190 



CATSKILL DIVISION. 



black Blate 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 Blate seems to terminate, or where the rock lias ceased 

 to be a decided Blate, or has become a thin flagstone, and then including in the Portage 

 group the whole mass above as u exists at Portage, we believe the thickness is not tar from 

 twelve hundred feet. Bui Mr. Hall, who has had better opportunities for determining this 

 question, has estimated it at cue 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. 



G _ r c in tltc Portage group. At Mountmorris the Genesee river issues from a gorge, 

 which is remarkable both for depth and length. It is m this deep cut, made h\ the river, 

 that these rocks may he observed U> the best advantage. -\l the bridge near .Mountmorris, 

 i> and even perpendicular cliffs bound and shut in the river on both sales. These cliffs, 

 in consequence of the increased thickness "I rock, rise up above the river three hundred 

 and tony reel on both sides. With these formidable hanks mi either hand, the river wends 

 its way from Portage. A part of the distance there is Bpace for a mad : hut the descent lo 



the ri\er is practicable in a few places only, while i t of the distance it is totally out of 



the question. The date is the onlj rock \\ Inch forms the cliffs for four or five miles !,,« aids 

 M' mnt inoins : 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-hedded 

 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 nee from slate, in order to resist the action of the weather. 



§ 2. Portage, ithaca and chemvng groups ok the central counties of new-york. 



The Chemung group is made up of flags and slates, whose beds arc thinner than those 

 of the Portage group upon which they rest. The flags are gray, olive and brown, with 

 impure calcareous hands of fossils; the shales are green and olive, but Bometimes black. 

 These forms of mineral matter are arranged without order. The stratification is usually 



Fig. 31 



