156 J. V. Lewis — Palisade Diabase of New Jersey. 



interstices between the interlacing lath-shaped feldspars, or 

 when greatly in excess it forms the ground mass in which the 

 feldspars are imbedded. In the coarser-grained portions of 

 the rock there is often developed a granitoid texture, in which 

 the two chief minerals occur in grains of approximately equal 

 size and of nearly equal dimensions in every direction. 



Augite, the most abundant constituent, is pale green to 

 colorless and sometimes exhibits distinct pleochroism — pale 

 green to light yellow.* It occurs in plates up to 3 or 4 milli- 

 meters in diameter, and in irregular grains whose forms are 

 determined by the accompanying feldspars. Crystal outlines 

 are rarely observed. In the denser contact facies augite of 

 two generations appears, the earlier as large plates scattered 

 through the denser gronndmass in which the augite of later 

 crystallization forms a fine granular rilling between the feld- 

 spars. Two forms of twinning often appear, both separately 

 and in combination. That parallel to the orthopinacoid (100) 

 usually produces paired halves, while the basal twinning (par- 

 allel to 001) is more commonly repeated in thin lamellae, which 

 are sometimes exceedingly minute. 



Plagioclase, the chief feldspar and the second constituent 

 in abundance, occurs in characteristic lathlike forms, ranging 

 up to 2 millimeters in length with a breadth one-fifth to one- 

 third as great. In the coarse textures of the granitoid, facies 

 these dimensions become more nearly equal, and diameters of 

 3 to 4 millimeters are often observed. Often the plagioclase 

 presents complete crystal outlines, but very commonly the 

 terminal planes are lacking, the elongated crystals abutting 

 irregularly against each other. They are made up of thin 

 twinning lamellse, chiefly according to the albite law, but peri- 

 cline and Carlsbad twinning also occur. Zonal structure is 

 quite commonly developed, and fringing the extreme acid 

 borders a graphic intergrowth of quartz and orthoclase is often 

 found. 



Maximum extinction angles in sections normal to the 

 albite twinning plane range a little under 30 degrees, corre- 

 sponding to acid labradorite. Analyses of feldspars separated 

 by heavy solution have shown that labradorite containing the 

 soda and lime molecules in about equal proportions is the 

 most abundant plagioclase ; but other members of the series, 

 present in considerable amount, ran^e to almost pure albite. 



Orthoclase and quartz in graphic intergrowth, as noted 

 above, frequently form a fringe about the plagioclases, and 

 fill many of the triangular and irregular interstices. These 



*In the examination of several hundred sections of Newark diabase from 

 New Jersey and neighboring states only monoclinic pyroxenes have been 

 observed. It seems highly probable that the hypersthene that has been 

 occasionally reported in these rocks is simply pleochroic augite. 



