CHAPTER X.. The Extrusive Processes.

THE EXTRUSIVE PROCESSES.

Outward movements, 590.

+Vulcanism+ 590 Phases of vulcanism, 591. 1. Intrusions 591 The heating action, 592. 2. Extrusions 592 Fissure eruptions, 593. Volcanic eruptions, 594. Intermediate phenomena, 596. Lunar craters, 598.

+Volcanoes+ 599 Number of, 599. Distribution of Volcanoes 599 In time, 599. Relative to land and sea, 599. Relative to crustal deformations, 601. In latitude, 603. In curved lines, 603. Relations of Volcanoes 604 Relations to rising and sinking surfaces, 604. Relations to one another, 605. Unimportant coincidences, 606. Periodicity, 607. Formation of Cones 608 Lava-cones, 608. Cinder-cones, 608. Subordinate cones, 610. Composite cones, 610. Extra-cone distribution, 610.

+Lavas+ 612 Their nature, 612. Consanguinity and succession of lavas, 614. Temperature of lavas, 615. Depth of source, 616.

+Volcanic Gases+ 617 Differences in gas action, 617. Spasmodic action, 618. Kinds of gases, 618. Residual gases in volcanic rock, 619. The source of the gases, 621.

+The Cause of Vulcanism+ 623 I. On the Assumption that the Lavas are Original 623 Lava outflows from a molten interior, 624. Lavas assigned to molten reservoirs, 624.

II. On the Assumption that the Lavas are Secondary 625 Lavas assigned to the reaction of water and air penetrating to hot rocks, 625. Lavas assigned to relief of pressure, 627. Lavas assigned to melting by crushing, 628. Lavas assigned to melting by depression, 629. Vulcanism assigned to the outflow of deep-seated heat, 629. Modes of Reaching the Surface 631 Additional Considerations Relative to the Gases 633 Thermal Considerations 635