ELISHA MITCHELJ^ SCIENTIFIC SOCIETY. ' 66 



further study, to belong to the class of apo-rhyolites, a 

 term introduced by Miss Bascom to denote a devitriiied 

 rhyolite. Emmons describes the type v^ry well under 

 the head of quartzite (p. 59). They resemble perfectly 

 cry pto-ciiys tall ine quartz, and on weathering- present an 

 earthy, yellowish surface. The color of the fresh rock is 

 drab, bluish to almost black; translucent on edges; frac- 

 ture flat conchoidal; sometimes banded, showing- flow 

 structure, as at the Silver Valley mine in Davidson 

 county, where the rock is locally called "gun-flint.'' It 

 often contains small crystals of metallic sulphurets, 

 chiefly pyrite with some galena, chalcopyrite and blende. 



At the Moratock gold mine in Montgomery county, a 

 siliceous rock occurs in large masses, which at first sight 

 resembles a compact, homogeneous hornstone, but which 

 on close investigation is found to be dotted with small, 

 dark-colored, glassy specks. These are minute quarts 

 crystals, and the microscopic examination of thin sections 

 shows the rock to be an undoubted quartz-porphyry. 

 Its true porphyritic character is best illustrated in the i 

 weathered specimens, the feldspathic groundmass be- 

 ing decomposed and altered, leaving the quartz phenc- 

 crysts clearly outlined. The flow structure is also beau- 

 tifully brought out in the weathered groundmass. 



Emmons, in his description of his quartzite, states that 

 it is often porphyritic and porphyrized, and that fre- 

 quently the fresh fracture i-^ '•< u^'d with small limpid 

 crystals of quartz (p. 59; . 



In the enumeration of the varieties of Lower Taconic 

 quartz rocks (p. 57) he mehtions a cherty or apparently 

 porphyrized quartz, which contains feldspar, which de- 

 composes -and leaves a rough porous mass similar to 

 burrhstone." Kerr sa3^s: "In Montgomery county, in 

 a very heavy ledge of siliceous slate, occurs a siliceous 

 conglomerate Vv^hich is filled for hundreds of feet with 

 very singular, siliceous concretions, some of which Dr. 

 Emmons has described under the name of Paleotrochis; 



