288 Cincinnati Society of Natural History. 
THE UPPER SILURIAN. 
The Upper Silurian is subdivided into the Medina Group, Clinton 
Group, Niagara Group, Onondaga Salt Group, Guelph Group, and 
Lower Helderberg Group. 
The Medina Group.—This group includes the Medina sandstone, 
Oneida conglomerate and Gray sandstone. It was named from 
Medina, New York. 
The Medina sandstone was described by Vanuxem, in 1842, and 
redescribed by Hall, in 1845. It usually consists of red, marly or 
shaly sandstone, or variegated light-red and yellowish sandstone, and 
gray quartzose sandstone. In its geographical distribution it extends 
from New York into Canada, and as far west as Lake Huron, and from 
New York across Pennsylvania, Maryland, Virginia, and into Tenn- 
essee. Between the mouth of the Niagara river and Lewiston, it is 
300 feet in thickness; at the west end of Lake Ontario, in Canada, 614 
feet; on Lake Huron, 103 feet; and in Pennsylvania and Virginia, from 
1,000 to 1,500 feet. 
The Oneida conglomerate was defined, in 1843, by Prof. James Hall, 
as a quartzose conglomerate succeeding the Hudson River Group in the 
eastern part of New York, and a gray sandstone in the western part. 
In 1842, Vanuxem described the gray sandstone, and showed that it is 
intimately connected with the Medina sandstone. Its thickness in 
the region of Lake Ontario is less than 100 feet, but it extends south- 
wardly increasing in thickness in Pennsylvania and Virginia, so that 
its maximum thickness in the latter States is not less than 700 feet. 
This thickness is included in the statement above of the thickness 
of the Medina Group. The character of the conglomerate is 
such that it indicates rapid deposition, and it is almost non- 
fossiliferous, though a few fragments of fucoids and shells too im- 
perfect for definition have been found init. The conglomerate and 
gray sandstone are included in the Medina Group, because the gray 
sandstone, in some places, graduates into the Medina so that they can 
scarcely be distinguished, except by the color, and for the still stronger 
reason that there are no fossils to characterize it. 
The accumulation of the deposits, in this group, seems to have been 
more or less affected by waves and currents, and probably where it 
has its greatest thickness, took place with some rapidity. The most 
that can be said of it is, that it indicates a vast and important chasm 
in geological time. The fossils are usually very poorly preserved. 
The only genera commencing an existence in this group are Arthro- 
