38 NEW YORK STATE MUSEUM 



crossed the beds from one horizon to another, it did so without 

 crushing or bending the strata to any extent. There must have 

 been some lateral crowding, as the beds were pushed aside to 

 make room for the ascending sheet of lava, but no evidence of any- 

 great dislocation has been found. Locally there are slight varia- 

 tions in the strike and dip of the beds near the trap, but these 

 are not abundant. Indeed, it is a source of surprise that the in- 

 trusion of so great a mass of trap was not accompanied by greater 

 dislocations in the inclosing strata. There is no evidence that 

 the beds were faulted by the intrusion of the trap, nor that the 

 prevalent westward dip is due in any respect to this cause. And 

 of course the idea, prevalent among so many, that the beds were 

 lifted above sea level, and that the present topography and eleva- 

 tion of the Palisade ridge itself is due to volcanic forces, is en- 

 tirely without foundation. 



Thickness. Estimates of the thickness of the trap sheet vary 

 greatly, partly because of great variations in its actual thickness 

 and partly because of the difficulty of making accurate estimates 

 in the case of a sheet, cut by faults, whose structural relations are 

 more or less obscure, and which has lost something by erosion. 



At Weehawken (N. J.) the thickness is estimated to be between 

 700 and 875 feet. At Fort Lee (N. J.) a well penetrated 875 feet 

 of trap before reaching the underlying shale, and probably 75 

 feet more must be added for the amount lost by erosion. At Pier- 

 mont the thickness is estimated to be about 850 feet. At Upper 

 Nyack, where the width of outcrop is over 2 miles, the structural 

 relations are too indefinite to permit an estimate of much value, 

 but the thickness seems to be at least twice that at Piermont. 

 East of Rockland lake the base of the trap is slightly below sea 

 level, and the crest of the ridge has an elevation of 610 feet. To 

 this must be added about 300 feet more, owing to the fact that the 

 trap here dips westward at an angle of 13°. Even then our 900 

 feet gives only a minimum thickness, since no allowance has been 

 made for loss by erosion on the crest. North of Rockland lake, 

 where the width of outcrop is about J of a mile, the minimum 

 thickness may not be more than 250 feet, though here the glacial 



