60 NEW YORK STATE MUSEUM 



face indicated anywhere within the map limits. The surface seems 

 to have been worn down to that of a gently sloping plain, above 

 whose level, occasional low, rounded hills arose. 



Though this evidence is meager in amount, and needs corrobora- 

 tion from adjoining districts, it seems specially important in view 

 of the fact that Professors Kemp and Smyth, and the writer als'o, 

 have found evidence to show that, in the St LaAvrence and Cham- 

 plain valleys and vicinity, the surface on which the Potsdam was 

 deposited was considerably more uneven than this. In other 

 words, the surface on the south was worn down to a nearer 

 approach to base level than was the case farther north. 1 This 

 may be accounted for in part by the fact that, since the Potsdam 

 lies on the old surface there and the Beekmantown here, the sur- 

 face here was a land area and undergoing wear during Potsdam 

 time, or was a land area longer. The probable length of time 

 involved would however seem insufficient to account for all of the 

 observed difference. 



A more probable explanation would seem to be that we are 

 dealing here with a plain of marine erosion, and that its sub- 

 sidence was slow, giving opportunity for the cutting of a consid- 

 erable submarine terrace ; whereas on the north the rather rapid 

 subsidence during the Potsdam did not permit of the production 

 of such a smooth and well defined bench, the district passing be- 

 neath the sea practically as subaerial erosion had left it, except 

 for the removal of the weathered material. 



Slope of the surface on which the Beekmantown was deposited. 

 The Beekmantown rocks at Middleville are 200 feet in thickness. 

 Seven miles to the northeast they have nearly or quite disappeared, 

 and the Trenton rocks are overlapping on the pre-Cambrian. The 

 pre-Cambrian surface has risen 800 feet in the interval, while the 

 base of the Trenton has only risen 600 feet, the difference of 200 

 feet representing roughly the inclination of the surface on which 



"^Professor Kemp states in a letter that he has come to the same con- 

 clusions about the greater evenness in the south, away from the higher 

 hills of the interior, from observations at the " Noses " and in the southern 

 Hudson-Champlain valley. 



