SCHISTS OF WESTCHESTEK COUNTY 481 



New Rochelle, Rye, etcetera, and collected specimens now in the geologi- 

 cal cabinets at Columbia University. The following is a brief descrip- 

 tion of the most important types from New Rochelle, with references to 

 similar kinds at other localities in that region. Putting aside from pres- 

 ent consideration the pyroxenic rocks or diabases which have been 

 reported, all these hornblendic forms fall naturally into three classes : 



A. Coarse to fine hornblendites and quartz diorites, the mother rock, varying 

 merely in grain and in proportion of the feldspar-quartz groundmass. 



B. Actinolitic diorites, hornblende gneisses , and hornblende schists. 



C. Chloritic and ophiolitic ampbibolites and schists, passing into serpentine. 



Original hornblendites and quartz diorite. — At Davenport's neck, New 

 Rochelle, occurs a coarse hornblendite. It is a heavy black rock (specific 

 gravity, 3.128) made up of shining black grains and prisms of hornblende, 

 0.5 to 6.0 centimeters in length, often over 1 centimeter in breadth, 

 entirely allotriomorphic and lying in all positions. A strongly marked 

 prismatic cleavage produces the high luster of the fractured surface. 

 These grains appear pure and free from inclusions, but in the interstices 

 lie whitish nests of a mixture of grayish quartz, white feldspar, reddish 

 garnets (rarely 4 to 5 millimeters across), reddish brown iron oxide, and 

 a very few scales of black and white micas. In some specimens man}^ 

 cleavage planes of the hornblende are coated by red films of ferruginous 

 marmolite, showing the first stage of alteration of the rock to serpentine. 



Under the microscope a thin-section appears to be made up chiefl}^ of 

 elongated grains of hornblende, with rude boundaries, eminent prismatic 

 cleavage, few inclusions of magnetite and sometimes epidote. Absorp- 

 tion scheme, C>b>a; C, bluish green; t), pale brownish green ; a, 

 greenish yellow. The small interspaces between the sides and ends of 

 these grains are occupied mostly by granules of a plagioclase, with twin- 

 ning after the albite and rarely the pericline law. These inclose minute 

 translucent scales of pale green secondary amphibole (actinolite) and 

 grains of epidote, magnetite, and hematite. Many show wavy extinction ; 

 on the albite twinning lines, by Levy's method, the maximum extinc- 

 tion lines approach 44 degrees, indicating anorthite. A very little 

 quartz may be distinguished among the feldspar grains, and in extinc- 

 tion often shows the concentric strain shadows elsewhere described. 

 Biotite is moulded around the ends of the hornblende grains in small, 

 irregular brown scales. Epidote is commonly dispersed through the 

 interspaces in orange to brownish red, cloudy crystals, nearly colorless 

 when minute, with high relief, often gathered into groups. Pleochroism, 

 yellowish, reddish, and colorless; the usual high interference colors. 

 The crystals are mostly elongated and six-sided, approximately in plane 



