iod undoubtedly prehistoric, but not necessarily
many thousands of years old.

[Footnote 2: _Second Annual Report of the Peabody Museum of
American Archaeology_, etc., p. 18.]

[Footnote 3: Visited in 1866-74 by Professor Jeffries Wyman,
and described in his _Fresh-Water Shell Mounds of the St.
John's River_, Cambridge, 1875.]

[Sidenote: The Glacial Period.]

The second group of discoveries carries us back much farther, even into
the earlier stages of that widespread glaciation which was the most
remarkable feature of the Pleistocene period. At the periods of greatest
cold "the continent of North America was deeply swathed in ice as far
south as the latitude of Philadelphia, while glaciers descended into
North Carolina."[4] The valleys of the Rocky Mountains also supported
enormous glaciers, and a similar state of things existed at the same
time in Europe. These periods of intense cold were alternated with long
interglacial periods during which the climate was warmer than it is
to-day. Concerning the antiquity of the Pleistocene age, which was
characterized by such extraordinary vicissitudes of heat and cold, there
has been, as in all questions relating to geological time, much conflict
of opinion. Twenty years ago geologists often argued as if there were an
unlimited fund of past time upon which to draw; but since Sir William
Thomson and other physicists emphasized the point that in an antiquity
very far from infinite this earth must have been a molten mass, there
has been a reaction. In many instances further study has shown that less
time was needed in order to effect a given change than had formerly been
supposed; and so there has grown up a tendency to shorten the time
assigned to geological periods. Here, as in so many other cases, the
truth is doubtless to be sought within the extremes. If we adopt the
magnificent argument of Dr. Croll, which seems to me still to hold its
ground against all adverse criticism,[5] and regard the Glacial epoch as
coincident with the last period of high eccentricity of the earth's
orbit, we obtain a result that is moderate and probable. That
astronomical period began about 240,000 years ago and came to an end
about 80,000 years ago. During this period the eccentricity was seldom
less than .04, and at one time rose to .0569. At the present time the
eccentricity is .0168, and nearly 800,000 years will pass before it
attains such a point