494 A. A. JULIEX AMPHIBOLE SCHISTS OF MAXHATTAX ISLAXD 



tite, in dikes of this region, probably indicate tlie two materials usually 

 corresponding in segregation from a common magma of neutral composi- 

 tion, it is a curious anomaly to find in the former, the basic extreme, 

 as much quartz (40 to 50 per cent near Harrison, according to H. Ries) 

 as in the acid pegmatite ; but the universal association of the hornblende 

 schists with gneisses saturated with pegmatite of the earlier diffusion, 

 the later intersection, both of gneisses and these schists, b}^ abundant 

 seams and lenses of quartz, and the invariable revelation through its 

 optical characteristics, that this was the last comer among the minerals 

 of the hornblende rock, together testify to the secondary character of the 

 quartz disseminated through the great tracts of quartz diorite and dioritic 

 gneiss near Harrison, through the coarse diorites and amphibolites at 

 New Rochelle, Rye and vicinity along the Sound, and through their 

 sheared equivalents the dioritic and hornblende schists and gneisses 

 of Westchester county and of Manhattan island. All are but forms of 

 a silicified diorite. 



