from the Pre-Camhrian of Neiv Jersey. 359 



The mineral is an iron-rich hornblende related to barke- 

 vikite, though it has a higher iron and a lower alkali 

 content than does typical barkevikite. 



Quarts. — Quartz occurs sparingly in grains of medium 

 size. 



Accessories. — The accessories, magnetite, biotite, zir- 

 con, and apatite, are very subordinate in amount. Mag- 

 netite is present in large, irregular grains. Biotite 

 occurs in a few ragged individuals, commonly associated 

 with the hornblende. In such cases, the longer axes of 

 the biotite grains are parallel to those of the hornblende. 

 The biotite is brown in color, and exhibits strong pleo- 

 chroism from light yellow through brownish-yellow to 

 deep reddish-brown. Zircon is represented by numerous, 

 small, prismatic sections, frequently shoAving perfect 

 pyramidal terminations. Many larger basal sections are 

 also present. The apatite individuals are of medium size, 

 and have rounded or elliptical outlines. Most of the 

 apatite and zircon is found in the pencils of hornblende. 



Chemical composition. 



The analysis of the rock, which was made by Dr. Wolff, 

 is given on page 363. The rock is an akerose of the rang 

 monzonase, order germanare, with the following norm : 



Quartz 5.94 



Orthoclase 24.46 



Albite 44.54 



Anorthite 10.01 



Zircon 0.37 



Apatite 0.34 



Ilmenite 1.22 



Magnetite 2.78 



Diopside 2.88 



Hyperstliene 5.95 



98.34 



The mode of the rock, determined by the Rosiwal 

 method, is given below. The calculation of the mode 

 from the chemical analysis gave results which checked 

 very closely with these figures. The rock may be classed 

 as an alkali quartz-syenite gneiss. 



Am. Jour. Sci.— Fifth Series, Vol. I, No. 4.— April, 1921. 

 24 



