CHAPTER X.

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