PHYSICAL AND FAUNAL EVOLUTION 231 
The regressional movement here indicated appears to coincide 
with that of North America, but the transgressive movement seems 
to have begun somewhat earlier, unless the Lower Ordovicic is 
regarded as ending with the Asaphus expansus zone. 
C. THE UPPER ORDOVICIC OR TRENTONIAN 
Most current classifications of the Ordovicic formations of North 
America unite the Black River and Trenton limestones under Clarke 
and Schuchert’s term Mohawkian, which is made synonymous with 
Middle Ordovicic. As we have seen, the Middle Ordovicic is repre- 
sented by the Chazyan, which in its maximum development includes 
some 2,500 feet of limestone strata, and is therefore comparable in 
Busfalo Rochester 
Lorraine 600= — 
=40 
i 
i 
: 
at = —————<—— == = 
= == : SSS SS 
C § Ue = = [ = x c = : — == —= 
L = q r ——=T 1 1 
I I 
I 
SS twa 1200S = 
Fig. 9.—Diagram showing the relationships of the Ordovicic strata of New York, 
between Saratoga and Buffalo. 
magnitude and, inferentially, in time value, to the Beekmantownian 
or Lower Ordovicic. The fauna of the Chazyan is, moreover, 
distinct from both preceding and succeeding faunas, and the natural 
dividing-line between the Middle and the Upper Ordovicic is shown, 
by paleontologic, stratigraphic, and diastrophic reasons, to be within 
or above the Black River horizon; a division coinciding with that made 
in the European series. ‘The Trenton limestone of America is not a 
stratigraphic unit, but, as has been repeatedly demonstrated by 
Ruedemann and noted by many observers, it is the limestone phase of 
a series which elsewhere is in part or mostly represented by Utica 
shale. In the Mohawk Valley the dividing-line between Utica and 
Trenton is a line constantly rising to the west, the transition being 
in some cases abrupt, though probably in most cases it is gradual. 
Ruedemann has pointed out the progressive increase in thickness 
