426 A. A. JULIEN AMPHIBOLY SCHISTS OF MANHATTAN ISLAND 



of '' schistose biotite-gneiss " from West One hundred and twenty-fifth 

 street, near Riverside drive. The specimen exhibited somewhat of a 

 flaser structure, 



" caused by flakes of mica grouped in lines curving about larger crystals of 

 feldspar and quartz. It was made up of biotite, quartz, and feldspar, all allotrio- 

 morphic, with a small amount of muscovite, apatite, and zircon. The quartz 

 grains are irregular, but the smaller are often rounded. The feldspar is chiefly 

 oligoclase, with a little orthoclase." 



It is added, 



"As the proportion of feldspar grows less, the rock approaches a gneissic mica 

 schist." 



A good exposure of this rock remains near Columbia University, at 

 West One hundred and eighteenth to One hundred and nineteenth street, 

 on Morningside avenue west. I am indebted to Professor J. F. Kemp 

 for the opportunity to present the following analysis (I) of a specimen, 

 made for him in duplicate by Dr C. H. Jouet, of this university. To it 

 may be added an analysis (II) of a specimen of the same gneiss, selected 

 for its fine grain and uniformity in texture and mineral composition, 

 from the corner of East Fifty-second street and First avenue.^ The 

 analyst points out its approximate identity with the composition (III) 

 of a quartzose mica schist from Tagilsk, Ural,t to indicate its probable 

 derivation from alteration of a clay slate. 





la. 



16. 



I. 



II. 



III. 



SiO^ 



Ti02 



AloO, 



64.11 



None. 



17.45 



6.54 



63.86 

 



17.49' 

 6.39 



63.98 



None. 



17.47 



6.46 



57.20 



"'I'g'.si' 

 '9.52 

 "5.73 



4.40 

 .28 



2.13 

 .48 

 .59 

 .67 



100.51 



56.99 

 .91 



18.98 



FeO 



Fe„Oo 



'9' 02 



MnO 



CaO 



MgO 



K„0 . 



Trace. 



2.85 

 1.87 

 2.84 

 2.25 

 .71 



""2'. 88' 

 1.93 

 2.87 

 2.62 



Trace. 



2 86 

 1.90 

 2.85 

 2.44 



.711: 



" " "4.90 



.75 



3 00 



NagO . . 



2 59 



Ignition loss . . . . 



2 48 



Fe 





s 







98.67 





. 







99.62 



Under the microscope the thin-section is found to consist mainly of a 

 mosaic of closely fitted grains of colorless quartz and feldspar, to an 



* p. Schweitzer: American Chemist, vol. iv, 1874, p. 443, a. id vol. vi, 1876, p. 457. 

 t Kjerulf : Bischoff, El. Chem. and Phys. Geol., vol. iii, 1859, p. Ui. 

 I COg undetermined. 



