376 The American Geologist. Junu. i»yy 
permit the development of coal swamps and the accumulation 
of enough carbonaceous material to form coal deposits. 
Again, the extensive extrusive lava sheets poured out while 
the series was forming and the intrusive sheets that were in 
place before the occurrence of the great faulting and dyking 
that marked the close of the period, together with the magni- 
tude of the faulting itself, all point to a deformation rapid 
enough to prevent the accumulation of coal deposits and 
strong enough to so greatly weaken the underlying crust as to 
pioduce widespread cracking through w'hich extensive extru- 
sive and intrusive lava flows welled up from the interior. 
Similar conditions are found in the Nova Scotia area and 
in the area from New York and New Jersey southward to 
northern Virginia, except that in the southern extremity of 
the trough the eruptive activity during Triassic time becomes 
less marked and the lava sheets become thin and local, or, in 
places, are wanting. 
In contrast to these conditions the southern-Virginia- 
North Carolina region is characterized by the presence of nu- 
merous coal seams reaching four, six, eight, ten, thirteen, and 
even locally twenty-six feet in thickness*, and also ruarked 
by the absence of intrusive and extrusive lava sheets. That 
is to say, subsidence in this region instead of being compara- 
tively rapid and continuous, as in the more northern areas, 
must have been not only slow but interrupted now and again 
by intervals of quiescence during which the relation of coal 
swamp and lagoon levels remained constant long enough to 
permit the accumulation of the great thicknesses of carbo- 
naceous material represented in the coal seams found there 
to-day. 
The absence of lava sheets may be viewed as indicating 
that the crustal depression that developed the Triassic trough 
was so slow as to permit a readjustment of the displaced matter 
by sub-crustal lateral flow instead of by extrusion to the sur- 
face, — a method of readjustment that was aided, perhaps, by 
the crustal portion yielding so slowly as to escape being as 
greatly weakened and cracked as was the portion farther north, 
and so offering during the development of the trough greater 
*Russell, I. C. Correlation Papers, The Newark System, Bull. No. 
85, U. S. Geol. Sur. pp. 36-42. 
