of their existence. European Shells now
confined to the Northern Ocean are found as fossils in Italy,--showing,
that, while the present Arctic climate prevailed in the Temperate Zone,
that of the Temperate Zone extended much farther south to the regions we
now call sub-tropical. In America there is abundant evidence of the same
kind; throughout the recent marine deposits of the Temperate Zone,
covering the low lands above tide-water on this continent, are found
fossil Shells whose present home is on the shores of Greenland. It is
not only in the Northern hemisphere that these remains occur, but in
Africa and in South America, wherever there has been an opportunity for
investigation, the drift is found to contain the traces of animals whose
presence indicates a climate many degree colder than that now prevailing
there.

But these organic remains are not the only evidence of the geological
winter. There are a number of phenomena indicating that during this
period two vast caps of ice stretched from the Northern pole southward
and from the Southern pole northward, extending in each case far toward
the Equator,--and that ice-fields, such as now spread over the Arctics,
covered a great part of the Temperate Zones, while the line of perpetual
ice and snow in the tropical mountain-ranges descended far below its
present limits. As the explanation of these facts has been drawn from
the study of glacial action, I shall devote this and subsequent articles
to some account of glaciers and of the phenomena connected with them.

The first essential condition for the formation of glaciers in
mountain-ranges is the shape of their valleys. Glaciers are by no means
in proportion to the height and extent of mountains. There are many
mountain-chains as high or higher than the Alps, which can boast of but
few and small glaciers, if, indeed, they have any. In the Andes, the
Rocky Mountains, the Pyrenees, the Caucasus, the few glaciers remaining
from the great ice-period are insignificant in size. The volcanic,
cone-like shape of the Andes gives, indeed, but little chance for the
formation of glaciers, though their summits are capped with snow. The
glaciers of the Rocky Mountains have been little explored, but it is
known that they are by no means extensive. In the Pyrenees there is but
one great glacier, though the height of these mountains is such, that,
were the shape of their valleys favorable to the accumulation of snow,
they might prese