FETTKE, MANHATTAN ISCHI8T OF NEW YORK 219 



canie quite an important accessoi-y constituent of the hornblende schist, 

 but this mineral was present just as abundantly in specimens collected 

 elsewhere, where they were not taken from the vicinity of any contact. 

 In the mica schist, however, just above the lower slieet of hornblende 

 schist, which is the smaller of the two, a dark brownish green hornblende 

 identical with the one present in the hornblende schist was found in oc- 

 casional crystals. Such a hornblende was not noticed in any other speci- 

 mens of mica schist and is not a normal constituent of this rock. Appar- 

 ently the presence of the hornblende schist explains its occurrence. 



Closely related to the hornblende schist is a variety of hornblende or 

 quartz diorite gneiss which occurs in the mica schist at various places but 

 not nearly as abundantly as the hornblende schist itself. Its mode of oc- 

 currence and structural relationship are the same as that of the horn- 

 blende schist, and sometimes it grades into the latter. Megascopically it 

 is seen to be made up of alternating light and dark bands, usually less 

 than an inch thick, which grade into one another. 



Such a gneiss occurs about three-quarters of a mile southwest of Mill- 

 wood along the road to Ossining. The lighter bands, when examined 

 under the microscope, are seen to consist largely of quartz, feldspar and 

 hornblende, together with small amounts of biotite. A little garnet is 

 also present, as well as an occasional zircon. Small amounts of epidote 

 and zoisite occur as secondary minerals. A somewhat cataclastic struc- 

 ture has been developed. The feldspar consists of both orthoclase and 

 plagioclase. The latter is optically positive and shows extinction angles 

 up to 5° 30' in sections at right angles to the albite lamellae, which would 

 indicate an albite-oligoclase. The darker bands owe their color to the fact 

 that the hornblende becomes much more abundant in them, being the 

 most important constituent. The other minerals of the lighter bands, 

 however, are also present but in smaller amounts. 



Actinolite and Tremolite Schists and Associated Types 



Another type of schist occasionally found interstratified with the mica 

 schists consists predominantly of actinolite or tremolite. This type is 

 very similar in its mode of occurrence to the hornblende schist just de- 

 scribed. A sheet was encountered in the Catskill Aqueduct tunnel just 

 north of Shaft 18 at Madison Square, New York City. The borders of 

 the sheet consist of a very coarsely crystalline biotite schist, in which bio- 

 tite makes up the greater part of the rock. Most of the sheet, however, is 

 an extremely foliated tremolite schist. Wlien examined in thin section 

 under the microscope, it is seen to be made up largely of tremolite, biotite 

 and a little talc. The tremolite occurs in long acicular crystals showing 

 good prismatic cleavage. Transverse fractures are also well developed. 



