GEOLOGY OF MOUNT MARCY I3 



that is, south from the northern edge of the quadrangle. It extends 

 also laterally for about 2 miles east and west but is cut by intrusive 

 masses of anorthosite and syenite. Indeed, except for the bottom 

 of the valley of the river the relationships with the intrusives are 

 much confused, both here and in the neighboring parts of the Lake 

 Placid quadrangle in the north. The river itself has washed clean 

 a series of ledges so as to afford an extraordinarily good series of 

 exposures (see plate 6), but as soon as one climbs the hills on 

 either side the anorthosites and syenites seem to have penetrated the 

 Grenville in the most intricate manner, to have produced contact 

 zones of unusual interest and to have given rise to some bodies of 

 magnetite in the zones, which have been the object of mining in 

 earlier years. 



The Grenville is also in evidence in a patch of limestone included 

 in anorthosite opposite the hotel on the Cascade lakes and across 

 the lakes from it. Apparently the Grenville limestone has been 

 caught up in the anorthosite, and has been charged with beautiful 

 green diopsides (coccolite) and small black garnets. Along the 

 Cascade lakes on the northwest side, and in the pass on the north- 

 west side of Pitchofif mountain we find again complex relations, 

 but this time, as nearly as one can determine, the anorthosite is 

 involved with members of the Syenite series of eruptives. These 

 curious phenomena will be taken up under the syenites and 

 anorthosites. 



A third area is in the extreme northwestern corner of the quad- 

 rangle. A central area of limestone with wollastonite is surrounded 

 by rusty gneisses and quartz-diopside rocks. The enormous thick- 

 ness of sands, and gravels in this area prevent tracing the exposures 

 east and south. 



The most easily and certainly recognizable of the Grenville strata 

 are the crystalline limestones which appear in beds of varying thick- 

 ness. They may be but i or 2 feet in section, or again may reach 

 20 or more feet. No great section, however, is free from included 

 masses of silicates or of fragments of the wall-rock or of intrusive 

 tongues torn off in the great compression to which the region has 

 been subjected. The limestones themselves are rather coarsely 

 crystalline in texture. When recrystallized they afford very coarsely 

 crystalline calcite, with cleavage faces over a square inch in area. 

 The faces are then usually striated with traces of the gliding plane 

 parallel with minus R. The limestone is also impregnated with 

 diopside crystals of the variety coccolite. Beautiful examples may 



