492 A. A. JFLIEX AMPHTBOLE SCHISTS OF MANHATTAN ISLAND 



products of alteration of volcanic tufif, it contains less silica, but the 

 structural differences are still more decisive against this view. 



Second, from amphibolization of limestone. Incipient stages of this 

 process are indeed shown in the limestones of the island, but, from the 

 paucity of the latter in alumina and iron oxides, these changes have 

 never resulted in production of hornblende or augite. The composition 

 of amphibole known to be of that origin from other regions may approxi- 

 mate that of Manhattan island, but the solution of the question must be 

 sought in otiier considerations. 



Third, from alteration of basic igneous intrusions. Tliis a])pears 

 established by the correspondence of the hornblende rock in chemical 

 composition to basic igneous rocks and to hornblende schists of that 

 derivation, by identity of its hornblende constituent with that found in 

 volcanic rocks, b}^ the discover}^ of many apophyses, isolated or in groups, 

 and other structural features, and by the survival of products of contact 

 alteration. The absence of pyroxene and of dike-like intersection of the 

 associated gneisses may be w^ell explained b}- the extent of shearing and 

 metamorphism. 



These hornblendic schists then represent the most ancient group of 

 intrusions into the Manhattan series, probably both as dikes and sills 

 and mainl}^ aloig one or two horizons, about 500 feet apart. In the 

 larger sheets the trap was apparently porphyritic, wath holocrystalline 

 ground mass, of extreme basic composition and probabl}" rich in pyrox- 

 ene. While no definite evidence appears of the survival of that mineral 

 in any rocks of the island, the remnants of phenocrysts point to the 

 gabbro-like character of these ancient dikes, and perhaps to their issue 

 as a fringe to the now well known center of eruption, 30 miles north, 

 near Croton point* — or more likeh', I think, at an independent center 

 of still greater antiquit}^ in the Manhattan region. 



During the initial shearing the ready flow of the basic material has 

 produced extensive folding and crumpling of sheets, perhaps assisted 

 in some cases by downward crushing of upright dikes and possibly by 

 rolling out of thick laccolitic masses. This was further advanced, even 

 to minute corrugation of laminae, by the softening w^hich attended the 

 beginning of pegmatization. 



But my latest consideration of the phenomena suggests that the basic 

 material, originall}^ injected probabl}^ in continuous thin sheets along only 

 one or two planes, has moved freely under the intense pressures, squeez- 



*In 1865 Dr Hermaua Ciedaer published his "Geoguostische Skizze der Umgegend von ^■e\v 

 York" (Zeits. d. D. geol. Ges., vol. xvii, 1«65, pp. 38S-398), iu which he diduot recognize the pres- 

 tsnce of ampliibole rocks on the island, though from several miles to the north up to the vicin- 

 ity of Peekskill he records the passage of hornblende gneiss and schist into syenite and 

 hypersthenite. 



