PHYSICAL AND FAUNAL EVOLUTION OF NORTH 
AMERICA DURING ORDOVICIC, SILURIC, 
AND EARLY DEVONIC TIME 
AMADEUS W. GRABAU 
Columbia University 
IV 
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 :? 
Upper Siluric or Monroan. 
. Middle Siluric or Salinan. 
. Lower Siluric or Niagaran. 
. Upper Ordovicic or Trentonian. 
. Middle Ordovicic or Chazyan. 
. Lower Ordovicic or Beekmantownian. 
Sselesl htc! [est sl 
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 
t This journal does not approve the terms ‘‘ Ordovicic,”’ ‘ Siluric,”’ etc. 
2 Science, N. S., Vol. XXIX, pp. 351-56, February, 1909. 
209 
