210 J. V. LEWIS NEWARK TRAP ROCKS OP NEW JERSEY 



flow would be roughly a measure of the force of eruption in excess of that 

 required merely to sustain the column. Solidification of the sedimentary 

 strata, in whatever degree, would also favor intrusion, unless the eruption 

 followed a previously formed fissure, since cohesion is always less along 

 bedding planes than across them. Overlying extrusive trap sheets, if 

 present, would greatly augment these predisposing conditions due to pro- 

 gressive induration of the sediments and great difference in specific 

 gravity, and might serve to divert an eruption of considerable violence 

 into subterranean channels. It seems entirely probable, therefore, since 

 the period of intrusion is known to have been in late Newark time, that 

 it was subsequent to one or more of the extrusions and possibly contem- 

 poraneous with one of the later extrusives. 



Furthermore, the investigation of the copper ores* deposited in many 

 parts of the Newark in New Jersey has shown that the origin of many if 

 not all of them is intimately connected with the intrusion of the great 

 Palisades sill and its numerous offshoots; but the relations of some of 

 these to the extrusive trap of First mountain are such that they could 

 have been deposited only after the formation of the trap sheet and some 

 of its overlying sediments. Hence these studies also have led to the 

 • conclusion that the period of intrusion was subsequent to at least the 

 first extrusive flow. 



On the other hand, several considerations make it clear that intrusion 

 preceded the deformation of the strata by faulting : 



(1) The numerous faults by which the traps, both extrusive and in- 

 trusive, are displaced throughout the area (plate 1). 



(2) The intrusives of Sourland mountain, mount Gilboa, and Byram 

 are brought up in a great series of strata twice repeated by the Hopewell 

 and Flemington faults. 



(3) The western extremities of the Cushetunk Mountain trap seem to 

 be determined by a fault or faults, probably northward extensions of the 

 Flemington fault. 



(4) Nowhere do the intrusives follow or send off branches along these 

 fault fissures, and hence there is no evidence of igneous activity either 

 during or subsequent to their development. 



Thus all considerations agree in placing the date of intrusion after the 

 formation of the first extrusive sheet and before the deformation of the 

 strata by faulting and tilting. Possibly, therefore, the great intrusive sill 

 is contemporaneous with one of the great lava flows of the second extru- 

 sive, or with the scant eruption of the third, or even subsequent to both. 



* J. Volney Lewis: "The Newark copper ores of, New Jersey." To be published in the 

 Annual Report of the State Geologist of New .Tersey for 1906. 



