H 



CONTENTS 



PAGB 



Geological Chronology . 381 



Order of Superposition. Chronology Determined by Fossils. Use of 

 Fossils in Determining Physical Conditions. Difficulties in Corre- 

 lating Strata. 



Divisions of Geological Time . . . . 383 



CHAPTER XIV 

 THE EARTH BEFORE THE CAMBRIAN 



Theories of the Earth's Origin 385 



Nebular Hypothesis. Planetesimal Hypothesis. Nebular and Planetesimal 

 Theories Contrasted. 



Pre-Cambrian Eras 388 



The Archeozoic Era 389 



Distribution of the Archaeozoic Rocks. Characteristics of Archaeozoic Rocks. 

 Thickness. Causes of Metamorphism and Deformation. Conditions 

 during the Archaeozoic Era. Duration. Bearing upon the Theories 

 of the Earth's Origin. 



The Proterozoic Era 393 



Archaeozoic and Proterozoic Contrasted. The Proterozoic in Different 

 Regions. Iron and Copper Deposits. Life of the Proterozoic Era. 

 Duration. Climate. Life before Fossils. 



CHAPTER XV 



THE CAMBRIAN PERIOD 



The Paleozoic Era 401 



The Cambrian Period 402 



Divisions of the Cambrian. Location of Cambrian Rocks. Physical 

 Geography of Ancient Periods. Basal Unconformity. Physical 

 Geography of the Cambrian. Character of the Cambrian Rocks. Pres- 

 ent Condition of the Sediments. Volcanism. Close of the Cambrian. 

 Other Continents. 



Life of the Cambrian 408 



Plants 409 



Animals 410 



Crustacea: Trilobites. Other Crustaceans. 



Mollusca: Gastropods. 



Molluscoidea: Brachiopods. 



Echinodermata : Cystoids. 



Worms. 



Coelenterata: Corals. Graptolites. Jellyfish. Sponges. 



Protozoa. 



Summary 4^ 



Evolution during the Cambrian. Climate and Duration. 



