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NEW YORK STATE MUSEUM 



of the sandstone base is exposed, owing to solution of the limestone 

 beneath. A sketch of the relations here is given in figure I, the 



Fig. i Potsdam contact on Grenville limestone, just 

 north of lower bridge at Theresa, showing the sloping 

 Grenville hillside on which deposition took place, and 

 the sand-filled cracks in the limestone. 



arrow showing the camera position for plate n. It is at this locality 

 that the best examples of sand extending down into widened joint 

 cracks in the Grenville limestone were seen. At the east end of the 

 outlier the limestone is cut out by granite gneiss, whose summit is 20 

 feet above the top of the sandstone, hence terminating the outlier in 

 that direction. Of course the full original thickness of the sand- 

 stone is not present in the outlier, but only the mere basal portion, 

 and formerly the sandstone extended over the granite as well. 



Another interesting contact occurs along the Potsdam front, 2^2. 

 miles northeast of Theresa. From a previous northeasterly trend 

 the front here turns and for a mile and more runs northwest across 

 the strike, crossing a prominent granite ridge and then dropping 70 

 feet in level into a limestone valley. Near the turn the contact 

 sketched in figure 2 is shown. A low knob of ferruginous, quartz 

 schist projects upward into the Potsdam to the amount of 20 feet. 



Fig. 2 Potsdam contact on Grenville quartz schist, 

 2J miles northeast of Theresa. The much contorted 

 and steeply dipping schists constitute a ridge over 20 

 feet high around which the Potsdam was deposited. 



The Potsdam here is more evenly bedded than in the cases described 

 at Theresa, the bedding abutting squarely against the sides of the 

 knob. Its small size as compared with the long ridge slopes of the 

 other contacts is thought to be the chief reason for this difference. 



