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