62 AMADEUS W. GRABAU 



portion of the "Stones River beds" of southern Pennsylvania; and 

 of Upper Chazy species, including Camarotoechia plena, in the Cham- 

 bersburg limestone of the same region. Whether this implies an 

 Appalachian extension of the Champlain gulf or a connection with 

 the Atlantic in the southern part of North America must be deter- 

 mined by further detailed study. It is probably true, however, that 

 the open passage along the west border of Appalachia and Taconia, 

 through which the mud-bearing currents swept in early Beekmantown 

 time, and which formed the route of dispersal for the graptolite 

 fauna of that age, was not re-established until late Chazy or Black 

 River time. This accounts for the slight development of the grap- 

 tolite-bearing shales referable to the Chazy in the Hudson and Levis 

 series. The disconformity which represents this interruption would 

 probably be difficult to trace in strata of such similar lithic characters. 



THE BLACK RIVER FORMATION 



This formation is widespread, having been traced by its fauna 

 from the Champlain Valley to the upper Mississippi and southward 

 to Oklahoma and the Appalachians. Over this area it forms an 

 excellent datum plane from which correlation of overlying and under- 

 lying formations becomes possible. Its thickness is never very great ; 

 it is only 7 feet in the type region, at Watertown, N. Y., 50-60 feet in 

 Minnesota, less than 100 feet in Oklahoma, 90 feet in southern Penn- 

 sylvania, and 70 feet in the Champlain Valley. Faunally, it repre- 

 sents a transition between the Chazy and Trenton, as will be seen by 

 consulting published lists. Its classification with either the Chazy or 

 the Trenton is therefore permissible. Since the formation represents 

 the unchecked continuance of the transgressive movement initiated 

 at the opening of Chazy time, its classification with that series of 

 strata as Mid-Ordovicic is perhaps most desirable. 



THE NORMANSKILL BEDS AND FAUNA 



The Normanskill shales are generally regarded as representing 

 the shale facies of the Lower or Middle Trenton. Ruedemann, in 

 his recently published monograph parallels them with the Lowville, 

 Black River, and Lower Trenton ^ In Rysedorf Hill, the shale includes 

 a conglomerate, the pebbles of which, regarded as nearly syn- 



I Graptolites of New York, Part II. 



