526 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



intrusive origin which must not be neglected. Moreover, masses of shale and sandstone have 

 been embedded in the trap near both the under and upper surfaces, and the trap itself shows 

 evidence in its texture of having cooled more slowly (and therefore presumably at greater 

 depths) than the overflow sheets. 



The overflow sheets are contemporaneous with the beds between which they lie — rthat is, 

 the upper third of the Brunswick shales. 



The intrusive masses extend, for the most part, well up into the Brunswick shales and are 

 therefore younger than these. Moreover, so far as the evidence goes, they antedate the dis- 

 turbances which closed the deposition of the Newark beds. There are good reasons for believing 

 that many, perhaps all, of the intrusive masses are younger than the extrusive sheets, although 

 the evidence is not conclusive. From a priori considerations it may be suggested that the lava 

 formed intrusive sheets after the formation became so thick that it could not readily rise to 

 the surface, whereas earlier in Newark time the lava was able to break through the inner beds 

 and overflow. 



Kummel ^"^ gives his first careful estimate of the maximum thickness of the 

 sedimentary formation of the Newark group as follows : 



Feet. 



Stockton 4, 700 



Lockatong 3, 600 



Brunswick 12, 000 



20, 300 



These figures his later investigations led him to modify, his revised estimate "'^ 

 being : 



Feet. 



Stockton 2, 300- 3, 100 



Lockatong 3, 500- 3, 600 



Brunswick 6, 000- 8, 000 



11, 800-14, 700 



The Newark group passes from New Jersey into Pennsylvania in the Trenton 

 quadrangle and is described in detaU by Darton ^* in the Trenton folio. 



The continuation of the belt northwest of Philadelphia is discussed by Dar- 

 ton, ^^° who says: 



The Newark group in Pennsylvania occupies a broad belt extending across the southeastern 

 portion of the State from Delaware River to the Maryland line south of Gettysburg. It is 32 

 miles wide on the Delaware, 12 miles on the Susquehanna, 4 miles in northern Lancaster County, 

 and 14 miles at the Maryland line. It is bounded by shales, limestones, sandstones, quartzites, 

 mica schists, gneisses, and granites, varying in age from Ordovician to pre-Paleozoic, which it 

 overlaps along irregular boundary lines. In some portions of the area its margin is determined 

 by faults, but these marginal faults appear not to be extensive either in throw or in length. 

 Over wide areas the dips are to the north and northwest, but in some districts there are flexures 

 of moderate amount and extent. The group is traversed by numerous normal faults, mostly 

 extending northeast-southwest and with downthrow on the east side of the fault plane. One 

 of these extending through Bucks and Montgomery counties has a vertical displacement of 

 several thousand feet. 



In the rocks of the Newark group in southeastern Pennsylvania, as in other regions, the 

 typical red-brown sandstone and shale predominate, and there are igneous rocks in intrusive 

 sheets and dikes. The classification which has been established in New Jersey is applicable 

 here, and comprises three formations — the Stockton, Lockatong, and Brunswick, the last 

 named being the youngest. A series of names proposed by the Pennsylvania Geological Survey 

 has not been found acceptable because of their indefinite application. 



