432 A. A. JULIEX AMPHIBOLE SCHISTS OF MAXHATTAX ISLAND 



hackl}^ projections of the torn rock along both sides, as if the}" had been 

 wrenched apart and rubbed together. 



At other localities, as, for example, at the huge bed on Spu3'ten Du^'vil 

 creek, portions of the hornblendic rock are traversed b}- innumerable 

 veinlets of quartz or pegmatite, indicating a shattered and even brec- 

 ciated mass. 



MACROSCOPIC DESCRIPTION OF HORXBLEXDE SCHIST 



The following is an abstract of a published description^ of a horn- 

 blende schist from West One hundred and twenty-fifth street, between 

 Claremont and Riverside avenues: Rather massive crystalline schist, 

 greenish black, with tendency to cleave in parallel plates. In thin- 

 section it is found to consist of dark green hornblende, with subordinate 

 amounts of feldspar, quartz, magnetite, biotite, apatite, and a little zircon 

 and pyrite, besides some secondar}" minerals in places — muscovite, epi- 

 dote, or zoisite. The lamination is produced b}" a somewhat parallel 

 arrangement of the stout cr3^stals of hornblende and b}" streaks of the 

 other constituents in smaller grains. 



Hornblende is strongly green in thin-section, with tinge of brown 

 and with pronounced pleochroism, the variation being from green 

 through brownish green to light brown. Man}" individuals exhibit no 

 cleavage. Extinction angle low. Twinning seldom observed. The 

 substance of the hornblende very pure, with inclusions of occasional 

 zircons in rounded crystals and considerable magnetite, besides feldspar, 

 quartz, and biotite. 



Feldspar in irregular grains, mostly plagioclase ; some extinction 

 angles suggest andesine labradorite ; lamellae in places curved, and some 

 undulatory extinction. A little orthoclase may be present. 



Quartz in irregular grains, sometimes rounded, especially when in- 

 closed in hornblende; substance very pure, with almost no undulatory 

 extinction. 



Biotite, brown, with strong absorption, in irregularly shaped pieces, 

 generally inclosed in the hornblende. 



Magnetite, relatively abundant, in clusters and streaks of irregular 

 grains. 



An interesting outcrop of the hornblende schist occurs near Columbia 

 University, and has been carefully studied by Professor J. F. Kemp and 

 by myself. At the southwest corner of \yest One hundred and nineteenth 

 street and Morningside avenue west a mass of micaceous gneiss, in large 

 part pegmatitic, rises to a height of 20 feet, with steep slope to the south- 



* IddingP, op. cit., p. 331. 



