SERPENTINE OUTCKOrs 487 



also swept clean, scored and rounded by glacier action, so that fresh 

 surfaces were everywhere exposed for examination. A section from Fifth 

 avenue westward along Fifty-eighth and Fifty-ninth streets, across the 

 prevalent northeast strike in this vicinit3^ presented the following fea- 

 tures : 



From Fifth to Sixth avenues the predominant rock was gray mica- 

 ceous gneiss, with many thin intercalations of mica schist and seams of 

 granitoid gneiss, on the western side of a broad anticlinal fold ; dip, 

 70° > northwest. Near Fifth avenue rose a thick bed of slaty black 

 hornblende gneiss, highly epidotic, some layers pure epidosite, 2 to 3 

 inches thick. Near Sixth avenue a bed of tremolite schist about 40 feet 

 thick was intercalated, with the same westward dip, followed by mica- 

 ceous gneiss and granitoid gneiss. 



We may here digress to consider another parallel section of the same 

 sheet by passing only a hundred yards along the strike to the southwest, 

 between Fifty-seventh and Fifty-eighth streets. 



At Fifth avenue occurred layers of micaceous gneiss and mica schist, 

 greatly plicated ; strike, north 63° east, with dip 60° north, some becom- 

 ing north 43° east, with dip 80° east, with occasional thin granite seams 

 on the east slope of a broad anticlinal fold at Sixth avenue. 



Between Sixth and Seventh avenues the center of the fold near the 

 middle of the block, was occupied b}^ a fine grained, compact micaceous 

 gneiss (quartzose, with black and white micas), inclosing a sheet of 

 hornblende schist, 2 feet thick, about 100 feet from Seventh avenue; 

 dip, 80° > west. 



From Seventh to Eighth avenues, on the west slope of the anticline, 

 fine and coarse gray gneisses prevailed, sometimes garnetiferous and 

 inclosing a very thin sheet of hornblende schist about halfway and a 

 sheet of pegmatitic granite at Broadway. 



Comparison of these two sections of the same bed at Fift3^-seventh and 

 Fifty-ninth streets establishes two important facts : 



First, that the two thin sheets of hornblende schist in the Fifty-seventh 

 Street section are the tapering southern edges of two thick beds in the 

 Fifty-ninth Street section ; the one of hornblende gneiss, the other of 

 tremolite schist — that is, the latter is but an altered form of hornblende 

 schist. 



Second, that the hornblende rock does not lie on a single horizon in 

 the Manhattan schists, but on at least two planes, about 500 feet apart 

 on a section of the strata. 



Continuing the main section along Fifty-eighth and Fifty-ninth streets, 

 the same westerly dip prevailed to Twelfth avenue and the Hudson river. 



