CO \ TENTS. xvii 



PAGE 



Relations to rising and sinking surfaces, 604. Relations 

 lio one another, 605. Unimportant coincidences, 606. Peri- 

 odicity, 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 as- 

 signed 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 



CHAPTER XL 



THE GEOLOGIC FUNCTIONS OF LIFE. 



I. The Distinctive Features of Organic Processes 638 



The Chemical Work of Life 638 



Life material chiefly atmospheric, 638. The non-atmos- 

 pheric factors, 639. 



(1) Changes in the composition of the atmosphere 639 



The consumption and restoration of carbon dioxide, 



640. The freezing and consumption of oxygen, 640. The 

 organic residue, 640. The meaning of the organic residue, 



641, The more inert factor, 642. Probable fluctuations 



