THE NEWARK ROCKS 5 I 



complementary to this subsidence of the trough. If the subsi- 

 dence were greater along one side of the trough than the other, 

 the central axis must have shifted toward the side of greater 

 depression, and the shore line on that side must have encroached 

 upon the land If this occurred to any marked degree during 

 the later stages of sedimentation, the shore conglomerates of 

 that time might rest upon the older rocks and so be basal con- 

 glomerates. At the same time they would be the correlatives 

 of the topmost shales found further from shore. In this sense 

 they would be the upper members of the series. This may 

 have been the case locally along the northwestern border, and 

 at the northern end of the estuary near Stony Point, but the evi- 

 dence is far from conclusive on that point. 



The lava flozus. — Before the completion of the Newark sedi- 

 mentation the quiet course of deposition was interrupted by great 

 flows of lava. Whether the lava issued from a single vent or 

 group of vents, or from a fissure is unknown. Certain it is, how- 

 ever, that the molten lava flowed over the soft mud in the bottom 

 of the estuary. Locally the mud was forced up into cracks in 

 the under side of the lava, or was thrown into low billows by the 

 enormous pressure of the overflowing mass. The molten rock 

 issuing forth into the water generated an enormous amount of 

 steam, causing the mud to froth up and mingle with the scori- 

 aceous lava. So, too, the more liquid lava flowed around and 

 over the cooled and broken masses of clinkers, forming the ropy 

 structure and the commingling of dense trap and breccia-like 

 scoriae sometimes seen in the Watchung trap sheets. Locally, 

 perhaps somewhat generally, the lava flows were so thick as to 

 rise above the sea level. In these cases the scoriaceous surface 

 was readily eroded : the waterworn trap fragments were com- 

 mingled with the red mud brought into the estuary by the rivers, 

 and a layer of trap conglomerate or sandstone was formed. 

 Such was the origin of the conglomerate in the back of the first 

 Watchung sheet near Feltville. In other localities the vesicles on 

 the upper surface of the lava were filled with the red mud as the 

 lava flow was buried beneath the succeeding sediments. The three 



