200 J. V. LEWIS NEWARK TRAP ROCKS OF NEW JERSEY 



trap or diabase. Both extrusive and intrusive types are abundantly rep- 

 resented — the former usually in the form of sheets that spread quietly 

 over the surface, with only occasional local beds of tufE; the latter as 

 sills, bosses, and dikes. Davis, Darton, and Kiimmel have quite clearly 

 differentiated the extrusive sheets from the sills. 



The distribution of these rocks is shown on the map, plate 1. They 

 are irregularly distributed over all parts of the belt and constitute more 

 than one-tenth of the total Newark area of the state. Their prevailing 

 northeast and southwest direction is notable. This is due to the prepon- 

 derance of sheets, both extrusive and intrusive, and to their general con- 

 formity to the structure of the inclosing shales and sandstones. 



RELATIONS OF THE EXTRUSIVE TRAPS 



General characteristics. — The Watchung mountains, including the 

 various trap ridges of three successive lava flows, are all extrusive, as are 

 also the semicircular ridges near New Germantown and Sand brook. 

 The grouping of these sheets in the west central portion of the area, 

 where no intrusives occur, and their almost total absence from the areas 

 of intrusives to the northeast and southwest are characteristics that are 

 clearly shown by the map, plate 1. The former is the area of upper- 

 most, and therefore of latest, Newark strata, while the deep-seated intru- 

 sive masses have been laid bare only where these later strata have been 

 removed to great depths by erosion. 



First mountain. — The narrowing down of First mountain north of 

 Somerville and its final termination near Pluckamin seem to be due to 

 the thinning out of the lowest extrusive lava sheet. Although no evi- 

 dence of faulting could be detected in the homogeneous red shale of this 

 region, Kiimmel* suggests that this ridge might have been sheared off 

 diagonally by an extension or a branch of the Hopewell fault, which may 

 be continued in the deep gorge of the North branch of the Earitan. 



The double flow of Second mountain. — Second mountain recurves in 

 hook form at its southwestern extremity until cut diagonally across by 

 the great boundary fault near Bernardsville. This fault continues north- 

 ward and terminates the curved extremities of all three of the extrusive 

 sheets beyond Pompton. 



The width of outcrop of the trap sheet of Second mountain varies 

 greatly, the hook-shaped southwestern portion being much the broadest 

 part (see maps, plate 2). Here also for a distance of 17 miles the 

 crest is distinctly double, and in the intervening valley shale has been 

 found at a number of places, either in wells or at the surface. The crest 

 is single, however, at both ends of the ridge., and the gorge of Passaic 



* Annual Report of the State Geologist of New Jersey for 1896, p. 81. 



