426 JULIEN 



as commonly prevailed throughout Manhattan Island and the 

 neighboring county, Westchester. For this result the influential 

 conditions seem to have been mainly an excess of iron oxide, ^ 

 and partly the presence of alkalies required for development of 

 the black hornblende, syntagmatite, identified in the hornblende 

 schist of Manhattan Island. ■ 



Where the ferromagnesian elements, probably bronzite or 

 other orthorhombic pyroxene and diallage, predominated in 

 the original rock, perhaps a pyroxenite, a pure diallagite, or 

 bronzite-diallage rock, as near Baltimore, Maryland,^ or bron- 

 zite-diallage-enstatite-rock, like that of Chester County, Penn- 

 sylvania,* or in part peridotite, such as the olivine-bronzite-dial- 

 lage rock or Iherzolite of Baltimore County, Maryland,"^ this 

 constitution tended, during metamorphism, toward the develop- 

 ment of an amphibolite or smaragdite-rock,^ often sheared into 

 actinolite and tremolite schist. 



In this region these schists were later subjected to hydration 

 by meteoric waters, and then to ophiolitic decomposition, in part 

 by heated waters, with the following processes and products : 



1. Partial absorption of water by remnants of pyroxenes, 

 olivine and amphibole, with conversion into bastite, villarsite 

 an^ hydrated amphibole. 



2. Dissociation of bronzite and olivine into brucite and col- 

 loidal silica. 



3. Solution and removal of colloidal silica and deposition as 

 adjoining masses of compact chalcedony, jasper or ferruginous 

 chert (as also noted in Cecil County, Maryland "). These 

 siliceous deposits have not been found on Staten Island, but at 

 Hoboken testify to the more acid composition of the original 

 eruptive rock in the siliceous seams and in the layer of so-called 

 **jasperoid" (Henry Wurtz) formerly visible near the river- 

 level. 



1 J. D. Dana, Am. Jour. Sci., (3), xx, 1880, 26. 



2Julien, loc. cit., 466-468. 



3 Williams, loc. cit., 58. 



* Chester, loc. cit., 95-100. 



5 Williams, loc. cit., 54-57. 



6W^illiams, loc. cit., 58. 



^ Bascom, loc. cit., 94. 



