14 TRANSACTIONS OF WAGNER 
MINERALOGY OF THE NEWARK GROUP IN PENNSYLVANIA 
and has produced the observed alteration at its lower contact. This is, however, 
not possible because (a) the exposures above the trap sheet show unaltered 
shale overlying the altered phase directly, without room for a trap sheet of more 
than a few inches thick; and (b) the alteration decreases upwards, as the main 
trap sheet is receded from, instead of downwards, as would be the case if pro- 
duced by a second, overlying sheet. 
This sheet is further holo-crystalline throughout, and most coarsely 
grained near the top, indicating the slowest cooling there, precisely the reverse 
of what would occur in an extrusive flow. At the immediate contacts it is 
fine, and shows by its texture that it has been rapidly chilled, as is to be expected 
when molten trap flows against cold rock surfaces. “The phenomena are alike 
at both upper and lower contacts, whereas if the sheet had been extrusive, and 
cooled with its upper surface exposed to air or water, and the lower against 
rock, very different textures must have been produced. There is not the 
slightest trace of vesicular texture at any place throughout the whole exposure 
of this sheet, yet present day lavas are usually prominently vesicular, owing to 
the escape of steam during solidification. 
Finally, the structure of the southern sheet indicates its intrusive char- 
acter, in that it appears both to be unconformable with the sedimentaries, 
and to vary in thickness, the effect upon the outcrop being well shown in 
the geological map. Such unconformity would, of course, be impossible in an 
extrusive sheet; so its intrusive character would seem to be proved, in so far as 
it is possible to prove any geological theory. 
The evidence in the case of the northern sheet points with equal force to 
the other mode of origin. The underlying sediments are practically unmeta- 
morphosed, the greatest effect observed being a slight darkening of the color 
to a distance of three feet from the contact, and no change at all being detectable 
in many places. But here again the conclusion drawn from the lower contact 
phenomena is of far less weight than that derived from the effects exhibited 
above the sheet. The uppermost portion of the sheet presents, as mentioned 
above, a trap breccia, cemented together with soft calcareous red mud, overlain 
by shales composed of the same material, failing to show at any point the 
slightest trace of metamorphism. The whole suggests very clearly an advance 
of water over the cold, solidified trap, the currents at first breaking up to a slight 
extent the surface of the latter and then depositing over it the red mud charac- 
teristic of Newark sedimentation. 
