farther west.
They are rarely metamorphosed to the point of recrystallization, though
locally shales are altered to roofing slates, sandstones are indurated,
limestones slightly marblized, and coals, originally bituminous, are
changed to anthracite in northern Pennsylvania, and to graphite in Rhode
Island. Igneous intrusions consist only of unimportant dikes of trap.
The most striking and uniformly characteristic geologic feature of the
mountains is their internal structure, consisting of innumerable
parallel, long and narrow folds, always closely appressed in the eastern
part of any cross-section (Piedmont Plateau to Great Valley), less so
along a central zone (Great Valley and Valley Ridges), and increasingly
open on the west (Allegheny and Cumberland Plateaus). Asymmetry of the
folds is a marked characteristic in the zones of closer folding, the
anticlines having long gently inclined easterly limbs, and short, steep
and even overturned limbs upon the west. The effect of such folds is
often exaggerated by thrusts, and faulting of this sort is prominent in
the southern section, where the existence of over-thrusts measured by
several miles has been established.

What may be termed the ancestral Appalachian system was formed during
the post-carboniferous revolution, though certain of its elements had
been previously outlined, and perhaps at different dates. Folding of the
rocks resulted from the operation of great compressive forces acting
tangentially to the figure of the earth. Extensive and deep-seated
crumpling was necessarily accompanied by vertical uplift throughout the
zone affected, but once at least since their birth the mountains have
been worn down to a lowland, and the mountains of to-day are the
combined product of subsequent uplift of a different sort, and
dissection by erosion. Produced by long-continued subaerial decay and
erosion, in later Cretaceous times this lowland extended from the
Atlantic Ocean well toward the interior of North America; since then the
whole continent has been generally elevated, and by successive steps the
Appalachian belt has been raised to form a wide but relatively low arch.
The crosswise courses of the greater rivers result from the rivers being
older than the mountains, which indeed have been produced by
circumdenudation. The master streams of the present have inherited their
channels from the drainage systems of the Cretaceous lowland, and though
raised athwart the courses