SUMMARY OF CONCLUSIONS 493 



ing back and forth along these division planes of the more rigid strata, 

 here pinching out and apart and tliere thickening up into larger isolated 

 lenses, as now found along the strike. If this be true, the present thick 

 beds are generally, at least, onl}' the result of such rolling together of 

 masses of the ancient paste and not of original intrusion. Even the ap- 

 parent apophyses also may be those of secondary injection along foliation 

 planes during the new condition of fluidity assumed during shearing 

 and metamorphism. 



By saturation with quartz and addition of a little orthoclase, consoli- 

 dation took place; so that during continued orogenic movements and 

 intrusion of pegmatite dikes these lenses of schist acted as rigid masses, 

 suffering fracture, dislocation, and sometimes mutual attrition along 

 faults to a friction breccia. 



Comparison with basic rocks in the neighboring region, of whose in- 

 trusive character there is no question, shows such a trap in all stages of 

 alteration, from a gabbro-like diorite to hornblende schist and gneiss, 

 identical with those of this island, even to microscopic peculiarities. 

 Similar series of ancient intrusions occur in the tracts of crystalline 

 rocks along the entire Atlantic belt, in whose metamorphism the earlier 

 phase is well illustrated in the pyroxenic gabbro-diorite series of Mary- 

 land, and one of the latest in the hornblende schists of Manhattan. 



PA RA GENESIS OF MINER A LS. ^ ' 



Finally, the facts above presented impress some conclusions concern- 

 ing the paragenesis of three minerals. 



First, the particular significance in the Manhattan series of the pres- 

 ence of hornblende. Notwithstanding a good proportion of all its re- 

 quired bases in the composition of the gneisses of the island (analyses I 

 and II), further confirmed by the general diffusion of biotite, and not- 

 withstanding the wide play of changing conditions which has prevailed 

 during alteration of the old sediments into the present o;neisses, not a 

 trace of hornblende has been anywhere developed. Its occurrence has 

 been no accident of metamorphism, but is found exclusively confined 

 to the thin intercalated sheets now shown to represent injected igneous 

 rock. 



Second, the generation of epidote only under conditions of press- 

 ure and strain. It has been shown to be the invariable accompani- 

 ment of extreme folding and minor crumpling of the hornblende schist, 

 its mere presence bearing the same testimon}^ as the strain shadows, 

 under the microscope, of the associated minerals. 



Third, the secondary character of the quartz, constantly found in close 

 intermixture with the hornblende. While the quartz diorite and pegma- 



LXVIII— Bum,. Geoi. Soc. Am., Vol. 14. 1902 



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