FETTKE, MANHATTAN SCHIST OF NEW YORK 217 



In general, liowever, as already shown, these sheets run parallel witli 

 the foliation of the mica schist and are just as folded and crumpled as the 

 latter. At times, the hornblende schist is even more plicated than the 

 mica schist. This usually occurs where the latter was a rather quartzose 

 variety. In such a case, the hornblende schist is the more pliant member, 

 and naturally it was more closely folded. 



In a hand specimen, the hornblende schist appears rather massive, show- 

 ing some foliation, however, and a tendency to cleave in parallel plates. 

 Its color is greenish black which serves to readily distinguish it from the 

 lighter colored mica schist. In mineral composition, it consists princi- 

 pally of a dark green hornblende, together with subordinate amounts of 

 feldspar, mostly plagioclase, and quartz. Minor accessory constituents 

 usually present are magnetite, biotite, apatite, titanite, zircon and pyrite. 

 In some cases, garnet also occurs in it in quite appreciable amounts. This 

 schist maintains a fairly uniform mineral composition from place to place 

 without much variation in the percentages of the constituents present. 



The hornblende schists are particularly well developed along the south- 

 ern shores of Croton Lake in the vicinity of the old Croton dam. A thin 

 section of a specimen taken from an outcrop exposed in a cut a short dis- 

 tance west of the bridge across the lake at this place when examined under 

 the microscope is seen to consist largely of dark green hornblende, together 

 with minor amounts of quartz and feldspar (PI. XII, Fig. 4). Titanite 

 is present in considerable amounts. Other accessory minerals are biotite, 

 magnetite, zircon and apatite. The hornblende is a deep brownish green 

 variety showing marked pleochroism from light greenish brown through 

 bro\\mish green to deep green. Prismatic cleavage is well developed. It 

 often contains inclusions of titanite and apatite. The feldspar and quartz 

 occur in irregular interlocking grains of comparatively small size. The 

 feldspar is mostly plagioclase, although some of it is unstriated. It shows 

 extinction angles up to 16° 30' in sections at right angles to the albite 

 lamellae and is optically positive. It is evidently andesine. The horn- 

 blende crystals show a roughly parallel orientation which gives the rock 

 its foliated structure. The chemical composition of this specimen of 

 hornblende schist is given in a later paragraph. 



An interesting phenomenon was noticed along a fault plane which in- 

 tersected, obliquely to the foliation, the sheet of hornblende schist just 

 described. On both walls of the fault a thin coating consisting of dark 

 greenish brown biotite flakes was developed. Evidently during the shear- 

 ing accompanying the fault movement conditions were favorable for re- 

 crystallization, and the hornblende along the fault was converted into 

 biotite. A little secondary quartz was also introduced. The hornblende 

 schist must still have been buried under a considerable thickness of over- 



