SECOND HYPOTHESIS OF DEEIYATIOX 449 



found nearest in composition to our schist is given above in column XII. 

 In view of the great amount and wide distribution of our hornblende 

 schist, even beyond Manhattan island, throughout Westchester county, 

 over a region of many square miles to the north and northeast, as well 

 as along the Appalachian belt, it must obviously owe its origin to a ma- 

 terial of common occurrence, likely to be included in recorded analyses 

 of basic rocks. 



Both forms of this hypothesis therefore seem to me untenable to 

 account for the origin of our schist. 



Second Hypothesis : Amphibolizatiox of impure Dolomite 

 the process of amphibolizatioy 



The efficiency of such a process, during metamorphism of limestone, 

 has been long and generally recognized, so far as regards the earlier 

 progress and products of amphibolization. The process has been thus 

 described by Dana, with application to the amphibole and serpentine 

 rocks at West Fift3^-ninth street and elsewhere on the island, besides those 

 in Westchester county, at Xew Rochelle, Rj^e, and elsewhere : 



" When the raagnesian Hmestones thus made were afterwards rendered meta- 

 morphic, part of the magnesia and lime . . . was in many eases made into 

 sUicates, such as tremolite, white pyroxene, and other species ; or, when iron has 

 also been present, into other related silicates of light or dark green tints, as 

 hornblende, actinolite, green pyroxene . . . the darker kinds resulting when 

 the all-pervading ingredient, iron, was present." ^ 



Van Hise also explains : 



*' Hornblende schist may be produced from an impure dolomite or ankerite, the 

 process being essentially the same as in the production of actinolite schist or ac- 

 tinolite-magnetite schist; only, on account of the greater multiplicity of bases 

 present, hornblende is produced rather than actinolite." t 



In application of this hypothesis to the present problem three facts 

 call for attention : 



LOCAL EVIDEXCES OF PARTIAL AMPRIBOLIZATIOX OF LIMESTONE 



The composition of the crystalline magnesian limestone in this region, 

 within which it is generally admitted silicated solutions have brought 

 about considerable metamorphism, has yet been shown by few published 

 analyses : 



*Am. Jour. Sci., vol. xxii, 1881, pp. 115, 116. 

 fOp. cit., p. TO.-^. 



