438 W. J. MILLER ADIRONDACK ANORTHOSITE 



of significant relationships, I shall enter somewhat into the details of 

 their description. 



The typical Keene gneiss presents a different appearance from any of 

 the other Adirondack rocks. In the Lake Placid and Schroon Lake quad- 

 rangles the typical rock is medium grained, gneissoid, notably granulated, 

 and looks much like a rather basic facies of the syenite except for scatter- 

 ing phenocrysts of bluish gray labradorites up to an inch long. These 

 phenocrysts, which are rounded and usually elongated parallel to the 

 foliation of the rock, doubtless represent cores of crystals which survived 

 the process of granulation. Locally the phenocrysts are absent or spar- 

 ingly present, and ledges of such rock are difficult to distinguish from a 

 basic phase of syenite. Under the microscope, however, the distinction 

 may generally be made. The fresh rock is greenish gray and it weathers 

 brown. 



The mineralogical composition of selected samples of various phases 

 of the rock from the Lake Placid quadrangle are shown in the table on 

 the opposite page. 



Labradorite and andesine are always present and oligocene usually. 

 Microperthite occurs in most specimens in varying amounts up to 30 per 

 cent, and orthoclase in most specimens in varying amounts to over 50 per 

 cent. A little quartz is generally present. All the thin sections examined 

 show greenish- gray monoclinic pyroxene; sometimes diallage. A little 

 green hornblende nearly always occurs up to 14 per cent. Garnet varies 

 from none to 12 per cent. Ilmenite or magnetite up to a few per cent 

 never fails. Apatite and pyrite nearly always occur in small amounts. 

 In his recent paper, Bowen states that he has observed, in the transition 

 rocks from anorthosite to syenite, inclusions of potash feldspar, which are 

 small patches, uniformly oriented, and in some cases surrounded by areas 

 of plagioclase differing from the crystal as a whole. A few slight sugges- 

 tions of this sort were noted by the writer, but certainly this is not a 

 characteristic feature of the Keene gneiss thin sections examined. 



SOME OCCURRENCES IN THE LAKE PLACID QUADRANGLE 



Occurrence near Keene village. — The type locality of the Keene gneiss 

 is a ledge by the side of the State road at the northern end of the village 

 of Keene, where an excellent opportunity is afforded for the study of the 

 rock and its relations to both anorthosite and syenite. All three of the 

 rocks show as unweathered material in this one ledge, which has recently 

 been blasted open. The anorthosite, which occurs in minor amount, is 

 the typical Marcy facies, consisting mostly of dark bluish gray labradorite 

 up to an inch across, embedded in some granulated labradorite, and asso- 



