248 
GEOLOGY OF THE FOURTH DISTRICT. 
Localities of Superposition. 
The junction of this group with the one above is nowhere so well defined as the instances 
already illustrated. The terminating mass of the Portage group is usually a heavy thick- 
bedded sandstone, marked by the presence of vertical fucoids. Above this we find shales 
and shaly sandstones, differing in some degree from those below, and manifesting the presence 
of Delthyris and Atrypa. 
The channel and lateral streams of the Genesee above Portage present a continuation of the 
series almost completely. The order is illustrated in the section passing through Allegany 
county (PI. xi.). The Portage sandstone is succeeded by olive shaly sandstone and shale, 
and this by black micaceous slaty shale, with septaria; to this follow shales and coarse sand¬ 
stones, with fossils of the Chemung group. 
On Lake Erie the thick-bedded sandstone terminating the Portage group is succeeded by 
coarse shales containing fossils of the Chemung group. This junction may be traced south 
of Laona. Again, on the Chautauque creek the sandstone is scarcely defined, and there is 
little change in lithological characters from one to the other. 
At many intermediate points the change here indicated can be readily observed, but the 
absolute contact of the two groups is rarely visible. 
The greatest development of this group, and the point where it is most distinctly sepa¬ 
rated from the next above, is on the Genesee river. Although it is obviously marked farther 
east, there does not everywhere occur the thick-bedded sandstone at the termination; and it 
would appear from the investigations in the Third District, that there are not the distinctive 
characters in the fossils which are so prominent throughout the western part of the State. 
Again, as we approach the western limit of the State, we do not find the distinctive features of 
the two groups so well marked. The lower part of the Chemung containing fossils typical 
of that group, possesses more of the lithological nature of the Portage group than it does 
farther east. Still farther west, where I have examined these rocks, in Ohio and Indiana, 
there is a closer resemblance in the lithological nature of the strata of the two groups, and 
the change is attended by a great diminution in the number of fossils in the higher one. In 
this part of the country, no distinction has been made between different parts of the mass ; 
neither is it there of much practical importance. 
The facts in relation to this group, and its connection with the one above, are not peculiar 
to these, but appertain more or less to all our sedimentary deposits. The distinction, therefore, 
between groups where the whole series is sedimentary, cannot be relied upon over extensive 
districts ; and all such subdivisions can be considered no more than those of convenience, which 
in some places are strongly marked and readily identified, while in others the lines of de¬ 
marcation are obscure, and the distinguishing characters fail in a greater or less degree. 
Whenever we have an opportunity of comparing a complete tabular list of the fossils, with 
their geographical distribution, we shall find, that what in one portion of the country are 
limited in their stratigraphical range, are nevertheless in other places not thus restricted; 
