66 



AMADEUS W. GRABAU 



The regressional movement here indicated appears to coincide 

 with that of North America, but the transgressive movement seems 

 to have begun somewhat earher, 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 



SeLrCLtOifO^ 



ILorraine ti 



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 



