AGE OF EXTEUSIVES AND INTRUSIVES 209 



stopped and its lateral extension determined by the overlying extrusives, 

 which have since been removed from this region by erosion. 



AGE OF TEE EXTRVSIVES 



From the position of the extrusive sheets near the top of the sedimen- 

 tary series it is evident they were formed near the close of the period of 

 deposition, so far as it is represented in the strata that now remain. It has 

 been shown that the small remnants of extrusives at New Germantown 

 and Sand Brook are approximately contemporaneous with the Watchung 

 extrusives and possibly actual fragments of First and Second Mountain 

 sheets. All extrusive activity was therefore very late in Newark time. 



AGE OF THE INTRUSIVES 



Several of the intrusive masses, as Eocky hill, Pennington, and Bald- 

 pate mountains, Cushetunk and Round mountains, penetrate strata far 

 above the middle of the series, and Kiimmel* states that "there are good 

 reasons for believing that many, probably all, of the intrusive sheets are 

 younger than the extrusive, although the evidence is not conclusive." 

 He further suggests that intrusives were formed only after the overlying 

 sediments became so thick that the lava could not readily rise to the 

 surface. 



According to any theory of deposition, however, the elevation of the 

 surface did not yrtj greatly during the whole of Newark time, and there- 

 fore the force necessary to lift the lava to the surface would be little 

 greater at the close of deposition than at its beginning. 



Doubtless many of the deeper strata had become appreciably consol- 

 idated, however, before the close of the period, and the extrusive sheets, 

 if of earlier origin, formed an additional barrier against further eruption. 

 But there is also a strong tendency to intrusion inherent in the characters 

 of the rocks themselves, and which must have existed even at the time of 

 the extrusive flows. The specific gravity of the trap rocks averages about 

 3.0, while that of the inclosing sediments is near 2.5. Thus for every 

 hundred feet in height of a column of lava there is an excess pressure of 

 about 20 pounds per square inch over that of the inclosing strata, and 

 this is the measure of the tendency to intrusion. A column of lava 

 rising through 10,000 feet of sediments would thus exert an excess pres- 

 sure of 2,000 poimds per square inch at the bottom of the series, and if 

 it could be maintained stationary in a liquid state it would all force its 

 way into the lower strata, lifting the overlying beds upon its surface. 



A gentle welling up of lava under such conditions would perhaps 

 always result in the formation of some intrusive masses, and the over- 



* Annual Report of the State Geologist of New Jersey for 1897, p. 99. 



