228 
GEOLOGY OF THE FOURTH DISTRICT. 
the annual reports, to distinguish this part of the formation ; and since this is a good point for 
investigation, it is described as a subordinate part of the Portage group. 
In the lower part of this subdivision the shales consist of alternations of green slaty and 
sandy shale with black slaty shale, one or two thin courses of sandstone occurring in the space 
of four or five feet. As we ascend, the arenaceous matter increases in quantity, the layers are 
thicker and more numerous, and the shale forms distinct alternations of black and green, 
often many times in succession, within the space of fifty feet. Towards the upper part the 
courses of sandstone become too thick for flagstones, and the shale is in thicker masses than 
below. 
These characters, however, which are sufficiently obvious in the gorge of the Genesee, are 
not constant for any great distance in either direction. Toward the east the arenaceous strata 
augment in a great degree, to the exclusion of the shales ; while in a westerly direction the 
sandstones are constantly disappearing, and the proportion of shale constantly increasing. At 
the western limit of the State, along the shores of Lake Erie, the Cashaqua shale is succeeded 
by a thick mass of black shale, and this is again succeeded by alternations of green and black 
shales for several hundred feet upwards, the flagstones having entirely disappeared from 
nearly the whole thickness. With the absence of sandy strata and the augmentation of shale, 
a few fossils, which were rather sparingly seen along the Genesee and in the eastern part of 
the district, become more numerous, and form a distinguishing feature of the rock. 
3. Portage Sandstones. 
The thick-bedded sandstones at Portage form the terminal rocks of the group. These are 
well exposed in the deep gorge below Portageville, where the perpendicular cliffs rise to the 
height of three hundred and fifty feet. The upper part consists of thick-bedded sandstone, 
with little shale ; while below, the sandy layers become thinner, with more frequent alterna- 
nations of shale. The thick-bedded character of the sandstones, and the presence of fucoids 
passing vertically through the strata, induced the separation from the rocks below, where the 
characteristic species of the same genus lies horizontally upon the surface of the strata. The 
lithological character of the sandstone, and the presence of the vertical fucoid, hold uniform 
over a considerable extent; and the presence of the latter alone is often sufficient to decide 
the position of the rock, where it is but slightly exposed. 
Toward Cayuga lake, the increase of arenaceous strata below has rendered a distinction 
between the two of little importance ; and farther west, where the Gardeau division has be¬ 
come very shaly, the higher division has taken the character which the former one has on the 
Genesee, being composed of alternating thin-bedded sandstone or flagstones and shale. 
Along the western limit of the State there are but few points where the thick-bedded sand¬ 
stone, like that at Portage, can be seen. One of these is at Laona, and another, which is the- 
terminating mass of the group, is at Shumla. Along the lake shore it appears at several 
places ; and in the excavation for the New-York and Erie railroad, west of Fredonia, the same 
rocks are exposed. 
