110 ON EROSIONS OF THE EARTH'S SURFACE. 



strata are crossed, viz : the Medina sandstone (lowest), the Clinton group next, 

 and the Niagara group highest. It is evidently the different hardness of these 

 groups, or varying facility of decomposition, that have produced these falls. In 

 such a case we have indubitable proof that the river has done the work. These 

 falls at first were but one, and at this time the lower ones are gaining probably 

 upon the upper one, and the time may come when they will unite again. 



A few miles east of the mouth of Genesee river, the Irondequoit creek empties 

 into the lake, flowing ,in a deeper channel than the Genesee. But it passes 

 through deposites of sand and gravel, and Professor Hall suggests, with much 

 probability, that the Genesee once ran in the channel of the Irondequoit. But 

 when that was filled with deposits of sand and gravel, and the region elevated, 

 the Genesee was turned westward and compelled to cut out its present rocky bed, 

 like the Niagara, of about seven miles in length. I am not able to state the 

 amount of descent in the three falls : my aneroid gave 107 feet for the height of 

 the largest at Rochester. 



3. Gulf of the Genesee River between Mount Morris and Portage. — We have at 

 this place a still more remarkable example of a postdiluvian cut in the rock in 

 consequence of the filling up of the old channel. From Rochester to Mount 

 Morris the Genesee river occupies for the most part a broad valley with no gorges 

 of importance. But at Mount Morris it issues from high walls of Devonian rocks 

 (the Portage and Chemung groups), and if we follow its course upwards to Portage, 

 fourteen miles, we shall find its bed to be a deep cut in solid rock much of the 

 way, with nearly perpendicular walls, but sometimes sloping so as to admit narrow 

 meadows. It is not till you get considerably above St. Helena that you come to 

 cataracts. In Portage, within a distance of less than two miles, are three falls, 

 whose whole amount, with the intervening rapids, by my aneroid barometer, is 

 370 feet. The falls are said to be 60, 90, and 110 feet. Am. Journ. Sci., vol. 

 XVIII., p. 209. The depth of the gorge in some places is not less than 350 feet, 

 and its width only about 000 feet, the banks being nearly perpendicular. Were 

 the quantity of water in Genesee river as great as in Niagara river, the scenery on 

 the former at Portage would be more imposing, on account of the greater depth of 

 the gulf. As it is, it is well worth a visit, now easily made, as the railroad from 

 Hornellsville to Attica crosses the river a little below the middle falls, if I rightly 

 recollect. 



In passing from Portage at the south end of this gorge, and near the upper falls, 

 towards Nunda, we rise upon a deposit of sand and gravel of great depth, accord- 

 ing to my measurements, 235 feet thick at the head of this upper fall. This 

 deposit extends to Nunda, which place, by the aneroid barometer, lies 135 feet 

 below the Genesee at Portage. From Portage a canal extends through Nunda to 

 Mount Morris, following down from the former place a tributary of the Genesee. 

 I fully agree with Professor Hall in his suggestion, that this was once the bed of 

 the Genesee river: which being filled with drift and terrace materials, while the 

 country was yet beneath the ocean, was compelled, upon the emergence of the 

 land, to find a new tortuous channel more to the left. The result has been that it 

 has cut out its present channel ; that is, the deep gorge between Portage and 



