CAMBRIAN GEOLOGY AND PALEONTOLOGY 



II 



ABRUPT APPEARANCE OF THE CAMBRIAN FAUNA ON 

 THE NORTH AMERICAN CONTINENT^ 



By CHARLES D. WALCOTT 



(With One Map) 



CONTENTS 



PAGE 



Introduction i 



Pre-Cambrian rocks 3 



Sources of sediments 5 



Origin of later Algonkian rocks 6 



Pre-Cambrian Continental Surface 9 



Unconformity between Cambrian and pre-Cambrian 11 



Extent of withdrawal of Seas in Late Algonkian Time 12 



Evidence of Fossils 13 



Resume 14 



Bibliography 15 



ILLUSTRATIONS 



Plate I. Map showing distribution of pre-Cambrian Rocks 4 



Fig. I. Theoretical profile of North American continent during later 



part of Algonkian time 10 



INTRODUCTION 



In accepting the invitation of the Representatives for Paleontology 

 of the Swedish Committee of the Eleventh International Geological 

 Congress, to take part in the discussion of " The Abrupt Appearance 

 of the Cambrian Fauna," I decided to confine the geological discus- 

 sion mainly to phenomena observed on the North American Conti- 

 nent and to limit paleontological conclusions to results obtained 

 from studies of material from the oldest fossiliferous Cambrian rocks. 



In summing up the Lower Cambrian fauna in 1891 [Walcott, 



^ Read at the Eleventh International Geological Congress, Stockholm, 

 Sweden, August 18, 19 10. 



Smithsonian Miscellaneous Collections Vol. 57, No 1 



