340 R. S. TARk — LAKE CAYUGA A ROCK BASIN. 



tance and then turns abruptly northwestward after having received the 

 waters of Oneida, the easternmost member of the series. Two or three 

 smaller lakes on the western margin of the district empty into Ontario 

 through the Genesee river. 



As is pointed out by Dr Lincoln,* Professor Brighamf and others, 

 these lakes are situated in valleys cut in the northern slope of the New 

 York portion of the Appalachian plateau, which is composed mainly of 

 Devonian rocks. Near the head of the lakes there is an irregular divide, 

 high and diverse in topography, which separates the waters of the Che- 

 mung and Susquehanna from the north-flowing tributaries to the Saint 

 Lawrence system. This plateau frequently attains an elevation of over 

 2,000 feet, while the valley bottoms are, in two cases at least, below sea- 

 level. Dr Lincoln X brings out the interesting fact that the plateau is 

 here axially depressed, that is, while the upland is everywhere a plateau, 

 the upland itself, and the valleys in it, are lower in elevation near the 

 center of the district than on the margins. This is brought out by him 

 in a diagram which shows a general sagging of the surface in the center 

 of the district, and in this central portion the two large lakes, Seneca 

 and Cayuga, are in notable depressions, much more pronounced than 

 those on either side. 



Topography of the central Area. — The topography of the central area is 

 very typical of the New York-Pennsylvania plateau. The valleys of 

 Cayuga and Seneca are broad, . gently sloping, smooth sided, linear 

 valleys, in all respects typical of mature rivers. One might well con- 

 clude from the form alone that they are nothing more than mature pre- 

 glacial river valleys. They wind in and out, their southern headwater 

 portions are markedly more diverse than their northern continuations, 

 and as the northern end is approached the hills become lower and lower, 

 until finally they are replaced by a comparatively low plain. Moreover, 

 there are tributaries of mature form, well pronounced both in the southern 

 portion and on the plateau at one side. 



Drainage. — In most cases the creeks flowing into these lake valleys 

 enter through gorges as a series of l:apids and waterfalls — in one case, 

 Taughannock, with a single fall of 190 feet. They drop down, as it were, 

 over the valley slope into the lake. Ordinarily a few miles from their 

 mouths these streams flow in well developed, mature, pregiacial valleys, 

 so that on the main valley side and at their mouths alone the gorges and 

 falls exist.§ In one or two cases, however, the stream enters the lake 



* Am. Jour. Sei., vol. xliv, 1892, p. 290. 

 t Ball. Am. Geograph. Soc, vol. xxv, no. 2, 1893, p, 3. 

 X Am. Jour. Sci., vol. xliv, 1892, pp. 290-293. 



gThis is a reversal of the usual condition and is just the opposite from the river system as de- 

 scribed in text-books, in which the upper course is torrential. 



