OCCLUSION OF IGNEOUS ROCK 401 



in evidence of bedding. If such evidences once existed along 

 those planes, they have become obhterated. The conformity 

 of the hmestone-beds to the foHation of the gneisses also appar- 

 ently indicates coincidence with a common bedding plane. 



Outcrop of Gabbro. — Through the courtesy of the Curator, 

 Dr. E. O. Hovey, L have been enabled to examine the series of 

 rocks from New York Island in the collection of the American 

 Museum of Natural History. Two important specimens were 

 found labelled " Syenitic gneiss, West 216th Street and 14th 

 Avenue," in one of which I detected the presence of bronzite. 

 With the kind assistance of their collector. Dr. A. Woodward, 

 and of old maps, I have searched that locality, on north of Inwood 

 Heights, in vain for the out-crop. It was probably a small one 

 and has been apparently covered up with debris during the twenty 

 years which have elapsed. The authenticity of the specimens 

 seems sufficiently established to call for a brief description, on 

 account of their special petrographic importance. 



No. 107 is a coarse biotite-quartz-gabbro, fresh and unaltered, 

 closely resembling a biotite-gabbro from Keeseville and coarser 

 norite from Westport, near the Adirondacks, New York, and 

 quite different from the gabbros of Westchester County, New 

 York. It is so feldspathic as to approach a norite or anorthosite, 

 as shown by its specific gravity, 2.778, being chiefly made up 

 of striated grains of greenish gray plagioclase, 10 to 15 milli- 

 meters long, whose crystal outlines in many places impart a 

 semi-porphyritic texture to the rock. The dark intervening 

 aggregates, of about the same size, consist of black hornblende • 

 brown bronzite, with silky fibration and high lustre, rarely reach- 

 ing 15 mm. in length; scales of brown biotite, up to 4 mm. 

 across ; abundant grains of gray quartz ; and a few Alining par- 

 ticles of iron ore. 



In thin section, the texture is found to be allotriomorphic. 

 The feldspars form over half the volume and comprise two kinds 

 of plagioclase. The one presents, between crossed nicols, a 

 twinning lineation in rather coarse bands whose large extinction 

 angle indicates a basic variety. The other, with lower index of 

 refraction, displays very fine lineation, often occupying but a 



