area built
tip, perhaps, through denudation of a yet more ancient polar continent,
whose existence is only conjectured. To the southeast an island that
is now the Adirondack Mountains, and another that is now the Jersey
Highlands rose above the waste of waters, and far to the south stretched
probably a line of islands now represented by the Blue Ridge Mountains.
Far off to the westward another line of islands foreshadowed our present
Pacific border. A few minor islands in the interior completed the
archipelago.

From this bare skeleton the continent grew, partly by the deposit of
sediment from the denudation of the original islands (which once towered
miles, perhaps, where now they rise thousands of feet), but largely also
by the deposit of organic remains, especially in the interior sea, which
teemed with life. In the Silurian ages, invertebrates--brachiopods and
crinoids and cephalopods--were the dominant types. But very early--no
one knows just when--there came fishes of many strange forms, some of
the early ones enclosed in turtle-like shells. Later yet, large spaces
within the interior sea having risen to the surface, great marshes or
forests of strange types of vegetation grew and deposited their remains
to form coal-beds. Many times over such forests were formed, only to be
destroyed by the oscillations of the land surface. All told, the strata
of this Paleozoic period aggregate several miles in thickness, and the
time consumed in their formation stands to all later time up to the
present, according to Professor Dana's estimate, as three to one.

Towards the close of this Paleozoic era the Appalachian Mountains
were slowly upheaved in great convoluted folds, some of them probably
reaching three or four miles above the sea-level, though the tooth
of time has since gnawed them down to comparatively puny limits. The
continental areas thus enlarged were peopled during the ensuing Mesozoic
time with multitudes of strange reptiles, many of them gigantic in size.
The waters, too, still teeming with invertebrates and fishes, had their
quota of reptilian monsters; and in the air were flying reptiles, some
of which measured twenty-five feet from tip to tip of their batlike
wings. During this era the Sierra Nevada Mountains rose. Near the
eastern border of the forming continent the strata were perhaps now too
thick and stiff to bend into mountain folds, for they were rent into
great fissures, letting out floods of molten