202 J. V. LEWIS — NEWARK TRAP ROCKS OF NEW JERSEY 



the alternative hypothesis of a double flow with intercurrent deposition 

 and warping proposed. This is practically the interbedded shale hypoth- 

 esis referred to above without the restrictions of stability of land surface 

 and uniformity of deposition between the lava flows, and in this form it 

 is believed to be entirely consistent with all ascertainable facts and to 

 involve no improbable assumptions. After the outflow of the first trap 

 sheet of Second mountain, which averaged probably 500 feet in thickness, 

 there began a gradual depression of what is now the southern axial region 

 of the great Passaic basin (or Watchung mountain) syncline, the region 

 northeast of Somerville and Bound Brook. Subsequent deposits were 

 concentrated in this region, tending to build it up to the level of adjacent 

 areas; but before this condition was finally attained another eruption 

 occurred, forming a lava sheet of very variable thickness. Over the 50 

 feet or so of shales in the area of subsidence the maximum thickness of 

 lava was at least 800 feet, while in the adjoining regions, where it over- 

 spread the bare flanks of the preceding flow, it probably did not exceed 

 200 feet in thickness. Thus the two flows, separated by a brief interval 

 of deposition, merged into one on the sides of the incipient syncline, but 

 in the trough of the depression were separated by a thin stratum of shale 

 (see maps and sections, plate 2). 



The sediments between Second mountain and the overlying trap sheet 

 of Long hill give evidence of continued subsidence in the same region. 

 There is a decrease of one- fourth in the thickness of the intervening shales 

 at Madison as compared with those at Millington, and a much more rapid 

 thinning out toward the west. The trap sheet of Long hill is also 

 thicker about Millington, but, considering the thin and discontinuous 

 character of this sheet, the variation in thickness may be due to other 

 causes than the warped surface over which it flowed. 



Third mountain, or Long hill. — Long and Eiker hills and Packanack 

 and Hook mountains, from tlieir structure and attitude with reference to 

 each other, are regarded as probably outcrops of a single extrusive sheet. 

 There is no question as to their extrusive character, and they are all 

 much thinner than the great flows that constitute First and Second 

 mountains. Their disconnected character may be due to projecting lobes 

 or tongues of a relatively slight flow of lava, or they may be the result of 

 several small eruptions from independent vents. On the other hand, 

 they may be regarded as the effects of glacial or preglacial erosion 

 whereby the continuous sheet of lava was deeply notched, the portions 

 that were most worn down being now buried by the thick glacial drift 

 and alluvial deposits of the Passaic river that cover so much of this 

 region. So far as known, all of these hypotheses seem to be consistent 



