32 The American Geologist. 
July, 
had been laid down and on it began the building of the mas- 
sive Appalachian strata which it carries and of which Devonia 
in this region for the most part consists. 
B. THE C0RNIFER0US-HAMILT.0N PERIOD. 
A change now came over the scene. The long Corniferous 
peace was broken and its monotonous limestone formation in- 
terrupted. The calcareous deposit which had been in process 
for millenniums gave way to mechanical sediments and over 
the whole Appalachian area the sea-bottom and its fauna un- 
derwent a radical, but not, save perhaps locally, a catastrophic 
revolution. The apparently severe and rapid, perhaps orogenic, 
upheaval to which the formation of the Medina sandstone was 
due gave place to a long continued but gentle elevation during 
the whole Silurian era, enduring in the southern Appalachian 
area even into the Devonian. 
Estra-Liiiiital Immigration. — The typical Corniferous in 
Xew York was succeeded by a heavy bed of black shale called 
the Marcellus, and this was again overlain by the Hamilton 
proper, consisting in different places of limestone, shale and 
sandstone. The Marcellus in Xew York seldom exceeds fifty 
feet in thickness, but in Pennsylvania it rises to three hundred 
feet. The Hamilton in eastern Xew York is one thousand feet 
thick but rapidly thins away to the westward. To the south- 
ward, however, it thickens until, in Pennsylvania, it attains fif- 
teen hundred feet and includes a bed of coarse sandstone eight 
hundred feet thick. 
Not only is there a sharp difference between the Cornifer- 
ous and the Hamilton in the nature of their component sedi- 
ments but the change is equally marked in their fossils. The 
new environment did not suit the organisms and they were 
compelled to migrate, to modify themselves to the changed 
conditions or to die. Palseontological evidence indicates that a 
sudden and great, but not complete, break occurs on this hor- 
izon. A fauna, in large part new, came upon the scene, doubt- 
les- migrating into the area from extra-limital regions, but a 
certain proportion of the previous inhabitants survived the 
change, adapting themselves to the circumstances. These 
amply prove that no catastrophe of wide extent marked the 
passage from the Corniferous to the 1 lamilton-Marcellus per- 
