145 



tion are, first, where the trap reposes unconformably upon the 

 secondary strata, as may be seen in a number of interesting 

 exposures; and, secondly, where it intersects or traverses the 

 strata in nearly vertical dikes, conceived, from the known volcanic 

 nature of the rock, to exist in every trappean ridge, but only 

 occasionally disclosed to observation. 



That the trap is of posterior origin to the red sandstone forma- 

 tion which it overlies and intersects, is obvious, but at what pre- 

 cise epoch we are to suppose it to have been etruded to the 

 surface, and whether all its numerous outbursts were contempo- 

 raneous or occurred at successive periods, are points open to 

 discussion. After reviewing all the phenomena of igneous action 

 in the region, we shall touch upon these interesting questions. 



Throughout the whole district where the trap abounds, we 

 notice instances of its superposition to the stratified shales and 

 sandstones through which it has burst its way to the surface. 

 Interesting proofs of this occur along the shore of the Hudson, at 

 the eastern base of the Palisadoes, where several varieties of the 

 sandstone are visible near the water's side, dipping at a gentle 

 inclination westward and into the ridge. The same thing is well 

 displayed in the bed of the Passaic river below the Falls at 

 Paterson ; the trap reposing over the westerly dipping beds of 

 the sandstone, and presenting some curious evidences of its 

 having reached this position while fluid and in a state of intense 

 ignition. 



Of the Local Features of the Trap and the Alterations induced by 

 it on the Adjacent Strata. 



I now proceed to a particular account of some of the more 

 striking features of these injected dikes, and of the modifications 

 they have produced in the contiguous stratified rocks. Many of 

 these changes in the texture, contents, aspect, and properties of 

 the neighbouring rocks, attributable to the intense heat originally 

 imparted by the intrusive trap, are of a kind familiarly noticed in 

 the vicinity of igneous veins and dikes; but there are some which 

 1 shall have occasion to describe, that are of a less common cha- 

 racter, and which tend, by the views connected with them, to 



13 



