14 NEW YORK STATE MUSEUM 



in determining just what the nature of the original rock may have 

 been. 



Limestone. An unusually large proportion of the exposed 

 Grenville on the Ogdensburg sheet consists of limestone. A great 

 belt of limestone comes on to the sheet at its southwest corner and 

 the rock is magnificently exposed along the east side of Mud lake, 

 and runs northeast from there in a prominent belt more than a 

 mile in width, its western portion overlapped and covered by Pots- 

 dam sandstone. The Grenville limestone is a weak rock in its 

 resistance to erosion, tends to form low grounds and to be heavily 

 covered with soil, so that outcrops are scarcer than in most of the 

 Grenville rock belts, that is, belts of other rock. To the east of this 

 belt are two narrower belts, one east and one west of the Oswe- 

 gatchie, but in these the limestone is less pure and alternates with 

 thin bands of quartzite and of schists. The areal distribution of the 

 limestone suggests a series of folds pitching to the northeast. 



Frequent knobs of granite are found cutting through the lime- 

 stone, especially in the belt which borders the Oswegatchie on the 

 east. These are more resistant to erosion than the limestone and 

 form the more prominent outcrops in the limestone belts; in fact 

 the great majority of the limestone outcrops are found on their 

 borders. These granite knobs in every case consist of white granite, 

 though the granite masses elsewhere are red. It seems to be the 

 same sort of bleaching of the granite at limestone contacts as has 

 been described in the Thousand Island region. 



Quartzite. There is no considerable belt of quartzite in the 

 Grenville series of the Ogdensburg quadrangle, though there are 

 narrow belts of it involved with impure limestone in alternating 

 layers, too narrow to map on this scale as separate from the lime- 

 stone. There is still more of it in narrow bands interbedded with 

 amphibolite and rusty gneiss, all too narrow to map separately and 

 hence mapped simply as Grenville schist. The quartzite is thin 

 bedded, is really quartz schist, and exhibits everywhere minute 

 folding and puckering, showing these features much better than 

 any other rock in the district (plates i and 2). 



Schists. The larger portion of the area which is mapped as 

 Grenville schists is occupied by the dark-colored rock conveniently 

 known as amphibolite, and made up chiefly of feldspar and horn- 

 blende, often with some pyroxene as well and commonly with black 

 mica in addition. When the mica becomes prominent the rock 

 cleaves readily and becomes rather weak. From these thinly 



