132 J. F. Kemp — Porphyrite Bosses in New Jersey. 



What appears to be the original rock is generally a very 

 finely crystalline ground-mass in which are developed numer- 

 ous porphyritic crystals of biotite. Its composition is shown 

 by analysis I in the following table. The crystals are often 

 very thickly distributed through the mass and again are com- 

 paratively few. They run from microscopic dimensions up to 

 two inches in diameter. The biotite is deep brown, almost 

 black in color and well-nigh uniaxial, affording in convergent 

 light an image, hardly, if at all, to be distinguished from a single 

 cross and rings. As is to be expected it is optically negative. 

 In the rock the mica is sometimes idiomorphic and sometimes 

 in irregular masses. It frequently exhibits beautiful illustra- 

 tions of crumpled and strained crystals and was evidently 

 formed before solidification had set in. It alters concentrically 

 from the edges. 



Augite is especially abundant in the' northern exposure, but 

 less frequent in the others. It is greenish in tint, and exhibits 

 well developed idiomorphic crystals bounded by the prism and 

 pinacoids. In upper A 5, it occurs altered in such a way as to 

 resemble most closely serpentinized olivine crystals and on 

 casual observation one would regard them as such. But by 

 searching out the small unaltered fragments and testing the 

 extinction angle they are seen to be augite. No hornblende 

 whatever was found. Plagioclase seldom occurs in large crys- 

 tals, but in the ground- mass is found by high powers to be 

 present in quantity. In the available specimens it is generally 

 in an advanced state of alteration and affords a light yellow, 

 feebly refracting alteration product. Together with small 

 masses of biotite, apatite needles and magnetite, it forms the 

 ground-mass. The absence or very small amount of soda 

 found in the anatyses (see below), would indicate a plagioclase 

 close to anorthite. Apatite is everywhere extremely abundant 

 in well defined hexagonal crystals, often of unusual size. It 

 constitutes one of the most remarkable features of the rock. 

 Magnetite is abundant, without however showing indications 

 of titanium. Pyrite is not infrequent. The ground mass is 

 an extremely finely crystalline base, composed of innumerable 

 doubly refracting elements which are chiefly plagioclase, biotite 

 and less abundantly augite. Two generative periods are thus 

 clearly shown for the last two. In the majority of cases alter- 

 ation is much advanced, yielding calcite in great quantity, so 

 that the rocks often readily effervesce. In middle B5, titanite 

 crystals are numerous but small. 



An analysis of the biotite gave the results in column I ; an 

 analysis of the rock from B2, those in column II ; another of 

 the rock from upper A5, the results in column III. 



