F. H. Bradley— Geological Map of the United States. 289 
merly continuous across much of the area which, from the 
known outcrops, we are obliged to color as mainly Upper Silu- 
rian, Indeed, it suggests the probability that the Michigan 
coal-field may once have been continuous with that of Indiana 
and Illinois; but the surface of that region is so flat and so 
deeply covered with drift, that it would hardly be possible to 
obtain the data for a certain solution of the problem. 
Noticing my own neglect of such points, I am led to ask 
geologists, generally :—Have we paid sufficient attention to the 
indications of former probable extensions of formations over 
areas where no portions of them now remain ?—a point of little 
practical importance, perhaps, but certainly of great theoretical 
interest, 
I regret that the scale of the map forbids the representation 
of the numerous trap dikes, probably of Triassic age, whic 
cut across the metamorphics of North Carolina, South Caro- 
lina, Georgia and Alabama. The dips of the sandstones and 
shales in the two Triassic areas of North Carolina give pretty 
strong evidence of these being only the border-remnants of a 
mi ge-anticlinal, over a hundred miles wide,* (the connecting 
arch being now removed,) which probably extended far to the 
southwestward, and possibly over much or all of the area inter- 
sected by the trap dikes. Kerr’s estimates show the erosionof 
from 10,000 to 25,000 feet of strata, along the back of the fold; 
and fully as much must be allowed for, between the New Jersey 
and Vonnecticut River areas of Triassic if we accept the oppos- 
Ing dips of these as evidence of another, or part of the same, 
great ge-anticlinal. There are, it is true, strong objections to 
this view for the latter region; but no other theory seems to 
€ writer so well fitted to meet the facts. ; 
We have, as yet, no certain evidence of distinctively Jurassic 
strata on the Atlantic slope of this continent; was then the 
ec of the continental mass. It may also be questioned 
ie ether the Permian period, equally destitute of distinctive 
2g and metamorphism of the Appalachians, which was 
I 
H. £00 stall to be of any importance on a chart of this scale: 
the river- and inks. baeder matin are sufficiently well located 
* Kerr, Geology of North Carolina, i, 141-6, 1875. 
