OCCLUSION OF K;NE0US rock 429 



the temperature was lowered as near faces of a fissure."^ Reefs 

 and islets in New York harbor have also been found to con- 

 sist of remnants of pegmatite dikes and of the gneisses which 

 predominate over Manhattan Island. Granite was struck at a 

 deep boring in Hoboken, and behind that city, at side of the 

 Weehawken hill road, in an outcrop " underlying the red sand- 

 stone."" The latter reference, however, is open to the sus- 

 picion that it may have related to the so-called white " arkose " 

 or bleached Mesozoic sandstone, often found below the trap in 

 that vicinity. 



The significant inference from these observations is that the 

 basic intrusion in the gneiss, now forming the ridge on the west 

 bank of the Hudson, was followed, as on Manhattan Island, by 

 intrusions of pegmatite. These found the tough material of the 

 overlying basic sill impenetrable, except perhaps on the north 

 line of the present Kill van Kull. Along this plane of weakness, 

 after elevation, the ensuing erosion of the acid rock resulted 

 in excavation of the mouth of the Kill and in separation of part 

 of theserpentinoid ridge, on Staten Island, from its continuation 

 on the mainland at Jersey City and Hoboken. 



Occlusion Tracts along the Appalachian Belt. — Intrusions of 

 the same general character and apparently of the same age have 

 been traced through the belt of crystalline schists all along the 

 coast, as far south as Alabama. It seems desirable to review 

 briefly certain results, at some points along this course, which 

 correspond to observations on Manhattan Island on processes of 

 occlusion. 



Maryland. — The Piedmont plateau in this State presents 

 two distinct tracts ^ of highly tilted schists in approximate par- 

 allelism to the coast line. The western consists of semicrystal- 

 line schists — slate and phyllite, limestone and marble, sand- 

 stone and quartzite — regarded as metamorphosed Palaeozoic 

 sediments (Cambro-Silurian) ; the eastern, of holocrystalline 



' T. G. Bonney and C. A. McMahon, Quart. Jour. Geol. Soc, XLVII, 1891, 

 483-490. 



2 1. Cozzens, A Geol. Hist, of N. Y. or Manh. Isd., 1843, 40-41. 



3G. H. Williams and C. R. Keyes, Bull. Geol. .Soc. Am., II, 1891, 301-322 



