CONTENTS Ii 



CHAPTER VII 

 THE STRUCTURE OF THE EARTH 



PAGE 



Structural Features of Rocks . 252 



Dip and Strike. Effect of Dip and Strike upon Outcrop. 



Folds . 254 



Effect of Folding on Competent and Incompetent Strata. How the 

 Structure of a Region is Determined. Origin of Folds. Warping. 

 Zones of Flow and Fracture. 



Joints 258 



Origin of Joints. Effect of Joints on Topography. 



Faults 261 



Normal or Gravity Faults. Examples of Normal Faults. Reverse or 

 Thrust Faults. Examples of Thrust Faults. Vertical and Hori- 

 zontal Faults. Influence of Faults on Topography. Minor Features 

 of a Fault Fracture. Detection of Faults. Origin of Faults. 

 Rapidity of Fault Movements. 



Conformity and Unconformity 270 



Importance of Unconformities. Overlap. 



Constitution of the Earth's Interior 272 



Zone of Variable Temperature. The Interior Heat of the Earth. 



Theories of the Physical State of the Earth's Interior 273 



Internal Fluidity Theory. Solid Interior. Gaseous Center. Radioactivity 

 and a Solid Center. Subcrust Theory. Summary. 



CHAPTER VIII 



EARTHQUAKES 

 Earthquakes and Attending Features . . . . . .' . . .275 



The San Francisco Earthquake. Distribution of Earthquakes. Summary 

 of the Causes of Earthquakes. Displacements. Depth of the Plane 

 or Point of Origin. Earthquake Waves. Amplitude of Vibration. 

 Vorticose and Twisting Movements. Duration. Frequency. Areas 

 Affected by Certain Earthquakes. Instruments for Determining and 

 Measuring Earthquakes. 



Effects of Earthquakes 287 



Faults and Fissures. Changes in Level. Landslides. Earthquake Topog- . 

 raphy. Sounds. Loss of Life. Effect on Underground Water. Gases. 

 Construction of Buildings in Earthquake Regions. Effect of Earth- 

 quakes on the Sea. Evidence that a Region has been Free from Severe 

 Earthquakes. 



CHAPTER DC 



VOLCANOES AND IGNEOUS INTRUSIONS 



Volcanoes 294 



How Volcanoes Begin. New Volcanoes. Classification of Volcanoes. 



