lava, remnants of which are
still in evidence after ages of denudation, as the Palisades along the
Hudson, and such elevations as Mount Holyoke in western Massachusetts.

Still there remained a vast interior sea, which later on, in the
tertiary age, was to be divided by the slow uprising of the land, which
only yesterday--that is to say, a million, or three or five or ten
million, years ago--became the Rocky Mountains. High and erect these
young mountains stand to this day, their sharp angles and rocky contours
vouching for their youth, in strange contrast with the shrunken forms
of the old Adirondacks, Green Mountains, and Appalachians, whose lowered
heads and rounded shoulders attest the weight of ages. In the vast lakes
which still remained on either side of the Rocky range, tertiary
strata were slowly formed to the ultimate depth of two or three miles,
enclosing here and there those vertebrate remains which were to be
exposed again to view by denudation when the land rose still higher,
and then, in our own time, to tell so wonderful a story to the
paleontologist.

Finally, the interior seas were filled, and the shore lines of the
continent assumed nearly their present outline.

Then came the long winter of the glacial epoch--perhaps of a succession
of glacial epochs. The ice sheet extended southward to about the
fortieth parallel, driving some animals before it, and destroying those
that were unable to migrate. At its fulness, the great ice mass lay
almost a mile in depth over New England, as attested by the scratched
and polished rock surfaces and deposited erratics in the White
Mountains. Such a mass presses down with a weight of about one hundred
and twenty-five tons to the square foot, according to Dr. Croll's
estimate. It crushed and ground everything beneath it more or less, and
in some regions planed off hilly surfaces into prairies. Creeping slowly
forward, it carried all manner of debris with it. When it melted away
its terminal moraine built up the nucleus of the land masses now known
as Long Island and Staten Island; other of its deposits formed the
"drumlins" about Boston famous as Bunker and Breed's hills; and it left
a long, irregular line of ridges of "till" or bowlder clay and scattered
erratics clear across the country at about the latitude of New York
city.

As the ice sheet slowly receded it left minor moraines all along its
course. Sometimes its deposits dammed up river courses or inequalitie