190 
CATSKILL DIVISION. 
black slate to a thick-bedded sandstone, that it is useless to attempt to draw division lines 
between the lower and upper strata. Drawing-, then, an arbitrary line along the strata, 
near to a plane where the Genesee slate seems to terminate, or where the rock has ceased 
to be a decided slate, or has become a thin flagstone, and then including in the Portage 
group the whole mass above as it exists at Portage, we believe the thickness is not far from 
twelve hundred feet. But Mr. Hall, who has had better opportunities for determining this 
question, has estimated it at one thousand feet. It must be recollected, however, that the 
cliffs from Mountmorris to Portage maintain an elevation of three hundred and fifty, and 
perhaps four hundred feet in some places, and that the dip is at the rate of about fifty feet 
to the mile. 
Gorge in the Portage group. At Mountmorris the Genesee river issues from a gorge, 
which is remarkable both for depth and length. It is in this deep cut, made by the river, 
that these rocks may be observed to the best advantage. At the bridge near Mountmorris, 
steep and even perpendicular cliffs bound and shut in the river on both sides. These cliffs, 
in consequence of the increased thickness of rock, rise up above the river three hundred 
and forty feet on both sides. With these formidable banks on either hand, the river wends 
its way from Portage. A part of the distance there is space for a road ; but the descent to 
the river is practicable in a few places only, while most of the distance it is totally out of 
the question. The slate is the only rock which forms the cliffs for four or five miles towards 
Mountmorris ; and the character of the mass, as indicated above, is preserved. The first 
change which appears, is produced by an increase of silex or sand. The layers are still 
thin ; but in the place of argillaceous layers, thin undulating shaly ones appear. If we 
trace the changes as we proceed towards Portage, we find the sand still increasing, and the 
strata becoming thicker, till finally at Portage the formation has become a thick-bedded 
sandstone. It is a gray fine-grained rock, and works well under the chisel; and, when 
wrought, it is durable. Some exceptions, however, ought to be made : the masses must 
be free from slate, in order to resist the action of the weather. 
§ 2. Portage, ithaca and chemung groups of the central counties of new-york. 
The Chemung group is made up of flags and slates, whose beds are thinner than those 
of the Portage group upon which they rest. The flags are gray, olive and brown, with 
impure calcareous bands of fossils ; the shales are green and olive, but sometimes black. 
These forms of mineral matter are arranged without order. The stratification is usually 
Fig. 31. 
