FREE INSTITUTE OF SCIENCE 
MINERALOGY OF THE NEWARK GROUP IN PENNSYLVANIA 
15 
The texture of this sheet also furnishes a similar inference as to its origin. 
While compact and fine grained below, it gradually becomes vesicular upwards, 
being toward the top crowded with cavities of all sizes, up to an inch in diameter. 
From observations made on modern lavas it is known that such cavities are 
produced by the escape of steam during the solidification of the molten material, 
and that they only form at or near the surface exposed to the air, where the 
cooling is rapid but at the same time the pressure upon it is low. In those por- 
tions of the sheets which cool slowly at some depth, so that a greater pressure is 
exerted, steam bubbles are of comparatively rare occurrence. The glassy 
character of the present rock, described fully below, is also in agreement with 
the assumption of rapid cooling at the surface. It stands in strong contrast 
to the rock of the southern sheet, which, throughout its whole extent, does not 
show the slightest trace of glassy material, but is everywhere completely crystal- 
lized. 
This northern sheet is also strictly conformable with the sediments surround- 
ing it. The outcrop of its northeastern arm is shown on the map to be rather 
narrower than that of the western one, but this does not indicate a variation in 
thickness, for the dip of the beds as a whole is greater in the latter place, the 
thickness being actually constant at 500 feet, throughout the length of the 
exposure. ‘This corresponds to what would be expected in the case of molten 
material flowing out over a perfectly level surface, whereas it would be quite 
exceptional in the case of an intrusion. The evidence for the extrusive origin 
of this sheet may thus also be regarded as complete. 
Recognizing the divergence in origin of these two closely associated sheets, 
the question now arises, Do they both belong to the same period of igneous 
activity, or are they of different ages, as has been established in the case of the 
intrusive and extrusive sheets in New Jersey? While no direct evidence bearing 
on this point can be obtained at the present locality, it would seem quite probable 
that the two sheets were produced by the same eruption. As shown on the map, 
a dike starts out from the upper side of the great mass of intrusive diabase 
forming Gibraltar Hill and those adjoining it, and can be traced continuously 
to the base of the intrusive sheet below Jacksonwald. The inference seems 
justified that the latter represents a small portion of the same magma which 
has found its way upwards. But it is hardly likely that it would stop a few 
hundred feet below the surface, for cracks or fissures through which further 
ascension could take place would be expected to become more abundant as the 
