PHYSICAL AND FAUNAL EVOLUTION 22% 
is characterized by Diplograptus dentatus and Cryplograptus antenna- 
rius. This zone has been correlated by Ruedemann with the Chazy 
limestones 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 formed. 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 (Lowville) and Black River 
graptolite fauna, were formed. In spite of some similarities, the 
Diplograptus dentatus 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, 
