298 On the Physical Geology of the United States, §c. 
Profs. Wm. B. and H. D. Rogers in their paper on the physical 
structure of the Apalachian chain of mountains* ascribe the 
elevation of that chain to an undulation of the fluid interior of 
the earth, combined with a lateral tangential force. 
e whole range of mountainous country from Alabama 
through Tennessee, Virginia, Maryland, Pennsylvania, New Jer- 
sey, New. York, and thence to Canada, has evidently been subject 
to an elevatory action, combined with a lateral or tangential force, 
that has elevated, broken, crushed, and plicated the strata, and in 
many places has folded them over, and reversed rocky strata of 
great thickness that were formerly horizontal rocks and formed 
beneath the ocean. Farther west, the plications and undulations 
become more and more gentle, until their angles of inclination 
become insensible to the eye 
On the east of the axis of principal disturbance, the primary 
and metamorphic rocks are also much broken and disturbed, and 
many of the sedimentary strata along the lines of disturbance 
have been much altered and modified in their aspect, structure, 
and mineral characters, by masses and veins of intrusive rocks. 
Their explanation of the tangential force that has acted over 
so great an extent in the eastern parts of the United States is in- 
genious.t If it be true, the tangential force must have acted in 
every direction from that part of the earth’s surface where the 
oscillations began, in the directions of the arcs of great circles 
radiating from an axis passing through the origin of the disturb- 
ance. The “ axis planes” also of the folded strata, will be found 
to dip inwards towards the same axis; and the crests of the fold- 
ed strata will all point outwards. ‘These would be necessary re- 
sults of an undulatory wave-like reciprocating motion flowing 
outwards from a centre, and a knowledge of this will aid in test- 
ing the truth of the theory of Profs. Rogers. 
I will here offer another explanation of the folded strata dipping 
to the $. E., showing the possibility of another cause for a tan- 
gential force, in addition to that arising from collapse of the crust 
of the globe by refrigeration, and it seems to harmonize with the 
facts observed among the disturbed strata of the United States. 
> halts of the Association of American Geologists, 1840—1842, pp. 474 
' 4 Profs. Rogets refer the whole effects to werent earthquake motion. 
Vide memoir, Trans. Assoc. Am. Geologists, 1840—1842, 7. 
