l8 NEW YORK STATE MUSEl M 



the granite mass whose relations to other rocks on the Alexandria 

 sheet seemed to indicate a Laurentian age for it. Its termination 

 here is due to its disappearance under the Paleozoics, which have 

 bordered its sides all the way from Chippewa bay, and which here 

 meet over it. Its further extent to the northeast is purely prob- 

 lematical. It is, however, quite possible that the granite mass in 

 Macomb, east of Black lake, is a part of the same bathylith. The 

 Macomb granite runs out to the northeast in the same manner, 

 namely, by having the bordering Potsdam close around and cover 

 its prolongation. (It is, however, very like the rock of the main 

 Alexandria bathylith in character ; we regard it as quite probably 

 a part of it, and are disposed to class it as of Laurentian age, along 

 with the Alexandria rock. Even without including it, the Alex- 

 andria granite extends for 25 miles in a northeast-southwest direc- 

 tion, to which must be added at each end an unknown amount under 

 the Paleozoic cover which hides its continuation. This continued 

 extent enables us to state that, on the Brier Hill sheet, a granite 

 is present whose proved relations to other rocks strongly suggest its 

 Laurentian age. Evidence of similar sort on the Ogdensburg 

 sheet itself regarding the age of the granites there present, is lacking. 

 The Macomb granite, which we regard as very probably a part of 

 the Alexandria bathylith, is in large part a fine-grained, red ortho- 

 gneiss, composed chiefly of feldspar and quartz, but with a variable 

 amount of black mica. Exposures are very frequent in the entire 

 rugged district which it occupies. Coarse phases appear, especially 

 at the margins, and there are frequent pegmatite and quartz veins. 

 In the vicinity of limestone its color is bleached from red to white 

 as described for the granites on the Alexandria sheet, and the 

 granite knobs and dikes found cutting the limestone are all of 

 white color. Inclusions are frequent, especially at the east and 

 south, on approach to the bordering Grenville belts there, and, as 

 usual, the inclusions are always of amphibolite, no matter what 

 the nature of the bordering Grenville rock is. In this particular 

 area, however, most of the inclusions are sharply bounded and 

 show little sign of becoming soaked by the granite and converted 

 into mixed rocks, as so many of the inclusions in the granites of 

 the Alexandria Bay region do. This we take to indicate that we 

 are here comparatively near the margin of the bathylith, and that 

 the inclusions had been incorporated in the igneous mass in its 

 late stage, when its fluidity was much diminished. 



