480 A. A. JULIEN — AMPHIBOLE SCHISTS OF MANHATTAN ISLAND 



these results seem to be plainly connected with later impregnation and 

 cementation of the trappean sheets b}' free silica. This has been illus- 

 trated by the quartz and pegmatite lenses shown in plates 61 and 68. 

 The later intersection of the strata b}'' pegmatite dikes has often followed 

 the same planes of weakness, splitting up these basic dikes with new 

 seams of pegmatite and quartz (as well shown in the upright section of 

 hornblende schist, 40 feet high, at West One hundred and thirty-fifth 

 street and Saint Nicholas avenue) and forming a second period of 

 silicious impregnation. The convolutions and corrugations of the folded 

 layers are themselves shattered and separated in such a way as to testify 

 that their plasticity was an antecedent and temporary condition. The 

 apparent incongruity of the two classes of phenomena was therefore 

 founded on successive conditions of constitution, the physical qualities 

 of the original, highly basic, crystalline trap, a gabbro or diorite, and 

 the later characteristics of the more acid quartz diorite. 



Schists at Other Localities 



westchester county 



Hornblende rocks. — We gain further light by reference to similar schists 

 in the region northeast of Manhattan island, and to those in other tracts 

 of crystalline rocks. 



A comparison with the hornblendic schists of Westchester county 

 yields very satisfactorj^ and decisive results. It had been already pointed 

 out by early observers that intrusives are of common occurrence in that 

 county.^ We owe to Heinrich Ries the recognition and description of 

 the tract of granite diorite, about 1 by 7 miles in extent, near Harrison, 

 and also smaller tracts east of Portch ester and Rj^e and south of Ma- 

 maroneck. This rock was found to consist of quartz (40 to 50 per cent), 

 in part in augen form ; plagioclase, sometimes predominating over the 

 quartz ; considerable orthoclase and biotite and less hornblende, and a 

 small admixture of garnet, titanite, rutile, muscovite, microcline, zircon, 

 apatite, and pyrite. The main mass was decidedly gneissoid, passed 

 into mica schist, carrying sillimanite along the border, and was there 

 generally seamed with numerous veins of coarse granite and pegmatite.f 

 Mr F. J. H. Merrill has also stated : '* Near the shores of Long Island 

 sound the Manhattan schist is ever3^where injected with bands, lenses, 

 and dikes of pegmatite, granite, amphibolite, and pyroxenite." J 



Man}^ years ago I had studied some of the prominent intrusions near 



* Matlier, op. cit., p. 23. 



t Trans. N. Y. Academy of Sciences, vol. xi.\, 1893, pp. 80 8t;. 



; N. Y, State Mus. Rep., vol, L, 1896, p. 2;-5. 



