3IO C. N. FENNER 



trace of water contained in the interstices of the ground would be 

 expanded by the heat, and having no other escape would force them- 

 selves into the molten lava and render it vesicular for a short distance 

 above the contact, but the great mass of lava would be left dense and 

 unaltered. At the surface of the sheet the escape of occluded gases 

 might also produce a slightly porous structure. But where the lava 

 poured over lake beds the case was different. The standing water 

 was probably not of sufficient volume to produce much effect, but the 

 underlying strata were thoroughly saturated. The heat of the lava 

 penetrated downward slowly but irresistibly, and the water was vapo- 

 rized. If there had been no means of escape, the pressure would have 

 gone on accumulating until it became enormous, but before this the 

 vapor began to push itself up through the over-lying lava, and thus 

 found vent. It continued thus to force its way long after the lava 

 congealed, and finally the temperature dropped to such a degree that 

 currents of heated water replaced the steam. The effects produced 

 constitute the record left and will be described in detail. 



Let us first, however, examine the structure of the trap in its less 

 affected portions. The gorge of the Passaic in the vicinity of Figs. 

 I and 2 offers good opportunities for this. It is seen that the contact 

 is practically conformable with the stratification of the underlying 

 sandstone series, but there are slight irregularities, and at the point 

 where Fig. 2 was sketched the trap drops about six feet. For a short 

 distance above the contact, that is, from a few inches up to four or 

 five feet, the trap is vesicular. Above this it is firm and dense to the 

 top of the cliff. At Garret Rock there is another good exposure of 

 the contact where the D. L. & W. R. R. rounds the point of the moun- 

 tain. The contact is perfectly sharp, but slightly irregular. There 

 is no mixture of the trap and sandstone. The vesicularity of the 



trap extends about three feet up 



and the sandstone also shows 



small passages through which 



gases were forced. Fig. 7 shows 



a typical example of the contact here. In some places the trap is so 



welded to the sandstone that the same hand specimen will show both. 



At the trap quarry southwest of Albion Place there are certain 



features of interest. At the contact there is a mixture of vesicular, 



fyg- 



