78 
GEOLOGY OF THE FOURTH DISTRICT. 
The other forms have never been met with, though they are abundant farther east. The 
Lingula oblonga is distinguished from either of those just described, by its straight parallel 
sides and abruptly rounded base, and also by a series of longitudinal striae, not seen in either 
of the others. 
Sections illustrative of the order of succession among the members of the Clinton group, and 
its connection with the rocks above and below. 
These sections illustrate the connection of the Clinton group with the overlying and under¬ 
lying rocks, and also show its variations in character and development at different points in 
the district. The same order of the strata, and nearly the same degree of development, occur 
in the eastern part of the district, as that exhibited in the section on the Genesee. The sec¬ 
tions of this part are omitted, as not showing, in the same conclusive manner, the connection 
of the group with those above and below. 
20. 
1, 2, 3. Upper, Middle and Lower falls. 
g. Shale of Niagara group. 
( f. Upper limestone of Clinton group. 
Clinton group. J Upper green shale. 
| d. Pentamerus limestone, 
t c. Lower green shale. 
6. Grey band, upper part of Medina sandstone. 
a. Medina sandstone. 
The section (No. 20) exhibits the order of superposition in the strata forming the Clinton 
group, a part of the Medina sandstone, and the lower part of the Niagara group. In the 
banks of the Genesee, this order of succession is as clearly seen as in the woodcut. The 
green shale rests directly upon the upper part of the Medina sandstone, and extending beneath 
the Pentamerus limestone, forms, together with that rock, the Middle fall, twenty-five feet. 
The limestone, however, is the resisting stratum, and about one half the lower part of the 
shale forms a slope extending towards the Lower fall. 
Above the Pentamerus limestone, the upper green shale forms the bed of the river nearly 
to the Upper fall, the limestone above producing rapids in the stream for a short distance. 
The whole height of the upper fall is over the shale and lower beds of impure limestone of 
the Niagara group. 
