CHAPTER IV 



PHYSICAL AND FAUNAL EVOLUTION OF NORTH AMERICA 



DURING ORDOVICIC, SILURIC, AND EARLY 



DEVONIC TIME 



AMADEUS W. GRABAU 

 Columbia University 



The following classification of the Ordovicic' and Siluric has 

 recently been published by the author and will be made the basis 

 of the present discussion of these systems:^ 



F. Upper Siluric or Monroan. 



E. ^Middle Siluric or Salinan. 



D. Lower Siluric or Niagaran. 



C. Upper Ordovicic or Trentonian. 



B. Middle Ordovicic or Chazyan. 



A. Lower Ordovicic or Beekmantownian. 



A. THE LOWER ORDOVICIC OR BEEKMANTOWNIAN 



At the beginning of Ordovicic time, as now generally recognized, 

 the great marine transgression or positive diastrophic movement, 

 which obtained throughout Upper Cambric time, was in progress, 

 so that the early Beekmantown strata overlapped the Upper Cambric 

 (Saratogan) and came to rest directly upon the crystalline basement. 

 The basal portion of the sedimentary series is generally quartz 

 sandstone of greater or less purity, or sometimes a conglomerate 

 with crystalline pebbles of local origin. This basal sandstone is 

 commonly referred to the "Potsdam," that term being used synony- 

 mously with Upper Cambric. Aside from the question as to whether 

 or not the Potsdam sandstone of the type locality is really of Upper 

 Cambric age, it must of course be apparent that in a normally over- 

 lapping series of strata deposited by a transgressing sea, the basal 

 sand member would naturally rise in the series in the direction of 

 transgression and overlap, and that hence a basal sand is not every- 

 where of the same age. In northwestern New York, in Ontario, and 

 in northern Michigan, these basal sands are probably in all cases 



I The editor does not approve the terms "Ordovicic," " Siluric," etc. 

 = Science, N. S., Vol. XXIX, pp. 351-56, February, 1909. 



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