PHASES OF METAMORPHISM 441 



with proportion of feldspar varying from maximum in a gneiss to min- 

 imum in a schist. Their common invasion by pegmatitic seams and 

 dikes has resulted in increment of orthoclase, and probably of menacca- 

 nite, titanite, and garnet. The rock thus varies from a quartz-diorite 

 schist or dioritic gneiss to a true hornblende-orthoclase gneiss, with epi- 

 dotic varieties. 



Phases of Metamorphism 



in general 



There are several interesting phenomena of structural change and of 

 mineral development, generated during the metamorphism of the horn- 

 blende schist and gneiss, which should be next differentiated from these 

 rocks in their original condition. 



DEVELOPMENT OF ACTINOLITE 



This mode of alteration of the hornblende is, particularly in micro- 

 scopic form, almost universal, represented by pale, greenish blades and 

 scales. That it is not of primary origin is shown by the appearance of 

 alumina, separated from the black hornblende by this ohange, sometimes 

 in the form of spinel, more commonly that of biotite or chlorite. With 

 further progress of the alteration, the following varieties of actinolite and 

 tremolite rocks have been thus produced. 



Green actinolite schist, as, for example, at West Seventy-eighth street, 

 near Amsterdam avenue, and at West One hundred and fifty-fifth street, 

 100 feet west of Tenth avenue, the present site of Trinity cemetery. This 

 consists of a fibrous, laminated rock of yellowish green color, made up 

 almost altogether of blades of actinolite, rarely exceeding 2 or 3 milli- 

 meters in length, parallel to the foliation, but imperfectly so to each other. 

 The intervening white seams, often 1 to 10 millimeters thick, are occupied 

 mainly by granules of quartz, but in part, at the West One hundred and 

 fifty-fifth street locality, by flakes of a pinkish white mineral of alumi- 

 nous odor on moistening and somewhat harder than kaolin. Some 

 planes are rich in glistening scales of biotite. 



Dana states : 



"A bed consisting chiefly of radiated actinolite occurs west of Kingsbridge, north 

 of the Harlem river. . . . The looseness of texture shows that something has 

 been removed, and this is probably, in part at least, calcareous material ; and, if 

 so, the bed should be classed with the limestone beds." * 



Since this outcrop, however, lies on line of the strike of the dioritic 

 gneiss on the south side of the stream, I think that this actinolite bed 



*Am. Jour. Sci., vol. xx,1880, p. 25. 



