BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 

 Vol. 21, pp. 227-248, PL. 5 JUNE 10, 1910 



ISOBASES OF THE ALGONQUIX AKD IROQUOIS BEACHES, 

 AND THEIR SIGNIFICANCE^ 



BY JAMES WALTER GOLDTHWAIT 



{Read hefore the Society December 28, 1909) 



CONTENTS 



Page 

 Introduction 227 



The Algonquin water-plane 229 



Stage recorded by the Algonquin beach 229 



Isobases of the upwarped portion of the Algonquin plane 231 



The horizontal portion of the Algonquin plane 236 



The "hinge line" or "isobase of zero" 239 



The Algonquin plane as a datum plane 240 



The Iroquois water-plane 241 



Relative ages of the Iroquois beach and the Algonquin beach 241 



Isobases of the Iroquois plane 242 



Comparison of the two water-planes 243 



The isobases and the pre-Cambrian bounaary 245 



Summary 247 



Introduction 



The study of epeirogenic movements by means of critical surveys of 

 raised beaches is one for which North America is a peculiarly rich field. 

 The investigations of Gilbert in Lake Bonneville, of IJpham in Lake 

 Agassiz, and of Gilbert, Spencer, Leverett, Taylor, Fairchild, Coleman, 

 Woodworth, and others in the Great Lakes region have contributed greatlv 

 to the understanding of post-glacial npwarpings. Among the many 

 papers which have been written on this subject, one is unique in that it 

 seeks to correlate observations over all eastern North America. This is 

 the paper read before the Boston Society of Natural History in 1892 by 

 Baron Gerard De Geer.^ 



1 Manuscript received by the Secretary of the Society January 12, 1910. 



2 Gerard De Geer : On Pleistocene changes of level in eastern North America. Pro- 

 ceedings of the Boston Society of Natural History, vol. 25, 1892, pp. 454-477. 



(227) 



