416 JULIEN 



where the nemahte is concentrated, as in the lateral bands of the 

 veinlets, with cross-fibration, its higher birefringence and relief 

 betrays its intercalation among the films of serpentine and 

 chlorite. 



Genesis of the Serpentinoid Scliists. — Four hypotheses have 

 been advanced to account for the origin of the rocks in this 

 western ridge and of their congeners on Manhattan Island and 

 in Westchester County. 



First, Alteration of a sedimentary rock} There is no longer 

 need of discussion of this view. 



Second, Direct scrpentinization of dolomite. But that mode 

 of reaction and alteration, by solutions carrying silica, though 

 actually observed in the limestones of this region, is here as 

 generally elsewhere a subordinate and limited process. To this, 

 however, has been probably due the origin of the ferruginous, 

 dark green serpentine near Port Henry, New York,^ as well as, 

 in part, of the serpentine at Montville, New Jersey^ (and at Rye, 

 Westchester county, New York, according to Dana^). 



As for replacement by injection of serpentine bodily into dolo- 

 mite, it has been long ago observed that the conveyance of the 

 mineral serpentine by solutions, if it ever occurs, is very limiited 

 in distance. 



All such deposits are confined to cracks and cavities within 

 or very near the decomposing silicates from which the mineral 

 has been usually derived. Its transference in quantity therefore 

 and injection into a body of limestone are, I believe, everywhere 

 unknown. 



Third, Alteration of amphibolized dolomite. 



Alteration of a magnesian limestone might pass through two 

 metamorphic phases : amphibolization, the rock becoming pene- 

 trated or even replaced by actinolite, tremolite, and other min- 

 eral silicates ; and subsequent ophiolitic decomposition, by pre- 

 liminary hydration and later change into hydrous magnesian 



1 H. Wurtz, Proc. Lye. Nat. Hist. iX. V., 1870, 102. 

 2G. P. Merrill, Proc. a S. Nat. Mus., XII, 1890, 597. 

 ^ Idem., XI, 1888, 105-111. 

 *]. U. Dana, A771. Jour. Sci., (3), XX, 1880, 31. 



