GEOLOGY OF MOUNT MARCY 49 



4 

 THE PRECAMBRIAN INTRUSIVES, CONCLUDED 



The Syenites. The syenites are developed in the northern eds^e of 

 the sheet. They present the usual dark green gneissoid rock, now 

 become widely familiar in the Adirondack area. Under the micro- 

 scope microperthite is the chief component. The spindles of albite 

 which give the feldspar the microperthitic character are sometimes 

 set in orthoclase, sometimes in microcline. Plagioclase appears in a 

 subordinate capacity in the typical cases, but in the syenite of Pitchofif 

 mountain on the borders v^^ith the anorthosite, it sometimes becomes 

 quite prominent. The syenite has acquired or developed blue 

 labradorite crystals and one does not know with which series of 

 rocks, syenites or anorthosites, to place the specimens. The most 

 typical dark silicate is emerald green augite, presumably of a soda- 

 bearing variety. Brown or green hornblende is also present, and at 

 times hypersthene. Stray bits of magnetite and the small accessories, 

 zircon and apatite, make up the balance. The characteristic syenite 

 has been collected on a shoulder of Scott's Cobble. It is illustrated 

 in plate 20A. Elsewhere in this hill it shades into siliceous varieties 

 v>nth much quartz, practically a granite. These variable characters 

 have, however, not infrequently been observed in the extended 

 exposures of syenite elsewhere. 



No analyses have been prepared of the syenites within the Mount 

 Marcy quadrangle but a number are given of specimens in Bulletin 

 138, page 45, and the rocks are treated at length by Professor Cush- 

 ing in Bulletin 115. 



Pegmatite. On the western shoulder of the summit of Mount 

 Porter, C. H. Fulton, the writer's assistant in 1897, noted in the 

 anorthosites a pegmatite vein mainly containing microcline and 

 quartz. In the summer of 1914 the writer and H. L. Ailing were on 

 the eastern shoulder of Mount Porter and were impressed with a 

 small series of pegmatites containing microcline as the feldspar. They 

 filled crevices in the anorthosite. The association is a peculiar one 

 in a rock so low in potash as the anorthosite. We know of no other 

 intrusive rock near, which might have supplied the pegmatite. A 

 parallel is mentioned in Bulletin 138 where orthoclase pegmatites in 

 association with the basic gabbro are mentioned from the Elizabeth- 

 town-Port Henry quadrangle. Prof. W. J. Miller describes the same 

 peculiar association in the North Creek quadrangle in Bulletin 170, 

 page 38. A pegmatite dike also appears east of the included lime- 

 stone at the Cascade lakes. 



