GEOLOGY OF OGDENSBURG 25 



Herkimer county, the irregularity is least, and there, in fact, the 

 surface is quite smooth. Because of abundant rock exposures the 

 phenomena are much better exhibited in the Thousand Islands 

 region than elsewhere, and were described at length in the report 

 on that district.^ These irregularities amounted to differences of 

 altitude of at least lOO feet in the various parts of the Precambrian 

 surface, the limestones and weaker schists forming the depressions, 

 and the granites and quartzites projecting as the knobs and ridges. 

 This irregularity in surface is just about equal to the greatest 

 thickness of the Potsdam sandstone, the first Paleozoic formation 

 to be deposited upon it, so that the sandstone is very thin on the 

 higher parts of this surface, and the very highest may even project 

 through it. 



Since the Alexandria Bay quadrangle corners on Brier Hill at 

 the southwest, it is to be expected that these phenomena will be 

 repeated in the Ogdensburg region. But they are by no means so 

 well shown. The Potsdam seems no thicker here than there, and 

 the irregularity in Precambrian surface seems somewhat greater 

 here. On the Brier Hill sheet, near the river and 3 miles south- 

 west of Morristown, is a small exposure of Precambrian rock, a 

 hard, pyroxenic Grenville quartzite, entirely surrounded by ex- 

 posures of the Theresa formation, nearly 40 feet above the summit 

 of the Potsdam. This is the only observed case within the limits of 

 the map of Theresa beds on Precambrian, but the Potsdam is very 

 thin, not over 20 feet thick, on the Macomb granite, and it also 

 seems to be thin as the Canton sheet is approached. On the other 

 hand, through Depeyster where it lies on Grenville limestone, it 

 seems at least 80 feet thick. 



CAMBRIAN (OZARKIAN) FORMATIONS 



Potsdam sandstone. The Potsdam occurs in two different 

 situations, the one as a continuous belt of sandstone bordering the 

 Precambrian rocks on the north, and the other as outliers of sand- 

 stone, within the Precambrian area. As the rock characters differ 

 somewhat in the contrasted situations, they are best described 

 separately. 



The belt which borders the crystallines is a very irregular one, 

 because of the irregular surface on which it was deposited. The 

 thickness varies greatly, only the upper beds overriding the Pre- 



iN. Y. State Mus. Bui. 145, p. 54-60. 



