GEOLOGY OF OGDENSBURG 2/ 



Point Comfort is a 25 foot thickness of Potsdam sandstone to the 

 river level, with the base not reached, a greater thickness of sand- 

 stone than was noted anywhere under the conglomerate on the 

 Alexandria sheet. It seems to vary in horizon, instead of being a 

 persistent band, and to exist in the sandstone after the fashion of 

 large channel-fillings. Above it there is a 20 foot thickness of 

 Potsdam sandstone, making altogether a 50 foot section of the 

 sandstone, with the base not seen. This is the thickest continuous 

 section of the formation that we have seen within the map limits. 



There is locally a little hard, red sandstone, mixed with the 

 ordinary white and buff rock, more particularly in the exposures 

 along Black lake, but there is no great amount of it anywhere. 



At the extreme summit of the Potsdam a thickness of some 10 

 feet of beds is slightly calcareous. The calcareous cement occurs 

 in rounded patches which have a light, pinkish gray tinge in con- 

 trast with the dead white of the rest of the rock, in fresh specimens. 

 But in the ordinary exposures this cement has been leached out, 

 leaving brown-stained spots in the rock. This brown-spotted sand- 

 stone is a characteristic feature of the upper beds. It is also 

 characteristic of the sandstones of the Theresa formation above. 



The Potsdam sandstone of the occasional outliers in the Pre- 

 cambrian rocks frequently shows phases quite unlike those of the 

 general mass of the rock in the continuous belt. In this respect it 

 accords closely with the outliers on the Alexandria and Theresa 

 sheets which have already been described.^ Occasional patches of 

 a red, very quartzitic sandstone were found, resting upon Grenville 

 rocks, usually limestone, and extending down into the limestone 

 along joint cracks which had been widened by solution. Other 

 patches of basal conglomerates of the Potsdam were found which 

 contained, along with numerous pebbles of Grenville quartzites, 

 pebbles of this red, quartzitic sandstone. There is no rock in the 

 Precambrian series which is anything like this red sandstone, and 

 the occurrences distinctly suggested a sandstone, older than the 

 ordinary Potsdam, which had largely been eroded away before 

 ordinary Potsdam deposition began. 



The Potsdam outliers on the Ogdensburg sheet, which are rather 

 remote from the main mass of the formation, are chiefly found on 

 the surface of the Grenville limestone belt which the Oswegatchie 

 follows for the first 4 miles of its course on the quadrangle. Two 



N. Y. State Mus. Bui. 145, P- 61--63. 



