PHYSICAL AND FAUNAL EVOLUTION 249 
1,200 feet of Helderbergian (St. Albans and Bon Ami limestones), 
the succession being a conformable one.’ 
The Oriskany of the United States is mostly a sandstone, often: 
of pure quartz grains, at other times calcareous. The source of the 
sandstone is to be sought in the sandstones of the eastern extension of 
the Siluric and Ordovicic formations, and perhaps in the exposures of 
the St. Peters and the Sylvania. It seems most likely that the distri- 
bution of the sand over eastern North America was largely effected by 
wind, during the long period of erosion preceding the submergence 
of the continent. On the westward extension of the Oriskany sea 
these accumulated sands were reworked and were transformed into 
the fossiliferous marine sands which they are found to be today. 
In the east, after a short period of sedimentation, an extensive accumu- 
lation of black muds occurred, forming the Esopus-Schoharie shale 
series. This has its greatest thickness at Port Jervis, whence it 
thins away in all directions, apparently by overlap. Since the source 
of the material was clearly in the east, and the overlap is toward 
the west, north, and south, the formation must be a subaerial fan. 
This is further indicated by the general absence of fossils, except for 
occasional intercalations, such as would be expected ina fan of this 
kind, probably rising but slightly above the level of the shallow Oris- 
kany sea. The continuance of the Oriskany invasion is found in 
the spread of the limestone with the Schoharie fauna and the succeed- 
ing Onondaga submergence. During Onondaga and Hamilton time, 
continuous deposition and spreading of the seas went on, but at the 
close of the Middle Devonic, renewed emergence affected most of 
southern and southeastern United States, accompanied by erosion. 
This again was followed by the slow resubmergence, which commenced 
from the north and slowly advanced southward and eastward. The 
basal member of this transgressing series is the black shale, which, 
in northern Michigan, is of Lower Devonic (Genesee?) age, but 
becomes of later and later age southward, at the same time resting 
always on lower strata. Thus late Upper Devonic (Portage) black 
shale rests on Lower Hamilton in southern Michigan and northern 
Ohio; still later beds (Chemung) on the Onondaga (Columbus) 
«See Clarke, J. M., “Early Devonic History of New York and Eastern North 
America,”? New York State Museum Memoir 9, 1908. 
