THIRD HYPOTHKSIS OF DERIVATION 477 



in Westchester county — for example, in black bornblendite, with grayish 

 tremolite rock, coarse diorite, and serpentine, on Davenport's neck, near 

 New Rochelle, there also doubtless a result of contact reaction. 



PECULIARITIES OF CRYSTALLIZATION 



Uniformity of texture. — Two forms may be sought for, characteristic of 

 igneous intrusions, of which one is general uniformity in texture, cor- 

 responding to homogeneity in chemical and mineralogical constitution. 

 This seems clearly to prevail through all the outcrops of our horn- 

 blende rocks. Besides the large series of rocks of the island in the col- 

 lections of the department of geology at Columbia University, I have 

 been enabled, through the courtesy of the curator, Dr L. P. Gratacap,of 

 the American Museum of Natural History, to study the series at that 

 museum, the more recent collections from excavations for 5 miles along 

 the subway, and also the collection of the New York Mineralogical Club. 



In texture the prevalent uniformity of the hornblende gneiss and schist 

 is very striking, the blades and scales of the predominant mineral, horn- 

 blende, rarely exceeding 3 millimeters in length, though sometimes reach- 

 ing 1 centimeter in certain thin layers — for example, at West Ninety-first 

 street and Riverside avenue — or becoming coccolitic or granular — for ex- 

 ample, at West Eighty-first street and Ninth avenue. It looks like a 

 curious anomaly that its smallest dimensions, 0.5 to 0.2 millimeter, occur 

 in nearly the thickest bed of the schist — that at West One hundred and 

 thirty-fifth street. This has been caused plainly by the excessive folding, 

 internal motion, mutual attrition of scales, and consequent comminution 

 which have taken place within the thicker mass. In this fine grained 

 schist the lessened tenacity shown by its peculiar brittleness and pul- 

 verulence is probably due to the minuteness of the binding scales. 



In mineral constitution the uniformity has already been shown in the 

 microscopical description of specimens from several localities (pages 435 

 to 438). 



In chemical composition we have as yet only the evidence of the ex- 

 cellent analysis of Jouet, but I anticipate its confirmation by all future 

 analysts. 



Survival of phenocrysts. — Another peculiarity of an intrusive igneous 

 material, often partly retained even in the product of its metamorphic 

 alteration, consists in the separate crystallization of certain mineral con- 

 stituents as phenocrysts. The detection of porphyroidal texture at sev- 

 eral localities bears important testimony in favor of the igneous origin 

 of this schist. The traces of this texture of occasional occurrence, how- 

 ever, need to be discriminated from an imitative metamorphic form. In 



LXVI— Bull. Geol. Soc. Am., Vol. 14, 1902 



