1885.] NEW-YORK MICROSCOPICAL SOCIETY. 131 



rocks of the island ; the fibrolitic gneiss^ near High Bridge, and 

 indeed all the other forms of gneiss, schists, and granyte ; the 

 smoky quartz which occurs in the granyte veins ; the quartz- 

 grains in many of the coarser granytes and sandstones which are 

 imported into the city, in great variety, as building stones ; and 

 even many of the quartz-grains in the sand of our seashore, as 

 at Coney Island. The smoky form of quartz is almost always a 

 vein quartz, of very common occurrence on our island, and 

 familiarly known to all our collectors of local minerals. Espe- 

 cially on the west side of the city, all the way up from Sixtieth- 

 street to the upper end of the island, very nearly, wherever 

 excavations are going on for opening streets or for the founda- 

 tions of buildings, the granyte-veins, carrying this form of quartz, 

 are conspicuous. 



The matrix of the quartz has been deposited by heated so- 

 lutions in fissures of the gneiss, often producing an alteration of 

 the gneiss in its neighborhood. One form of the deposit is the 

 smoke-colored variety of quartz, which is filled with minute fluid- 

 cavities. Its color is not necessarily due to the presence of cavi- 

 ties : it has been attributed to a bituminous substance dissemi- 

 nated through it. Dr. Lea, of Philadelphia, has stated'* that the 

 fluid-cavities " are in smoky quartz much rarer " than in trans- 

 parent rock-crystal — evidently referring to the larger cavities 

 visible to the eye. However, the minute cavities are invari- 

 ably present in vast abundance through smoky quartz. 



Interesting localities elsewhere, at which quartz of this variety, 

 containing fluid-cavities of remarkably large size, is to be found, 

 are Branchville, Conn.,* Chester, Penn,, and White Plains, N. C. 

 The Branchville material has been already very fully described. 

 In the material from Chester — which was sent to me by Mr. T. D. 

 Rand, of Philadelphia — the cavities are occupied by brine, hold- 

 ing cubes of salt and occasional hexagonal crystals of another 

 mineral, and by liquid carbon dioxide and its gas. The quartz 

 from White Plains — which locality has been described by Mr. 

 W. E. Hidden^ — has yielded the largest and most remarkable 

 cavities, holding carbon dioxide, on record. Thin sections of 



»A. A. Julian : " On the Fissure-Inclusions in the Fibrolitic Gneiss of New Rochelle, 

 N. Y." Am. Quar. Mic. Jour., Jan., 1879, pp. 3-15. 



*" Further Notes on 'Inclusions' in Gems, etc." Proc. Acad. Nat. Sci. Phila., 

 1876, p. 6. 



•G. W. Hawes : Am. .Jour. Sci., 1881, XXI., p. 203.— A. W. Wright : Idem, p. 209. 



" Note on " Fluid-bearing Quatz Crystals." Am. Jour. Sci. and Arts, (3), 1883, XXV., 

 p. 393. 



