422 A. A. JULIEN AMPHIBOLE SCHISTS OF MANHATTAN ISLAND 



Page 



Chemical composition of the schist 457 



Identification of the hornblende 460 



Comparison of methods of graphic analysis 460 



Graphic analysis of tlie hornblende schist 466 



Dike-like linear extension of outcrops 468 



Intersection of gneisses by apophyses ■ 470 



Evidences of contact alteration 474 



In general 474 



Sharp line of demarcation 474 



Induration of a contact band 475 



Development of contact minerals 476 



Peculiarities of crystallization 477 



Uniformity of texture 477 



Survival of phenocrysts . 477 



Structural evidences of igneous origin 478 



Schists at other localities 480 



Westchester county 480 



Hornblende rocks 480 



Original hornblendites and quartz diorite 481 



Metamorphic diorites 483 



Chloride and ophiolitic amphibolites 484 



Appalachian belt 485 



Serpentine outcrops 486 



Summary of conclusions 491 



Review of the genetic hypotheses 491 



Paragenesis of minerals 493 



Introduction 



The common distribution of hornblende schists of undetermined or- 

 igin, exactly or essentially like those of Manhattan island, along the whole 

 Appalachian belt, in the Adirondack region of New York, throughout 

 the crystalline rocks of Canada and the Northwest, as well as in Eu- 

 rope, has led me into a rather full discussion of the genesis of our 

 local rock, in the hope that it may satisfy a more general interest. Al- 

 though nearly twenty years have elapsed since the careful investiga- 

 tion of similar schists in Maryland, we are still in need of "additional 

 light upon one of the much-mooted questions of Archean geolog}^ 

 namely, the origin of lenticular beds of hornblende rocks so often inter- 

 bedded in the old gneisses."* A majorit}^ of the outcrops of this rock 

 on Manhattan island have been covered by buildings, but many of the 

 finest ex])0sures (at least thirty-five) still remain open in street cut- 



* G. H. Williams: Preliminary notice of the gabbros and associated hornblende rocks in the 

 vicinity of Baltimore. Johns Hopkins Univ. Circ, no. 30 (1884). 



