merican Museum of Natural History_

Glacial striae on Lower Helderberg limestone]

[Illustration: Glacial grooves in the South Meadow, Central Park, New
York]

[Illustration: _By permission of the American Museum of Natural History_

Mt. Tom, West 83d St., New York]

Glaciers are small to-day compared with what they were long ago, in
Europe and in America. The climate became warmer, and the ice-cap
retreated. Old moraines show that the ice rivers of the Alps once came
much farther down the valleys than they do now. Smooth, deeply scored
domes of rock, the one in Central Park and the bald head of Mount Tom,
are just like those that lie in Alpine valleys from which the glaciers
have long ago retreated. There are old moraines far up the sides of
valleys, showing that once the glaciers were far deeper than now. No
other power could have brought rocks from strata higher up the
mountains, and lodged them thus.

Nearer home, Mt. Shasta and Mt. Rainier still have glaciers that have
dwindled in size, until they bear little comparison to the gigantic
ice-streams that once filled the smooth beds their puny successors flow
into. Remnants of glaciers lie in the hollows of the Sierras. We must go
north to find the snow-fields of Alaska and glaciers worthy to be
compared with those ancient ice rivers whose work is plainly to be seen,
though they are gone.

THE GREAT ICE-SHEET

Greenland is green only along its southern edge, and only in summer, so
its name is misleading. It is a frozen continent lying under a great
ice-cap, which covers 500,000 square miles and is several thousand feet
in thickness. The top of this icy table-land rises from five thousand to
ten thousand feet above the sea-level. The long, cold winters are marked
by great snowfall, and the drifts do not have time to melt during the
short summer; and so they keep getting deeper and deeper. Streams of ice
flow down the steeps into the sea, and break off by their weight when
they are pushed out into the water. These are the icebergs which float
off into the North Atlantic, and are often seen by passengers on
transatlantic steamers.

Long ago Greenland better deserved its name. Explorers who have climbed
the mountain steeps that guard the unknown ice-fields of the interior
have discovered, a thousand feet above the sea-level, an ancient beach,
strewn with shells of molluscs like those which now inhabit salt water,
and skeletons of fishes lie buried in th