470 NEW YORK STATE MUSEUM 



slight differences in their chemical composition also. Much of the 

 red gneiss is of the type which contains green pyroxene and deep 

 colored titanite, like that described from the Grampus lake area, and 

 some of the green gneiss has the same minerals in abundance. There 

 are also other gneisses present in minor quantity. It is quite likely 

 that the green gneiss actually belongs with the main syenite mass, 

 its differences being due to the granite intrusion. If so then in all 

 probability most of the Long lake and Grampus gneiss should be 

 classed with this granite as a great bulk of granite intrusive, later 

 than the syenite, as Ogilvie has argued for the Paradox lake area. 

 Owing however to the differences between the two rocks the writer 

 hesitates to adopt this view without more decisive evidence, and 

 has again taken refuge in noncommittal mapping. 



Great intrusions 



Anorthosite. A great mass of this rock lies in the northeast 

 portion of the quadrangle and comprises about one fourth of its 

 area. It is but a small segment of a great batholite of the rock 

 which has a wide extent in Essex and Franklin counties, and forms 

 the larger part of the surface of th^^ three quadrangles, Santanoni, 

 Saranac and St Regis, which bound the Long Lake quadrangle 

 on the north and east. It represents the earliest of the gr^at in- 

 trusive masses which invaded the region in Postgrenville times. It 

 is one of the most easily recognizable rocks of the Adirondacks, 

 and its area is accurately mapped, so far as surface exposures will 

 permit. 



This great mass of molten rock ascended to its present position 

 and solidified, not at the surface but underneath a great thick- 

 ness of overlying rock. This cover, and the upper part of the 

 anorthosite itself, have since been removed by slow surface erosion. 

 The present surface extent of the rock is simply the area of the 

 original mass at the horizon where the present erosion surface 

 cuts it. We can only conjecture as to its extension downward, 

 though it no doubt runs deep and broadens downward. The 

 amount worn away from the surface is less conjectural. Sections 

 of the rock of above 3000 feet in thickness are exhibited in some 

 of the mountains which it composes, suggesting the removal of 

 at least that amount from the neighboring valleys, with an addi- 

 tional unknown amount from the summits. This however necessi- 

 tates the assumption that the original upper surface of the mass 



