These three principal phenomena of the ice-fields, the mammals, and the human life and industry, together establish the chronology of the Age of Man. In other words, there are four ways of keeping _prehistoric time_: that of geology, that of palæontology, that of anatomy, and that of human industry. Geologic events mark the grander divisions of time; palæontologic and anatomic events mark the lesser divisions; while the successive phases of human industry mark the least divisions. The geologic chronology deals with such immense periods of time that its ratio to the animal and to the human chronology is like that of years to hours and to minutes of our own solar time. 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.