GEOLOGY OF MOUNT MARCY II 



Pleistocene i l^t'ltas and modified drift 

 ( Moraines and eskers 



ItS^^^L!'"^^" "! Basaltic (Camptonite) dikes 



Basaltic (diabase) dikes 



ordovician 



Precambrian 



Algoman 

 intrusives 



Gabbro-syenite dikes 

 Gabbro 

 Syenite-granite series 



( Whiteface type 

 Anorthosites | Marcy type 



Crystalline limestones 

 Grenville J Quartzites 

 series | Paraschists 



(^ [_ Paragneisses 



General summary. The glacial drift either in its original and 

 unsorted condition or else modified by the action of water is very 

 general in its distribution. It appears in largest amount in the 

 valleys. There are no Paleozoic exposures, so far as is known. 

 No positive indications of any beneath the drift have been observed 

 but an extraordinary number of flat slabs of Potsdam is in the 

 drift above the old Weston mine. There are a few basaltic dikes, 

 but the number is not great. They are entirely unmetamorphosed 

 and have clearly entered the wall rocks after the general meta- 

 morphism. Whether they are Precambrian or later in age, it is 

 impossible to state but their petrographic characters are in some 

 instances like those of the pre- Potsdam series described by Prof. 

 H. P. Gushing,^ and in others like those of the Postordovician dikes 

 described by the writer^ and V. F. Marsters. 



The gabbro syenite dikes have been observed in only one or two 

 places. The most interesting is on the shores of Avalanche lake 

 where a powerful dike appears in the gulch between Avalanche moun- 

 tain and Mount Golden. It cuts the anorthosite. Another has 

 been observed in the northern entrance to Indian pass. We have 

 no direct evidence of their ages except that they cut the anorthosites ; 

 but inasmuch as they are closely related to the basic gabbros of the 

 Elizabethtown quadrangle on the east, and the latter are be.ieved to 

 be later than the syenites, these dikes are also placed later. They 

 have strong mineralogical affinities with the syenites. 



The syenites have been chiefly observed in the northern central 

 portion of the quadrangle, where they are profoundly involved 



1 On the Existence of Precambrian and Postordovician Trap Dikes in the 

 Adirondacks. N. Y. Acad. Sci. Trans. 1896, 15 :248-S2. 



2 Trap Dikes in the Lake Champlain Valley. Bulletin 107, U, S, Geol, 

 3urvey, 



