28 NEW YORK STATE MUSEUM 



of the sandstone in close proximity to the Precambric. Conglom- 

 erate was not observed here but dolomitic and calcitic beds even 

 within a few feet of the Precambric are more prominent than at 

 Edinburg. The dip varies considerably but is mostly toward the 

 southeast and apparently some thirty or forty feet of Potsdam is 

 present. The sandstone containing rotten calcareous beds may be 

 seen along the road due east of Northville. Typical sandstone 

 outcrops one and one-half miles southeast of Northville and also 

 in the south end of Giff ord valley where the dip is about 4 degrees 

 southwestward. 



On Bunker hill the sandstone is finely exposed in a quarry where 

 a thickness of about fifteen feet of the rock is shown. Certain 

 beds are beautifully ripple-marked and occasionally there are yel- 

 lowish to reddish-brown iron stained layers. The dip is 10 degrees 

 southeast. 



On Roberts creek, two and one-half miles southwest of Cran- 

 berry Creek village, and also three-fourths of a mile due south of 

 this locality there are good outcrops of typical thin-bedded Potsdam 

 near the Precambric and dipping five degrees to the southwest. 

 The thickness is estimated at about forty feet. 



From the above description it is seen that the Potsdam varies in 

 thickness from nothing to about fifty feet and that the lithologic 

 character changes rapidly in short distances. 



THERESA FORMATION 



The Theresa formation constitutes a perfect transition series be- 

 tween the Potsdam sandstone and the Little Falls dolomite. It was 

 named by Cushing three years ago in the Thousand Islands region 

 and, as modified from its original usage, it now applies only to 

 the Potsdam-Little Falls passage beds so commonly present in New 

 York. Lithologically the formation consists of alternating beds of 

 pure, gray, fine-grained sandstone and bluish-gray, fine-grained, 

 dolomitic limestones. The beds are seldom more than a foot thick. 

 The sandstone layers bear a remarkable similarity to the underlying 

 Potsdam, while the limestone layers are practically indistinguish- 

 able from those of the overlying dolomite formation. Cavities, 

 lined with calcite and quartz crystals and similar to those of the 

 Little Falls formation, are frequently found in the lim.estone beds. 

 The sandstone beds often, and the limestone beds sometimes, show 

 cross-bedding and ripple marks. Toward the base of the forma- 

 tion the limestone layers are occasionally sandy and contain pebbles 

 up to half an inch in diameter. 



