358 N. E. A. Hinds — An Alkali Gneiss, 



well developed crystal outlines. The hornblende is 

 imperfectly segregated into pencil-like areas, and hence 

 is responsible for the gneissic structure of the rock. The 

 grains in these segregation-areas have their long axes 

 roughly parallel, although a rather wide diversity of 

 direction exists. 



The hornblende is generally fresh, except for the 

 development of small amounts of greenish decomposition 

 products along cleavage cracks. The sections are brown- 

 ish-green, and are strongly pleochroic ; Y = dark brown- 

 ish-green to greenish-brown ; X = light brownish-green ; 

 Z == deep yellowish-brown ; and Z > Y > X ; Y is nearly 

 = Z. The pleochroism is similar to that of a section of 

 barkevikite from the type locality in Norway. Sections 

 near (010) give 12 of to Z. 



An analysis of the mineral by Dr. J. E. Wolff is given 

 below. Analyses of two other hornblendes, also by Dr. 

 Wolff from the Byram gneiss of Hamburg Mountain and 

 of Wa^^vayanda Mountain, near the northern border of 

 Xew Jersey, are included in the table. Analyses 2 and 

 3 are incomplete, but they will serve to show the simi- 

 larity of the hornblendes in rather widely separated 

 exposures of this group of rocks. 



Table I. 



1 2 3 - 



SiO 39.10 38.78 34.38 



ALO. 7.61 8.72 13.39 



Fe.0 5.72 0.22 1.16 



Feb ' 25.49 28.08 26.72 



MgO 3.23 0.58 3.30 



CaO 8.99 9.98 9.26 



Na.O 1.53 .... 1.51 



KoO 2.02 3.85 1.87 



H'oO-f 2.88 



H:0 — 0.34 



TiOo 2.13 2.71 5.53 



MnO 0.56 0.69 0.45 



99.60 93.61 97.57 



Sp. G 3.440 3.436 3.434 



1. Hornblende from alkali quartz-syenite gneiss. Van Nest 

 Gap, N. J. 



2. Hornblende from Byram gneiss. Hamburg Mountain, 

 N.J. 



3. Hornblende from Bj^ram gneiss. Waywayanda Mountain, 

 N. J. 



