12 CONTENTS 



PAGK 



Materials Erupted 295 



Gases. Fragmental Materials. Lava. Lava Streams. Effect of Compo- 

 sition on Fluidity. Temperature. Surface of Lava Flows. Velocity 

 of Lava Flows. Nature of Lavas. 



Types of Volcanoes 302 



The Explosive or Vesuvian Type: Vesuvius. Krakatao. Katmai. Mt. 



Pelee. Bandai-san. 

 The Quiet or Hawaiian Type: Crater of Kilauea. Eruptions. Lava 



Streams. Origin of Calderas. Steep Lava Cones : Volcanoes of the 



Chimborazo Type. 

 Fissure Eruptions : Recent Icelandic Lava Sheets. 



Characteristics of Volcanic Cones 311 



Profiles of Volcanoes. Shape of Craters. Erosion of Volcanic Cones. 

 Necks and Plugs. Age of Volcanoes in the United States. 



Distribution and Number of Volcanoes 318 



Number of Volcanoes. Distribution. Cause of Distribution. Ancient 

 Volcanoes. 



Importance of Volcanism to Man 321 



Beneficial Effects. Harmful Effects. Volcanoes and Climate. 



Subordinate Volcanic Phenomena 322 



Mud Volcanoes. Solfataras. 



Intrusive or Plutonic Rocks 324 



Injected Masses : Dikes. Sills. Laccoliths. 



Subjacent Masses: Stocks. Batholiths. Some Effects of Intrusions. 



Igneous Rocks 329 



Subdivisions Depending upon Chemical Composition. Subdivisions De- 

 pending upon Texture. 



Classification of Igneous Rocks . 330 



Coarse-grained Igneous Rocks: Granite. Syenite. Diorite. Gabbro. 



Peridotite. 

 Compact or Fine-grained Igneous Rocks : Felsites. Basalts. 

 Glassy Rocks : Obsidian or Volcanic Glass. Pitchstone. 



Fragmental Volcanic Rocks 332 



Tuff. Volcanic Breccia. Columnar Structure of Lava. 



Age of Igneous Rocks 334 



Theories of Volcanism . 334 



Theory Based upon the Assumption that the Interior is Molten. 

 Theories Based upon the Assumption that the Earth is Solid: Heat by Fric- 

 tion. Formation of Lava Reservoirs by Relief of Pressure. Liquid- 

 thread Theory. 

 Abyssal Injection Hypothesis. 



1.' of Present Knowledge of Volcanism 337 



Origin of Volcanic Gases. Cause of the Ascension of Lava. Cause of 

 Periodicity. Influences of the Atmosphere, etc. 



