r)EVEL0PM7^:NT OF ACTlXOLlTK 443 



A thin-section of the tremolite schist first referred to showed the fol- 

 lowing composition under the microscope : Tremolite largely predomi- 

 nates in sheaves and fasciclesof fine fibers, 13'ing in all directions ; })artly, 

 also, in well defined fibrous prisms, with cross-partings. Betw^een the 

 cross-nicols, the usual bright interference colors; extinction angles 3 to 

 6 degrees. The interstices between the fibers are occupied by films of 

 iron oxide, yellow and red to gray. There is also a small amount of 

 groundmass, in minute patches, colorless and feebly anisotrope, with 

 grays of the lower first order as interference colors — probably talc. 



Greenish gray mottled amphibolite, at West Fifty-eighth street, near 

 Eleventh avenue. A very finely granular rock, almost aphanitic in 

 texture, which is quite uniforml}^ spotted with irregular blackish green 

 grains, generally 1 to 3 centimeters long. There is commonly a coarse 

 schistose structure (specific gravity, 2.844) ; but at many points the rock 

 becomes almost massive, often shot through in all directions with fibrous 

 prisms and blades of yellowish gray tremolite, glistening plates of talc, 

 broad scales of blackish green chlorite, and rarely blades of actinolite. 

 Under the pocket lens the predominant groundmass also reveals minute 

 tremolite fibers and white scales of talc. 



The additional varieties of this rock produced by hydration and ser- 

 pentinization through a more recent process of decomposition are further 

 considered under the head of serpentine. 



DEVELOPMENT OF BIOTITE OR CHLORITE 



The partial to complete alteration of hornblende, during shearing, into 

 biotite Avas shown to some degree in every outcrop of the rock (plate 61). 

 The initial stages of the process appeared in scattered scales of brown 

 mica, inclosed in or adhering to the hornblende, often only distinguish- 

 able under the microscope. Exceedingy common are shining black films 

 over the surfaces of division planes. Sheets of glistening black biotite 

 schist or biotitic gneiss, often garnetiferous, mark the next stage, still re- 

 taining some scattered blades or scales of hornblende. Good examples 

 were observed at West Ninety-second street and Riverside avenue ; at 

 East One hundredth street, between Third and Lexington avenues, and 

 on the Speedway, near Dyckman street. 



Then follows a purely biotitic gneiss or schist, or spangled or mottled 

 micaceous gneiss, carrying both black and white micas. The latter dif- 

 fers from the prevailing gneiss of the Manhattan series, generally in a 

 somewhat finer texture, greater richness in micas, and retention of many 

 scales of both micas, which are long and bladed like those of the ante- 

 cedent hornblende. Excellent examples were found at East Sixty-fourth 



