56 AMADEUS W. GRABAU 



is characterized by Diplograptus dentatus and Cryptograptus antenna- 

 rius. This zone has been correlated by Ruedemann with the Chazy 

 Hmestones of the Champlain region, but it probably is also referable 

 to the Beekmantown, since most of its characteristic types occur in 

 the Upper Arenig of Great Britain. The world-wide distribution 

 of these graptolite faunas suggests that they were dispersed by strong 

 currents sweeping through an open channel along the inner or western 

 side of an Appalachian continent and its New England extension 

 (Taconia). The fauna was most likely spread from Australia by 

 strong currents passing up the west coast of South America and 

 entering the Appalachian synclinal trough, along which it flowed 

 northeastward to Newfoundland. Northwestward of this zone of 

 mud-deposition we find the limestone of the Beekmantown grading 

 down, by the addition of quartz grains, into the basal quartz sand, 

 without intervening mud deposits (see map. Fig. 2). 



With the progress of Beekmantown retreat the channel was 

 closed, a land bridge connecting Taconia with Laurentia. Thus 

 the mud deposition was checked and only a moderate thickness of 

 Beekmantown strata of this type was formied. This represents, there- 

 fore, largely the lower part of the Beekmantown. As has been stated, 

 it is probable that the Chazy is unrepresented by deposits of mud, 

 the channel remaining closed until the end of that period, when it 

 reopened through the progress of Chazy transgression, and the 

 Normanskill beds, with a late Chazy (Low'ville) and Black River 

 graptolite fauna, were formed. In spite of some similarities, the 

 Diplograptus dentakis and the Coenograptus gracilis zones are quite 

 distinct, the important genera, Odontocaulus, Thamnograptus, 

 Corynoides, Azygograptus, Leptograptus, Nemagraptus (Coeno- 

 graptus), Dicellograptus, and Dicranograptus, appearing suddenly. 

 In like manner, the characteristic Beekmantown genera, Dendro- 

 graptus, Goniograptus, Loganograptus, Dichograptus, Tetragraptus, 

 Phyllograptus, and Didymograptus, continue through the third Deepkill 

 zone, only the last of them extending into the Normanskill zone. 

 Certain long-lived genera, Desmograptus, Diplograptus, Clonograptus, 

 Climacograptus, and Cryptograptus, begin in the third Deepkill zone 

 and extend through all or most of the remaining Ordovicic. Of the 

 genera in common between the third Deepkill and the Normanskill, 



