476 A. A. JULIEX AMPHIBOLE SCHISTS OF ^FANHATTAX ISLAND 



Duyvil creek, indicates that whenever present it must have been very 

 thin. 



Distance in centime- 

 ters from contact 

 line. 



K\u(] of rock. 



Specific gravity. 



60 



30 



3 



Pegmatitic hornblende gneiss 



Black hornblende schist 



Hornblende gneiss 



2.993 

 3.270 

 3.119 





Pegmatite seam, 1 centimeter thick . . . 

 Laminated micaceous gneiss 



U a ii 

 ii 11 11 





•> 



5 



6 



10 



14 



2.811, 2.797,2.776 

 2.837 

 2.779 

 2.762 

 2.776 



15 

 19 



ii a ii 



2.846 

 2.789 



22 



ii ii i( 



2.770 



25 



ii ii ii 



2.808 



37 



i4 ii (( 



2.777 



52 



ii ii ii 



2.796 



67 



Pegmatitic gneiss 



2.806 









Development of contact minerals. — Such alteration might consist of satu- 

 ration of adjoining gneiss by the basic feldspar of the hornblende rock, 

 or injection by augite or hornblende, or generation of new minerals 

 along the contact line, such as tourmaline, biotite, or garnet, as would 

 imply reaction through heat, fumarole vapors, or mineraHzers. The 

 amount of metamorphism of the Manhattan series could have hardh^ 

 caused utter obliteration of any such naturally durable records in the 

 marginal bands. From the absence of such observations b}^ others as 

 well as by myself, after a long search, it became apparent that certain 

 necessary conditions — for example, presence of moisture — had been gen- 

 erally wanting ; but a recent re-examination of sheets of hornblende 

 schist, 2 to 18 inches in thickness, at the northeast corner of West One 

 hundred and eighty-sixth street and W^adsworth avenue, revealed an 

 abundance of biotite and also of garnets, up to 1 centimeter in diameter, 

 both within the hornblende schist and in the contiguous pegmatitic 

 gneiss, but only within a distance of 2 or 3 centimeters from the contact 

 line. In thin-section the garnetiferous band of the hornblende schist 

 shows an abundance of quartz and hornblende, with much biotite, gran- 

 ular garnet intimately mixed with quartz, and almost no feldspar. 



It will be noted that garnet has never been found in the hornblende 

 schist elsewhere on the island, though a notable constituent of the horn- 

 blende and biotitic gneiss. With this fact should be associated the abund- 

 ant occurrence of garnet, in lumps 5 to 8 centimeters across, at outcrops 



