426 NEW YORK STATE MUSEUM 



erosion surface of different slope and origin. This has its rise 

 in the very resistant character of the Precambric rocks when 

 compared with the overlying paleozoics, specially in districts 

 where the Potsdam sandstone is thin or is absent, as it is on the 

 west and south. During a cycle of wear, these weaker rocks are 

 stripped away from the underlying Precambric, whose old ero- 

 sion surface thus reappears, and tends to maintain itself for a 

 time, owing to its extraresistant character [fig. 8]. Thus is pro- 

 duced a considerable strip of Precambric rocks on these two 

 sides of the region, with an even hilltop line, which comes to this 

 level, rather than to that of the Cretaiceous peneplain'. It, how- 

 ever, slowly rises to meet, and insensibly merges into that. 



Quite a number of monadnocks, as residual hills which are not 

 worn down to the general peneplain surface are called, exist in the 

 region, their summits reaching a few hundred feet above the gen- 

 eral hilltop level. They are mot sufficient in number to obscure 

 the general level, though they do somewhat disguise it. - 



In the same districts the later base level is also indicated by the 

 rather broad valleys and their comparatively uniform levels. It 

 is also observable that, in passing toward the heart of the region, 

 the valleys are deeper cut, in other words', that the vertical in- 

 terval between the two plains increases, indicating that the Cre- 

 taceous peneplain was tipped when elevated, and that the uplift 

 was greatest on the northeast, so that it is now canted consider- 

 ably to the west and slightly toward the south. 



In the eastern Adirondacks the above features can not be satis- 

 factorily made out [pi. 16]. There is little concordance in the 

 summit elevations, so that either the district was not reduced to 

 a Cretaceous peneplain, or else that surface ha® been dislocated, 

 and given varying altitude, by subsequenit mbvementsi. Possibly 

 both may be true, and there is some evidence which points to the 

 dislocation having actually taken place. 



Recent uplift has affected the entire Adirondack region, in 

 common with a much larger area, and this uplift has been greatest 

 on the northeast. It has amounted to at least 400 feet at the 

 south end of Lake Champlain, and at least 550 feet at the north 

 end, and to 250 feet or more at the east end of Lake Ontario. 

 And these are minimum figures, which must likely be much in- 

 creased when the entire movement is taken into consideration. 



