424 A. A. JULIEN — AMPHIBOLE SCHISTS OF MANHATTAN ISLAND 



Fifty-fifth street and Tenth avenue, and probably underneath the Bat- 

 tery, at the southern end of the island. On the east shore of Staten 

 island it apparently underlies the serpentine. Apophyses of pegmatite 

 and granite, in the form of dikes, sometimes 10 feet or more in width, 

 penetrate abundantly upward through all the foregoing strata on Man- 

 hattan island, and in less number throughout Westchester county. An 

 eruptive boss of subordinate character seems to be represented by the 

 granite-diorite exposures at Harrison, Rye, and on Long Island sound 

 between Portchesterand Greenwich, Westchester county, as well as near 

 Ravenswood, on Long island, and in the adjoining part of the bed of 

 East river. 



Evidences of Sedimeintary Origin 



In regard to many of these schists the following characteristics have 

 been generally accepted as evidences of their sedimentary origin : 



Granular and often loose sandy texture, prevalent in the quartzitic 

 layers of the schists and gneisses and in the quartzites, suggestive of im- 

 perfectly altered clastic rocks, " underdone," as Dana described it. The 

 particles of quartz and feldspar are generally angular, as if from crush- 

 ing, but the smaller quartz granules may be rounded. 



Marked alternation of thin layers of sandy gneiss and mica schist, the 

 one or the otlier predominating in the gneisses below or above the lime- 

 stone. The three materials offer a strong correspondence to altered 

 layers of quartzose sandstone, sometimes argillaceous — of shale — and 

 of more or less impure magnesian limestones. The thin layers of horn- 

 blende schist, in Dana's opinion, resemble original layers of highly 

 ferruginous magnesian shales. 



A schistose structure, so characterized by the regularity of the above- 

 stated alternations as to imply its general relation to true bedding by 

 sedimentation rather than to foliation b}^ shearing and dynamic meta- 

 morphism during the folding of the strata ; in some cases, however, only 

 evidence of the latter agency remains. 



These characteristics are best preserved in the Fordham gneiss, as 

 shown in the fine section at One hundred and fifty-third street and 

 Seventh avenue. 



Stages of Metamorphism 



Without considering for the present the basic schists and limestone, 

 four stages are, in my opinion, plainly indicated in the alteration of the 

 acid or silicious schists below and above the limestone. 



The first was concerned in the early consolidation of the sediments, 



