254 
GEOLOGY OF THE FOURTH DISTRICT. 
This thinning out of some portions of the group is not fully compensated by increase of some 
other parts, (the green shale ;) and there is an evident diminution in thickness, and a constantly 
decreasing number of fossils, many species which are common farther east having disappeared, 
while scarcely any new forms are seen. Fossils, however, are numerous in many localities, 
and the same general appearance in their manner of aggregation and imbedding is preserved 
throughout. 
Besides the general diminution of sandy matter and the absence of the brownish sandstones, 
the red sandy micaceous shales have not been recognized : and they are nowhere visible in 
the ravines west of the Genesee river, so far as observed. 
The frequent alternations of shales and sandstones, the interlamination and mixture, all 
prove the operation of similar causes as in the group below. Although there is scarcely any 
definable difference in the products of the two, yet we see an immense increase in the number 
of species and individuals of organic forms. Judging, therefore, both from the condition of 
the deposition and the nature of the organic forms of the two, the lower part of the Portage 
group was deposited in a deeper ocean ; and although there were numerous oscillations toward 
the conclusion of the period, yet the conditions favorable to the production of numerous 
species of shells did not supervene till after the final deposition of the Portage sandstones. 
These remarks apply to the Fourth District; how far they are applicable farther east, I am 
unable to decide. It is very clear, however, that as we progress westward there is a dimi¬ 
nution of the Avicula, Cypricardia and allied forms, while the Brachiopodous fossils are at 
first more abundant, and finally themselves diminish also. It is also equally plain that the 
origin of the materials of this deposition was to the east of central New-York, and, probably, 
to the southeast, as is evidenced by the thinning of the deposits, and the diminution of sandy 
strata at the west. The increase of shaly strata in the same direction, which finally dimi¬ 
nish also, proves the same ; for this being longer suspended, was transported beyond the sand. 
We have here a corroboration of the same view as presented under the Hamilton group, viz. 
a position in eastern New-York, near the margin of this ancient sea, while towards the south¬ 
west we approach that part of more profound depth and greater distance from shore. The 
evidence continues throughout the Hamilton, Portage and Chemung groups ; for in all these, 
and the intermediate beds of shale and limestone, we find a constant diminution southwesterly. 
There is also another fact of the same import, viz: In eastern New-York we find, both 
in the Hamilton and Chemung groups, specimens of land plants, or such at least as did not 
grow beneath an ocean. These are rare in central New-York, one or two fragments only 
having been found ; and at the southwestern part of the State, and in Ohio, I have seen nothing 
of similar character. The inference naturally follows, that these were derived from land on 
the eastern margin of this ocean; and that some fragments floated westward, and were depo¬ 
sited with the sand and mud. Many of the thin sandy laminae throughout the district are 
often almost completely covered with small fragments of carbonaceous matter, apparently 
derived from terrene vegetation. These seem to have been comminuted fragments of vege¬ 
tables brought down by streams frorq the continent or islands on the east, and being spread 
