Induration under ordinary pressures and temperatures, 432. Cavity filling, 436. Fissure filling; veins, 437. Solution as well as deposition, 437. Concretions, 438. Replacements and pseudomorphs, 439. Incipient crystallization, 439.

+Reconstruction under Exceptional Conditions+ 440 Slaty structure, 441. Foliation, schistosity, 443. Metamorphism by heat, 446. Metamorphism by heat and lateral pressure, 448. Deep-seated metamorphism, 449. Completion of the rock cycle, 449.

+Various Classifications and Nomenclatures+ 449 New System of Classification and Nomenclature 451 The Proposed Field System 451 The phanerites, 451. The aphanites, 452.

+The Proposed Quantitative System+ 454

+Reference List of the More Common Minerals+ 460

+Reference List of the More Common Rocks+ 467

+Ore Deposits+ 474 Concentration, 474. Exceptional and doubtful cases, 474. Original distribution, 475. Magmatic segregation, 475. Marine segregation and dispersion, 476. Origin of ore regions, 477. Surface residual concentration, 478. Purification and concentration, 478. Concentration by solution and reprecipitation, 479. Location of greatest solvent action, 480. Short-course action, 481. Long-course action, 481. Summary, 483. The influence of contacts, 484. The effect of igneous intrusions, 484. The influence of rock walls, 484.