The Glacial Epoch when first revealed by Charpentier[(39)] and Agassiz,[(40)] between 1837 and 1840, was supposed to correspond to a single great advance and retreat of the ice-fields from various centres. The vague problem of the antiquity of Pliocene man and Diluvial man soon merged into the far more definite chronology of glacial and interglacial man. As early as 1854, Morlot discovered near Dürnten, on the borders of the lake of Zürich, a bed of fossil plants indicating a period of south temperate climate intervening between two great deposits of glacial origin. This led to the new conception of cold glacial stages and warm interglacial stages, and Morlot[(41)] himself advanced the theory that there had been three glacial stages separated by two interglacial stages. Other discoveries followed both of fossil plants and mammals adapted to warmer periods intervening between the colder periods. Moreover, successive glacial moraines and 'drifts,' and successive river 'terraces' were found to confirm the theory of multiple glacial stages. The British geologist, James Geikie (1871-94) marshalled all the evidence for the extreme hypothesis of a succession of six glacial and five interglacial stages, each with its corresponding cold and warm climates. Strong confirmation of a theory of four great glaciations came through the American geologists, Chamberlin,[(42)] Salisbury,[(43)] and others, in the discovery of evidence of four chief glacial and three interglacial stages in northern portions of our own continent. Finally, a firm foundation of the quadruple glacial theory in Europe was laid by the classic researches of Penck and Brückner[(44)] in the Alps, which were published in 1909. Thus the exhaustive research of Geikie, of Chamberlin and Salisbury, of Penck and Brückner, and finally of Leverett[(45)] has firmly established eight subdivisions or stages of Pleistocene time, namely, four glacial, three interglacial, and one postglacial. These not only mark the great eras of European time but also make possible the synchrony of America with Europe.

PLACE OF THE OLD STONE AGE IN THE EARTH'S HISTORY

(Indicated in heavy-face letter.)

Compare Schuchert's Table, 1914.

Major DivisionsPeriods and EpochsAdvances in LifeDominant Life
Quaternary.HOLOCENE.Recent alluvial.Rise of world civilization.Age of Man.
. . . . . . . . . . . .Industry in iron, copper, and
polished stone.		Iron, Bronze,
and
New Stone Ages.
PLEISTOCENE,
or
ICE AGE.		Postglacial stage.Extinction of great mammals.Men
of the
Old Stone Age.
Glacial stages.Dawn of mind, art, and
industry.
Tertiary.PLIOCENE.Late Tertiary.Transformation of man-ape into
man.		Age of
Mammals
and
Modern
Plant Life.
MIOCENE.Culmination of mammals
OLIGOCENE.Early Tertiary.Beginnings of anthropoid ape
life.
EOCENE.Appearance of higher types of
mammals, and vanishing of
archaic forms.
PALÆOCENE.Rise of archaic mammals.
Late Mesozoic.Cretaceous. Extinction of great reptiles.Age
of
Reptiles.
Extreme specialization of reptiles.
Comanchian. Rise of flowering plants.
Early Mesozoic.Jurassic. Rise of birds and flying reptiles.
Triassic. Rise of dinosaurs.

Since most of the skeletal and cultural remains of man can now be definitely attributed to certain glacial, interglacial, or postglacial stages, vast interest attaches to the very difficult problem of the duration of the whole Ice Age and the relative duration of its various glacial and interglacial stages. The following figures set forth the wide variations in opinion on this subject and the two opposite tendencies of speculation which lead to greatly expanded or greatly abbreviated estimates of Pleistocene time:

DURATION OF THE ICE AGE

1863.Charles Lyell,[(46)] Principles of Geology 800,000years.
1874.James D. Dana,[(47)] Manual of Geology 720,000"
1893.Charles D. Walcott,[(48)] Geologic Time as Indicated by the Sedimentary Rocks of North America 400,000"
1893.W. Upham,[(49)] Estimates of Geologic Times, Amer. Jour. Sci., vol. XLV 100,000"
1894.A. Heim,[(50)] Ueber das absolute Alter der Eiszeit 100,000"
1900.W. J. Sollas,[(51)] Evolutional Geology 400,000"
1909.Albrecht Penck,[(52)] Die Alpen im Eiszeitalter 520,000-840,000
1914.James Geikie,[(53)] The Antiquity of Man in Europe 620,000 (min.)

We may adopt for the present work the more conservative estimate of Penck, that since the first great ice-fields developed in Scandinavia, in the Alps, and in North America west of Hudson Bay a period of time of not less than 520,000 years has elapsed. The relative duration of the subdivisions of the Glacial Epoch is also studied by Penck in his Chronologie des Eiszeitalters in den Alpen.[(52)] These stages are not in any degree rhythmic, or of equal length either in western Europe or in North America.