Geology of Hudson County, Xeiv Jersey. 41 



The joints of the slate are frequently rounded off and slick- 

 ensided. Some of the layers of slate near the trap are covered 

 with a net-work of intersecting ridges, looking like casts of 

 shrinkage-cracks ; sometimes the under surface of the trap has 

 taken an accurate cast of these markings. The markings men- 

 tioned should probably be referred to the action of the heated 

 trap on the material now forming the slate, causing it to crack 

 in imitation of shrinkage-cracks produced when wet mud is 

 allowed to dry in the sun. No ripple-marks, rain-drop impres- 

 sions or foot-prints were observed, or have ever been reported, 

 from these exposures of Triassic rock. 



Bull's Ferry is on the northern boundary of Hudson County ; 

 the trap-sheet continues, as we have already stated, northward 

 of this point, forming the Palisades. 



On the western side of Bergen Hill, at West End, and far- 

 ther northwest of Union Hill, there are long ranges of traji 

 pai'allel with Bergen Hill, now worn and rounded, and separated 

 from it by a narrow area of level land ; these appear to be the 

 outcropping edges of thin trap-sheets that branched off from the 

 main sheet on the upper side, in the same manner as those 

 exposed beneath the trap-sheet at Weehawken were formed on 

 the lower side. 



Throughout the whole section which we have examined along 

 the bank of the Hudson, there is abundant and cumulative evi- 

 dence that the main sheet for the most part rests unconform- 

 ably upon the broken edges of the stratified rocks, but still has 

 followed in a general way the planes of bedding of the sandstones 

 and slates. 



Another phase that the trap-sheets present, is illustrated in 

 the First Newark Mountain, at Plaiufield, N. J., Avhere meta- 

 morphosed shale is exposed on the top of the trap-ridge three 

 hundred feet above the surrounding plain, wiih trap both above 

 and below it. 



In the same ridge, west of Bound Brook, eight miles south- 

 west of Plaiufield, there is a deep valley in the trap which runs 

 parallel with the strike of the ridge. Such a valley could only 

 have been formed by the removal of a mass of stratified rock, 

 which at one time must have divided the trap-sheet, in the same 

 manner as the shale does that is now exposed at Plaiufield. 



