24 NEW YORK STATE MUSEUM 



of a mappable belt of Potsdam on the north side of this crystalhne 

 ridge depends on whether the overlapping Theresa has been suf- 

 ficiently stripped away to reexpose it. 



It must be observed that the concealing drift is not in the nature 

 of a morainal ridge, but that the outcropless zone is mostly low 

 and extensively swampy, giving the impression of a refilled valley. 

 That a belt of Potsdam of considerable thickness and of resistant 

 character might lie buried in such a valley is shown by its occur- 

 rence along the shores of Black lake on the Brier Hill quadrangle, 

 with the Precambrian and Theresa rising above on either side. 

 (Compare the alternative cross sections in figure 4.) There is 

 even a possibility that the Potsdam underlies the drift throughout 

 nearly all the area mapped as undifferentiated Precambrian, with 

 the crystalline outcrops merely fenestrated through it. (Com- 

 pare figure 3.) The deep wells at Mr Crary's and Mr Sizer's 

 (localities 84' and 88) both found sandstone, and there are abund- 

 ant drift blocks of it all over the district. New outcrops may 

 reward further search both here and in the blank zone, and well 

 records will be of incalculable aid. 



Theresa Mixed Beds 



The series of alternating beds of blue dolomitic limestones and 

 rather weak grayish white or creamy sandstones that succeed the 

 Potsdam is the Theresa formation of Professor Cushing.^ On 

 the Canton quadrangle these beds are seldom well exposed, and 

 are nowhere seen in contact with the Potsdam sandstone. Their 

 thickness here is therefore unknown though probably somewhere 

 between 50 and 100 feet. The well at Mr Cunningham's on the 

 Ogdensburg state road one mile west of our limits (Woodbridge's 

 Corners) starting below the summit of the Theresa, which crops 

 out in the adjacent road-gutter, was still in these mixed beds at 

 50 feet depth. 



About 20 feet of the top beds, in contact with the overlying Heu- 

 velton sandstone, show on the west bank of the Raquette above 

 the Sissonville dam (localities 31 to 33). This is a short distance 

 below Potsdam village, with no exposure of Potsdam sandstone 

 known between these Theresa outcrops and those of the Pre- 

 cambrian gneisses in the falls at that village (see figure 3). It 

 is then 3 miles farther up the Raquette, without intervening out- 



' Geol. Soc. of Amer. Bui. 19, p. 160. N. Y. State Mus. Bui. 121, p. 12. 



