436 A. A. JULIEN AMPHIBOLE SCHISTS OP MANHATTAN ISLAND 



dant, its disposal around the hornblende grains suggests an iron cement 

 approaching the '' sideronitic " texture of Duparc* The smaller plates, 

 up to 0.02 millimeter in diameter, are generally rounded or pear-shaped. 

 The larger, about 0.1 millimeter in diameter, show rounded or imperfectly 

 hexagonal outlines, sometimes marked with fine parallel lines, as from 

 edges of overlapping lamellae. Little rhombic forms also occur, as if pro- 

 duced by fracture and cleavage. 



Magnetite in very small amount, as dull, iron-black fringes to hema- 

 tite scales, rarel}^ as minute isolated cubes, rectangular grains, or groups. 



P3a-ite, rarely, in minute granular coatings of brass-yellow color, around 

 a hornblende fragment — usually a single instance in each thin-section. 



Biotite, in a few brown plates, 0.2 to 0.6 millimeter long, sometimes 

 two or three in the same thin-section ; always imbedded in hornblende, 

 lying in schist plane or obliquely. They inclose a few black angular 

 particles (magnetite ?), and present the usual dichroism and strong 

 absorption in plates normal to cleavage. 



Zoisite, in colorless, elongated grains, almost idiomorphic, rarely reach- 

 ing 0.6 millimeter in length, lying in close association with hematite, 

 both as inclusions in hornblende scales or around their margins (plate 

 62). Though its prisms are mostly arranged with longer axes parallel to 

 both foliation and fibration of the schist, those included in hornblende 

 often lie in other positions, thus presenting cross-sections of rounded, 

 manj^ sided, six-sided, and rhombic forms. A majority are imperfect 

 prisms, lath-shaped, roughly octagonal, and occasionally terminated, 

 P (111) and 2 P "oo (021), with obelisk-like outline from the hemi- 

 morphic habit. Some traces of the brachypinacoid cleavage occur, 

 00 P ^ (010), usually in sections parallel to the prism faces ; but cross- 

 partings are common, much like those of thesillimanite in the adjacent 

 micaceous schist, by which the mineral seems to have been largely broken 

 up into angular fragments during rock movements. Though as an in- 

 clusion its boundaries are sharply defined, a transition appears in man}^ 

 places from colorless zoisite to green hornblende, around the margin of 

 the latter, perhaps denoting a secondary growth of the former mineral • 

 its relief is decidedly greater, and this excess in refractive index is well 

 and simply shown by use of the Becke method. The central part of a 

 prism is generally clouded by characteristic liquid cavities of irregularl}^ 

 rounded, elongated form, tubular, curved, and often branching, from 

 0.004 to 0.017 millimeter in length. Two methods of arrangement of 

 these commonly prevail within a prism, fluid cavities along one side of 

 the axis being disposed with their longer dimensions parallel, and on 

 the other side normal to the vertical axis of the prism. This may also 



* Rech. g6ol. et p6t. sur I'Oural du Nord, Geneve, 1902. 



