202 PHYSIOGRAPHY, GEOLOGY AND MINERAL RESOURCES

westward, they reappear again in the granite and volcanic rocks of
the Blue Ridge district. Even before the close of Archean time the
Algonkian rocks had already been greatly crumpled, altered and
metamorphosed by the intrusion of the igneous masses.

The oldest shore-line of Paleozoic time must have stood somewhere
to the east of the Frederick valley, perhaps along the western flanks of
Parr's Ridge, although smaller troughs penetrated the continental
margin, as shown by the sandstone of Deer creek and the long band of
phyllites and crystalline limestones which extend across Baltimore and
Harford counties into the Peach Bottom region of the Susquehanna.
Towards the close of the lower Silurian period occurred the first great
mountain-making movement of Paleozoic time. It is known in New
England as the "Green Mountain uplift. " Although less pro-
nounced toward the south, its influence was profoundly felt at least
as far as Maryland, for it was probably during this period that the
slates and limestones of the western Plateau region were so highly
folded and metamorphosed.

During later Silurian time the shore-line of the inland sea was
pushed considerably westward, and it is doubtful whether it ever again
reached a point east of the Blue Ridge, as this barrier was doubtless
raised in part at least by the disturbances of the lower Silurian, which
have just been described. Through the upper Silurian, Devonian and
Carboniferous periods comparative quiet reigned. While subject to
continual oscillation, the sea-floor in the Appalachian Region was, in
the main, sinking, although there was a gradual recession of the shore-
line westward. Toward the end of Paleozoic time the great Appa-
lachian trough gradually shallowed, and during the later part of the
Carboniferous period became to a large extent occupied by swamps,
in which the vegetable life of that time flourished luxuriantly. The
remains of these ancient forests form the great coal seams of the
Appalachian Region, although to-day we possess but their smallest
remnants in Maryland.

At the close of the Paleozoic occurred probably the greatest mountain
upheaval that eastern America has ever known. The vast sequence
of sediments, which had accumulated in the great Appalachian