58 NEW YORK STATE MUSEUM 



dark rocks in the valley of Johns brook, and in the bed of Roaring 

 brook near the trail to the summit of Giant and at many other cited 

 places, as included, and impregnated Grenville gneiss, is correct, we 

 have these various outlying fragments. Other cases are known on 

 Baxter, Gothics, Rooster Comb and other peaks, while probably 

 many additional instances of the same exist and may from time to 

 time be observed as fires or floods expose ledges not visible in 

 former years or in localities unobserved as yet. The exposures 

 are of limited extent and may be missed in the wooded areas. 



An exposure of the Grenville of a very characteristic sort, con- 

 stitutes the extreme northwestern hill of the quadrangle and extends 

 into the Santanoni sheet to the west. Limestones, quartz-diopside 

 schist, rusty gneisses and even quartzite, if we cross the border a 

 short distance, are all present. 



The anorthosites cover almost all the quadrangle. They are the 

 predominating rock of this core-area of the mountains. They are 

 less in relative amount in the surrounding quadrangles. In the Lake 

 Placid and Ausable areas in the north and northeast, the geology is 

 more complex. 



The syenites are developed along the extreme northern border and 

 project southward into the valley of the East branch for about 

 2 miles. They are strongly gneissoid and to what extent Grenville 

 gneisses of similar mineralogy^ may be mapped in with them, it is 

 impossible to say. The mineral compositions of these two approach 

 each other so closely that despite microscopic work the writer has 

 often been puzzled. Dark green rocks, consisting of microperthite, 

 augite, hornblende and sometimes hypersthene have been mapped as 

 of the syenite series, even though having varying amounts of quartz 

 and showing gneissoid foliation. Differences of opinion might easily 

 arise regarding them; the more naturally because the syenitic rocks 

 favor the limited area containing the Grenville limestones. 



Of the undoubted basic gabbro series, we have discovered only the 

 two exposures of gabbro-syenite ; the great dike at Avalanche lake, 

 and the one to the northwest of it at the entrance to Indian pass in 

 the edge of the quadrangle. In both cases the exposures are dikes. 



The basaltic dikes are widely distributed. The following tabula- 

 tion will best describe them. The table brings out the fact that all 

 but the two camptonite dikes in the Ausable river, follow the north- 

 east fault-lines. The two camptonites run east and west or nearly 

 so. The thickest dike is 9 feet. Where marked by an asterisk, they 

 have been microscopically determined. 



