FETTKE, MANHATTAN SCHIST OF NKW YORK 207 



not always. The fine-textured matrix in wliicli the biotite occurs consists 

 of muscovite, smaller biotite flakes, quartz and iron oxides. The little 

 flakes of muscovite and biotite are usually oriented parallel to tlie folia- 

 tion and often curve around the larger biotite crystals. 



Going north from the above locality the schist seems to show slightly 

 more severe metamorphism. Garnet and in some cases staurolite become 

 important mineral constituents. Tourmaline lias been introduced. A 

 specimen collected about one mile north of Croton-on-the-Hudson along 

 the road is made up largely of a matrix of flne muscovite in which numer- 

 ous garnet and staurolite crystals are imbedded. Under the microscope, 

 the matrix is seen to be made up largely of small flakes of muscovite to- 

 gether with some quartz and a little orthoclase and plagioclase (PI. XIII, 

 Fig. 5 ) . Most of the biotite present occurs in much larger flakes than the 

 muscovite. A light pink garnet and a 3^ellowish brown staurolite are 

 quite abundant, occurring in idiomorphic crystals. For a chemical analy- 

 sis of this schist see analysis 3 on page 212. This was the only place 

 south of the Highlands where staurolite was found as an abundant con- 

 stituent in the schist. It is only present elsewhere in very small amounts. 

 The rock here has a little more coarse-grained crystalline texture than 

 that described above. 



Another specimen taken from near the same locality shows upon ex- 

 amination in thin section under the microscope abundant little lath- 

 shaped crystals of dark brown tourmaline. The rock is also much more 

 quartzose and feldspathic. The feldspar is largely plagioclase of an ande- 

 sine varietv, showing extinction angles up to 25° in sections measured at 

 right angles to the albite lamellae. 



North of this area, no schist is again encountered until the southern 

 margin of the Cortland intrusive series is reached. A belt of gneiss and 

 limestone inten^enes. A specimen of the mica schist from a point one- 

 quarter mile west of Crugers, not far from the river and a short distance 

 south of the contact with the diorites of the Cortland series, is seen, under 

 the microscope, to be made up of biotite, muscovite, quartz and garnet, 

 associated with small quantities of orthoclase and plagioclase and a little 

 apatite. The biotite shows marked pleochroisin from light yellowish 

 brown to deep brown. It and the muscovite are often intimately inter- 

 grown with their basal sections in the plane of foliation. The irregiilar 

 interlocking quartz grains are also usually elongated ])arallel to the schis- 

 tosity. The garnet is very abundant in small crystals, rarely exceeding a 

 diameter of .2 millimeter. 



The schist northeast of Crugers along the railroa.d near the contact 

 shows verv much the same structure and mineralogical composition. 

 Feldspar, mostly orthoclase, is most abundant. Some of the quartz is 



