UNIVERSITY OF CALIFORNIA 



Bulletin of the Department of Geology 



Vol. I , No. 11, pp. 313-336. ANDREW C. LAWSON, Editor 



CRITICAL PERIODS 



IN 



THE HISTORY OF THE EARTH.* 



BY 



Joseph Le Conte. 



i 



CONTENTS. 



Page. 



Introduction 314 



Inapplicability of European Standards 314 



Early Views 3 14 



How Far May Divisions Be Made General . 316 



Critical Periods 317 



Signs of Critical Periods 318 



The Glacial Revolution 319 



Effect of New Dominant Types 320 



Psychozoic Era 322 



The Post-Cretaceous or Rocky Mountain Revolution 324 



The Post Palaeozoic or Appalachian Revolution 325 



The Pre-Cambrian Revolution 327 



Progressive Changes in Successive Critical Periods 328 



General Formal Laws of the Evolution of the Organic Kingdom 329 



General Advance by All Factors 329 



Origin and Increase of Geograpical Diversity 330 



Critical Periods Diminish Geographical Diversities 330 



Re-isolation 331 



Suddenness of Changes and Rarity of Transitional Forms 331 



Lost Records 332 



Rapid Steps in Evolution 332 



Few Generations Represented 332 



Few Individuals in Each Generation 332 



Migrations and Consequent Unconformity of Faunas 333 



Effect of Specialized and Generalized Forms 333 



Periodic Law Universal 333 



Concluding Reflections 335 



*The following article, in a more condensed form, opened the discussio 

 by the Congress of Geologists, at Chicago, August, 1893, on the question, "Are 

 there any natural divisions of the geological record which are of world-wide 

 extent? " 



