64 
GEOLOGY OF THE FOURTH DISTRICT. 
though in some localities they may have been drifted, still, as a general rule, they have not 
suffered violent removal.. This furnishes, among others, an example of a formation which, 
after thinning partially or entirely, reappears in the same direction, even better exhibited than 
in the first locality. Although the group is much more developed in the Third district, still 
the Pentamerus appears but rarely, and attains its greatest perfection in Wayne county. 
4. Second Green Shale. 
This is readily distinguished from the lower green shale by its less deep color, and from 
being everywhere fossiliferous, while the lower is so only in a few localities. The change is 
very abrupt from the limestone below to this soft green shale. At Rochester, this mass has 
a thickness of twenty-four feet, and it maintains nearly the same in other places. At Wol¬ 
cott, however, it appears to be somewhat thicker, though its upper limit w r as not distinctly 
ascertained. It is well exposed at Wolcott furnace in the banks of the creek ; it also appears at 
the ore bed six miles farther east. At Sodus point it is well exhibited, and large specimens 
may be obtained covered with Graptolites. Its whole thickness is seen in the Genesee river 
below the upper falls at Rochester. From this point it diminishes westward, and entirely 
disappears, as before mentioned, leaving the two limestones in contact. 
At Rochester and other places, the mass embraces a band of purple shale three or four 
feet thick, differing from that around it only in color. A few feet higher it is marked by a 
line of black, containing Graptolites. This portion is usually exceedingly brittle, apparently 
from the presence of carbonaceous matter. 
About six feet from the top of the green shale, are two, or in some places three, thin bands 
of calcareous matter, three or four inches thick, composed entirely of shells, principally the 
Atrypa hemispherica, which still preserves its original lustre, giving the whole mass a pearly or 
silvery hue. Very beautiful specimens are obtained from these bands, presenting an entire 
surface of the shells ; it is rarely, however, that perfect shells are found. They seem to have 
been drifted together in great numbers, and the valves are often separated. Like the Penta¬ 
merus below, they indicate a period when the tranquillity of the waters was disturbed, or 
when from some other cause this portion had become the margin of the sea. 
Throughout the deposition of this shale, however, the ocean appears to have been quiet: 
the mass consists of finely levigated mud, which would not have been deposited during a pe¬ 
riod of much disturbance. The condition of the fossils usually indicates a very quiet state of 
the waters; when imbedded in the shale, every part is found in great perfection. The deli¬ 
cate structure of the Graptolites, which apparently the least agitation would destroy, is gene¬ 
rally preserved very entire. 
The upper part of this mass is often deeply stained externally by oxide of iron, though the 
bed is entirely wanting. The stain of iron prevails more or less throughout, penetrating all 
the slaty divisions, and often forming a thin enamel over the green surface. I have seen no 
minerals in this rock, except in the thin calcareous bands, where sulphuret of iron, sulphate of 
