GEOLOGY OF MOUNT MARCY 15 



the northwestern corner of the quadrangle on the north side of an 

 unnamed brook. The overlying rock is not shown. 



The gneisses. The limestones are associated with well- foliated 

 gneisses, sometimes of light-colored acidic character, sometimes 

 dark and basic. In these respects the exposures are closely similar to 

 those in neighboring quadrangles. The chief contrasts in the large 

 way are due to the great abundance of igneous rocks in the Mount 

 Marcy area and to the contact effects which they have produced on 

 the Grenville sediments. It is difficult in many cases to draw 

 the line between regional and contact metamorphism and to be 

 certain as to original composition and the effects of saturation and 

 partial digestion of the sediments by the igneous masses. The 

 gneisses are also in many cases such close parallels in mineralogy 

 with the members of the syenite series, or else so intimately involved 

 with the syenites that one may be sometimes in doubt as to where 

 the metamorphosed ancient elastics end and the syenites begin. 



The most acidic phase of the sediments is a rather finely granular 

 aggregate of predominant quartz, with very subordinate plagioclase 

 and a very little orthoclase. About one-eighth of the slide consists of 

 irregular shreds of colorless pyroxene, presumably diopside. Plate 

 9 A illustrates the relations and relative amounts. The quartz, 

 which makes up fully three-fourths of the slide, is plentifully sup- 

 plied with minute needles of rutile. The specimen was found along 

 the edge of the sheet and less than one-half of a mile northwest of 

 Owls Head. It probably represents an old clastic sediment, asso- 

 ciated with the limestones and originally a sandy, somewhat calcare- 

 ous shale. It is similar to rocks described by Professor Gushing in 

 Museum Bulletin 115, pages 504-8. 



A slide of another gneiss, appearing as one of many included 

 blocks in the anorthosite at the foot of the last steeg rise of Owls 

 Head, revealed bands of pale green, granular pyroxene, in parallel 

 arrangement with other bands of both twinned and untwinned feld- 

 spar. The plagioclase has the extinctions of varieties somewhat 

 more basic than the labradorite series. A few titanites and an 

 occasional magnetic complete the mineralogy. The rock is 

 illustrated in plate 9 B. It is reminiscent of inclusions already 

 described from the Elizabethtown quadrangle next east (Museum 

 Bulletin 138, pages 34-35) in which, however, quartz replaces the 

 feldspar, here noted. These inclusions were of varying size, but in 

 the case cited were of a foot or less in diameter. They were sharply 

 angular and showed no corrosion or absorption. The foliation of 

 the different inclusions ran in all directions. The phenomena are 



