Devonian Era in the Ohio Basin. — Claypole. 35 
groups — the Corniferous and Hamilton — are less sharp in 
Ohio than in New York, where the transition from the pure 
limestones of the Corniferous to the black shales of the Hamil- 
ton is comparatively abrupt. The corresponding palseontological 
break is there, as might be expected, much more strongly- 
marked than it is in Ohio, and on this fact professor Hall was 
enabled to base his definition of the two groups. Moreover, 
the fauna of the Corniferous in New York is rich, while that 
of the Marcellus is exceedingly poor and the species are in 
general of small size. Even there, however, not a few species 
pass up across the plane of separation, so that the characteristic 
number of each fauna is much less than the total. 
Collecting the data, as we now possess them, from Ohio, we 
may compile the following section : 
Shale at Prout's Station Hamilton 
Blue limestone, thin bedded Corniferous-Hamilton 
Dublin blue shale Marcellus 
Bone-bed Corniferous 
Gray and buff limestones Corniferous 
The period was, in Ohio, evidently one of transition. Be- 
ing far distant from the eastern shore of the Appalachian gulf 
and the influx of eastern muddy waters, Corniferous condi- 
tions continued long after they had ceased to the eastward. 
The Marcellus fauna discovered by professor Whitfield marks 
the earliest of mechanical sediment, or at least the earliest of 
invasions of the area by members of the oldest Hamilton fauna. 
The purity of the Corniferous water was then for the first time 
sullied by the fine wash from the land, and with this change 
the mud-loving species were enabled to press in and to supplant 
those which had so long flourished in the pure, clear sea of the 
western gulf. Leiorhynchus and its comrades migrated west- 
ward and colonized the area to an extent at present unknown. 
But the slow rate of migration attained by similar forms com- 
pels us to regard this thin layer of shale as the memorial of no 
short period of time. 
The flood that swept the mud so far out to sea at length 
subsided and the purity of the waters was restored. Counter- 
migration ensued and the Leiorhynchus fauna succumbed to the 
survivors and to the descendents of the displaced Corniferous 
fauna, which had been temporarily driven to seek an asylum in 
the west. It was long before they again gave way, but when 
