486 A. A. JUJ.IEX AMPHIBOLE SCHISTS OF MANHATTAN ISLAND 



Third. There is a limited amount of alteration resulting from pressure, 

 as explained b}^ Williams, who states that 



" Only such rocks within the Baltimore gabbro area exhibit a schistose structure 

 as have their pyroxene entirely replaced by green fibrous hornblende. By no 

 means all of the plagioclase hornblende rocks, however, are schistose. The larger 

 proportion of these areas are as massive as the gabbros. . . The rocks of the 

 Baltimore gabbro area do not exhibit in their individual mineral constituents the 

 effects of having been subjected to an enormous strain. The bending and break- 

 ing of crystals and the disturbance of their optical constants, which is so often 

 observed in the rocks of some much more contorted regions, are here rarely 

 noticed. . . The very fact that the pressure has not been as great as in certain 

 other more disturbed areas may perhaps itself be sufficient to account for the 

 abundant masses of the unchanged pyroxene rock occurring in the midst of its 

 hornblendic derivative." 



It may be added that the same close association of biotitic schists 

 attends the hornblende rocks at Baltimore* as those on Manhattan island. 

 Both their resemblances and their very differences in the two regions 

 only bring out the more satisfactorily the fact of their common origin. 



Serpentine Outcrops 



The decomposed and hydrated form of ferromagnesian silicate, dis- 

 tinguished as serpentine, is also found in this district, and its genesis is 

 interwoven with that of the amphibole schists. Its direct derivation 

 from them is plainly shown b3^ constant accompaniment and close inter- 

 mixture, as recognized b}^ Dana, Gratacap, F. J. H. Merrill, and others, 

 though referred by Dana back to antecedent limestone.t 



Only one area of importance occurred, which occupied several acres 

 in a long belt, with a width of 3 to 30 rods, J between West Fift3^-fourth 

 and Sixty-third streets, from Tenth avenue to the Hudson river (figure 

 9). On the southwest it was in contact with the most extensive boss of 

 gneissoid granite on the island, whose conversion from a gneiss into that 

 form by pegmatization seems to have been simultaneous with that of the 

 hornblende rock into actinolite schist. In regard to this outcrop, now 

 inaccessible, it may be of interest to consider some details taken from 

 old field notes, recorded during man}- visits in 1878. A large part of its 

 area occupied a basin-like depression, with swampv bottom and deeply' 

 gullied sides, about Fifty-eighth street, which seemed to have been exca- 

 vated in the softer material during some ancient time by a small stream 

 running westward to the Hudson. The bosses of tough rock had been 



*G. H. Williams, op. cit., p. .36. 



t Am. Jour. Sci., vol. xx. 1880, pp. 3it-32. 



* Mather, op. cit., p. 461. 



