SECOND HYPOTHESIS OF DERIVATION 



453 



As to stage 3, direct serpentinization of limestone, which is a marked 

 feature of its alteration in Massachusetts, finds here but slight examples. 

 The ophicalcite at West Fifty-eighth street will be considered beyond. 



On the other hand, the amphibolites after limestone in Massacnusetts 

 show no tendency to form serpentine and steatite, as do those in the 

 same region, associated with gabbro-like beds, olivine, and enstatite rocks 

 of apparentl}^ eruptive origin. On Manhattan island a similar resist- 

 ance to ophiolitic decomposition is shown by the dense black horn- 

 blende schist; traces of serpentine have been observed at East Ninety- 

 fifth to One hundred and second street, and at West One hundred and 

 twenty-second street, near Tenth avenue ; but the serpentinization of 

 amphibolite, actinolite, and tremolite schist, with development of much 

 talc and chlorite, has been abundantly noted from West Fifty -eighth to 

 Sixty-third street, near Tenth and Eleventh avenues (figure 9, page 

 488), as well as in the coarse hornblendites, diorites, etcetera, at New . 

 Rochelle, Rye, and vicinity. 



A comparison of the characteristics and relationships of the lime- 

 stones, amphibole rocks, and serpentines of Massachusetts, Avith those of 

 Manhattan island, yields no reason therefore to favor the view of the 

 derivation of our amnhibole rocks from alteration of limestone. 



DISCRIMINATION BY CHARACTER OF THE AMPHIBOLE 



Some light might be thrown on the origin of amphibole rocks in 

 Massachusetts through evidence as to the relationship between the green 

 amphibolites and black hornblendites. Emerson observed the actual 

 passage of black hornblende schists into limestone, and not only in- 

 closure but intermixture therewith, so that in undecom posed rock a 

 remnant of carbonates formed a common constituent. From the analyses 

 by Eakins* of green and of black amphibolites the following averages 

 of four analyses of each kind have been prepared: 



SiOa- 



Ti02. 



AI2O3 



Fe^Oa 



FeO 



MnO 



CaO . 



MgO 



K2O 



Na^O 



XVI. 



97.78 



XVII. 



Green. 



Black. 



50.02 



49.82 



.89 



1.13 



17.46 



16.19 



2.11 



2.66 



7.93 



8.82 



.14 



.12 



8.21 



9.21 



7.74 



8.07 



.52 



.45 



2.76 



2.74 



99.21 



* Emerson, op. cit., p. 303. 

 LXIII— Bull. Geol. Soc. Am., Vol. 14. 1902 



