SERPENTINE OUTCUOJPS 491 



Gale has also recorded the occurrence of ophicalcite in beds at West 

 One hundred and fifty-seventh street, about 100 feet west of Tenth 

 avenue, exactly on the line of strike of the actinolite schist on West 

 One hundred and fifty-fifth street. Indications of other outcrops of 

 serpentine have been since discovered at East One hundred and twent}^- 

 third street and Lexington avenue, and at Aqueduct shaft number 26, 

 both in coarsely crystalline dolomite. Of the latter it is stated, " It 

 plainly originates through the hydration of a white monoclinic pyroxene 

 . accompanied with the formation of abundant secondary cal- 

 cite."* Its analysis has been given (XXXVIII) in the preceding table 

 and differs sharply from the others by its lack of iron oxides. 



In the chemical reactions which have attended the latest change in 

 the antecedent actinolite beds, serpentinization, there has been an ob- 

 vious succession : First, hydration and bleaching as tremolite, with 

 removal of iron oxide or its partial recombination with alumina in 

 chlorite ; second, the development of serpentine, or sometimes talc, with 

 utter excretion of the lime of the original amphibole and its concentra- 

 tion in rude, layer-like deposits, almost massive, of ophicalcite. The 

 true ophicalcite, familiar to the petrographer, is a rock of very diff*erent 

 character and origin, whose serpentinization was preceded by the sat- 

 uration of a crystalline limestone with amphibole or pyroxene. Its 

 discrimination from calciferous serpentine derived from alteration of 

 gabbro has already been drawn ; f but we have here, on Manhattan 

 island, an instance of the latter which has acquired an imitative struc- 

 ture which bears some resemblance to that of a true ophicalcite. The 

 essential distinction between the two is that in true ophicalcite or ophi- 

 dolomite the calcite or dolomite is primar}^ — a survival of grains of 

 the original limestone; in the ophicalcite of Manhattan, secondary, a 

 chemical deposit in interstices of cellular serpentine ; the structural evi- 

 dences correspond. 



Summary of Conclusions 

 review of the genetic hypotheses 



The following are my chief conclusions as to the three hypotheses of 

 derivation of the hornblendic rock of Manhattan island : 



• First, from alteration of ferruginous sediments. It differs from them 

 widely in chemical composition — less silica, greater amount of iron oxides, 

 and excess of alumina ; also in structure and form. Compared with the 



* G. P. Merrill, Proc. U. S. Nat. Mus., vol. xii, 1889, p. 598. 

 tRoth : Op. cit., vol. ii, p. 190 et seq. 



