450 A. A. J U LIEN AMPHIBOLE SCHISTS OF MANHATTAN ISLAND 



XIII. Dolomitic limestone. Inwood cross- valle}^ Manhattan island. J. F. 



Kemp. 



XIV. Dolomite. Westchester county. J. W. Alsop, Jr. 



XV. Dolomite marble, Tackahoe, Westchester county. W. F. Hillebrand. 





XIII. 



XIV. i 



XV. 



CaO 



38.42 



10.29 



1.16 



30.75 ' 



20.30 : 



1 



30.68 



MgO 



20.71 



AloOo and FeoOo 





FeO 



.77 •; 



46.66 1 



.21 



COj. 





46 66 



H2O . . ... 





36 



Si02 and insol 



3.27 



1.30 



1.33 











99.78 



99.75 



Though the specimens analyzed contain veiy little alumina and iron 

 oxide, dolomites have been shown to possess a large amount, reaching 

 7.68 per cent in one from Gharlemont, Massachusetts, and even 9.60 per 

 cent in a dolomitic marl from the vicinity of Stuttgart.^ In the study 

 of sedimentation, the passage of lenses of limestone into calcareous shales 

 is familiar to every geologist, with still greater variation in the inter- 

 mixture of carbonates with iron oxides and alumina. It would also 

 appear that during the process of metamorphism of limestones no con- 

 .dition could be more unstable than the relationship of lime and mag- 

 nesia carbonates to intermixed ferruginous, aluminous, and silicious 

 impurities — no reaction, perhaps, in nature more ready, rapid, and thor- 

 ough than the combination of these components into amphibole and 

 pyroxene. Accordingly we find tremolite and diopside commonly dis- 

 tributed in little nests through the crystalline limestone at all its out- 

 crops on Manhattan island, as well as at those north of the Harlem river, 

 at Tremont, Tuckahoe, etcetera. Phlogopite is still more abundant near 

 Inwood and Kingsbridge, evidently through the introduction of potassa, 

 much of the rock passing into a phlogopitic calcareous shale. To the 

 presence of alkalies the reaction may be attributed, resulting in feldspar 

 and mica, at a point a little farther north, about which Mather says : 



" Northeast from Sing Sing, the limestone becomes micaceous, and so modified 

 as not to be easily recognized. Much of it is very fissile, and so much intermixed 

 with mica and feldspar that it miglit with propriety be called a calcareous mica 

 slate; but much of it contains so little carbonate of lime as to be distinguished 

 with difficulty."! 



*Roth, op. cit., vol. ii, p. 577. 

 t Mather, op. cit., p. 548. 



