GEOLOGY OF OGDENSBURG . 23 



Folds. If the Grenville rocks are folded, as they assuredly are, 

 the folding must be of the close and overturned type in order to 

 give rise to dips so prevailingly in one direction as they in general 

 present in northern New York. The general northwesterly dip in 

 this district, for example, seems to us to imply that the folds are 

 all tipped over toward the southeast, so that the dips on both limbs 

 incline to the northwest. The distribution of the limestone on the 

 Ogdensburg quadrangle suggests that we are dealing with one great 

 bed of limestone, pinched into a syncline, the two sides diverging 

 southward, meeting and joining on the north, and hence indicating 

 that the syncline pitches to the southwest, and that the inclosed 

 amphibolite is younger. But the mapped area is too small, the 

 adjacent area too little known, and the structure too complicated, 

 to warrant anything more than this suggestion, to be conhrmed or 

 disproved by later work. 



The very contorted and complex crumpling and plication which 

 certain beds of the Grenville show, is of itself indication of larger 

 scale folding. Of all the rocks of the group the thin-bedded quartz 

 schists best exhibit such plications. Plates i and 2 are from photo- 

 graphs of such quartzite, 2 miles south of Kindrews Corners. 



Close folding is also shown in the combination of thin-bedded 

 impure limestones, pyroxene schists and garnet gneisses which 

 occur north of the Dekalb granite on the east margin of the Ogdens- 

 burg sheet. 



PALEOZOIC ROCKS 



General statement. The Paleozoic formations found within the 

 mapped area are the Potsdam and Theresa formations, of upper 

 Cambric age, and the Tribes Hill and Ogdensburg formations, of 

 Lower Ordovician (Beekmantown) age. They rest with marked 

 discordance on the irregular surface of the Precambrian rocks. 

 There is some evidence that still another formation will have to be 

 separated from the upper part of the Theresa, including what we 

 have mapped as the Heuvelton sandstone lentil of the Theresa 

 formation, the beds between this sandstone and the base of the 

 Tribes Hill, and the 30 feet (more or less) of sandy beds just under 

 this sandstone. But the evidence is not yet decisive, and lithologi- 

 cally the beds are much like those of the Theresa, with which they 

 form a convenient lithologic unit. 



It is quite possible that beds of later Ordovician age, Chazy, 

 Black River, even Trenton and Utica beds, may have once been 

 deposited in the district, but if so, erosion has left no trace of them. 



