FREE INSTITUTE OF SCIENCE 
MINERALOGY OF THE NEWARK GROUP IN PENNSYLVANIA 
and has then been buried beneath further accumulations of the same material. 
In the other, the intrusive type, the magma has never reached the surface, 
but has lifted the layers of sediment previously formed and flowed out between 
them, by virtue of the buoyant force due to its greater specific gravity. The 
criteria for distinguishing these two modes of origin are numerous, and although 
the presence of one or two of them might not be sufficient to decide a doubtful 
case, when several occur in conjunction, there can be little question as to the 
correctness of the origin indicated. 
These criteria, first worked out in detail by Prof. W. M. Davis,* may be 
grouped in the order of their relative weight as follows: 
TYPE OF PHENOMENA, 
Chemical. 
Lithological. 
Structural. 
EXTRUSIVE SHEETS. 
Slight alteration of under- 
lying, and no alteration 
of overlying sediments. 
Fine grained, with vesicular 
texture prominent, at 
least near the top, and 
usually throughout the 
mass. 
. 
Must be strictly conformable 
with the enclosing sedi- 
ments. May be over- 
lain by trap conglom- 
INTRUSIVE SHEETS. 
Intense metamorphism of un- 
derlying, and especially 
of overlying sediments. 
* ч 121 r 
Coarse-grained, except at 
contacts, rarely showing 
trace of vesicular texture, 
and then only locally. 
May break across the beds of 
the enclosing sedimen- 
taries and send offshoots 
into the overlying ones. 
erate, or by їшї beds. May be bounded by 
May contain xenoliths friction breccias. May 
near base only. contain xenoliths both 
at base and top as well. 
From the descriptions of the two ‘sheets in the present area given above 
it appears that they are of different origin, but in view of the divergence of 
published views on the origin of the trap sheets of Pennsylvania in general, it 
seems desirable to discuss the evidence still further. ‘Taking up first the southern 
sheet, this shows more intense metamorphism of the underlying sediments 
than has ever been observed in connection with any known extrusive sheet. 
This, however, would not be sufficient to decide the question, were other evidence 
not available. It was shown above that the overlying shale is also intensely 
metamorphosed. There can not be the slightest doubt of the correctness 
of this observation, for chemical and petrographical examination shows this 
shale to be changed from the normal types of Newark sediments in exactly 
It might be suggested that 
88 
the same way as the shales underlying the sheet. 
another hidden bed of trap exists at a horizon just above this altered shale, 
