52 NEW YORK STATE MUSEUM 



cott, on various occasions. Since Paleozoic deposition ceased in 

 the region, and it became anew a land area, it has been de- 

 capitated by the prolonged erosion which has followed. The 

 Paleozic cover has been entirely worn away from the heart of the 

 Adirondacks, these rocks now appearing as a fringe about the 

 district. In the past they extended farther in than they do now ; 

 the erosion of the future will remove them from districts which 

 they now cover, increasing the extent of the area in which the 

 older rocks form the surface exposures. The conditions along 

 the edge of the fringe, so far as they dift'er, depend not only on 



Fig. 10 A reproduction of fig. 3. to illustrate the supposed condition in the Adiron- 

 dack region at the close of the Utica period. Subsequent erosion has worn off the 

 region down to the line AB, reexposing the pre-Cambrian rocks over a wide area, 

 and leaving the Paleozoic rocks confined to the flanks of the region. 



Fig. 11 The region after the wearing away of the upper portion, the line AB of the 

 previous figure forming the surface. Erosion has cut more deeply on the right hand 

 side than on the left. On the right the basal sandstone is exposed, lying uncom- 

 formably on the old surface, with the overlying limestone appearing farther to the 

 right. On the left the limestone appears lying on the old surface, erosion having 

 nowhere cut deeply enough to expose the underlying sandstone. 



possible differences in the conditions of original deposition on 

 different sides of the region, but also on the depth to which 

 erosion has since cut. If we should assume that the Potsdam, 

 Beekmantown, Trenton and Utica formations were successively 

 deposited all about the Adiroiidacks, progressively overlapping 

 toward the center of the district, then it is theoretically quite 

 possible that we might today find the Potsdam resting on the pre- 

 Cambrian here, the Beekmantown there, and the Trenton or Utica 

 elsewhere, for the reason that more rock had been removed by 

 erosion in the former case than in those following, that in them 

 erosion had not yet cut deeply enough to bring the edge of the 

 Potsdam to daylight from underneath the overlying and over- 

 lapping Beekmantown [fig. 10, 11]. 



Within the map limits the Potsdam sandstone is wholly absent, 

 the Beekmantown resting on the old, pre-Cambrian surface. The 





