68 
GEOLOGY OF THE FOURTH DISTRICT. 
previously existing in the mud, and filled by the subsequently deposited matter. These are 
of more common occurrence in the rocks of the Portage group, where they will receive illus¬ 
tration. Whatever may have been their origin at the period of the Clinton group, they appear 
to be widely extended. When in Ohio, Dr. Locke showed me specimens of precisely similar 
character, and from the same position in that State. They were likewise associated with thin 
layers, containing numerous crinoidal joints, identical with those so abundant in the same rock 
at Medina, Lockport and elsewhere. The influences operating upon the bottom of the ocean 
at these two distant points, are thus proved to have been the same. 
Organic Remains of the Clinton Group. 
This group is for the most part well marked by its fossil contents, and presents an inte¬ 
resting series of organic remains. From the varying character of its lithological productions, 
however, the fossils are not so constant in their occurrence as in other rocks ; and if the group 
be examined at distant intervals, scarcely a fossil will be recognized as common to the two 
localities. A portion of the mass at one place well marked by certain species, may be desti¬ 
tute of fossils at another, or present, for the most part, a different assemblage. It becomes 
very evident, therefore, that lithological development has had much to do with the occurrence 
and nature of the fossils, as well as their abundance. It has already been seen that the 
products of the group fail in a western direction, and at the same time the character of the 
fossils changes in an equal degree. On the Niagara river, the only fossil which can be said 
to continue from the eastern extremity of the district, is the peculiar Crinoid of which we 
find the numerous joints ; and these indeed may have been drifted to their present situation. 
This crinoidal joint (fig. 5, woodcut No. 16), is abundant in nearly all situations, and though 
apparently an insignificant object, the observer will find it of the greatest use in tracing the 
lower limestone through the district. The same likewise appears in Ohio in equal abundance. 
In the Third District this group is much more extensively developed than in the Fourth, and 
its products are even more variable. Its most obvious fossils are marine plants, or Fucoids, 
which everywhere mark its shales and shaly sandstones; but in the Fourth District, rarely 
more than one species is seen, and this is common in the thin layers of sandy shale succeeding 
the lower green shale. This species closely resembles one in the Hudson river group, and 
bboth ear much analogy to the F. antiquus as figured by Hisinger. The following figure 
is from a nearly perfect specimen as usually seen, and it may be considered the lowest fossil 
of the group on the Genesee river. 
