This period fixed, we have a datum for estimating the duration of the interglacial periods. The moraines of the Wurm glaciation present everywhere a very fresh appearance, and the chemical change which the boulders they contain have undergone is slight, while weathering extends to a depth of scarcely a foot. The moraines of the Riss glaciation are weathered somewhat more deeply, and those of the Mindel glaciation very much more. Assuming that chemical weathering has proceeded uniformly during the interglacial periods and ceased during the glaciations, Penck and Brückner, who have studied exhaustively the glaciation of the Alps, find that the Riss-Wurm interglacial lasted about three times as long as the interval between the Wurm glaciation and the present day, or 60,000 years, and the Mindel-Riss interglacial about twelve times as long, or 240,000 years. No data are available for the Gunz-Mindel interglacial, but it is provisionally made equal to the Riss-Wurm, another 60,000 years. No possibility of such direct measurement of the duration of the glacial periods themselves has yet been found. Penck and Brückner assume that the duration equalled that of the Riss-Wurm interglacial, or 60,000 years in each case. This seems unnecessarily long. The Yoldia Sea, the deepest part of which coincided with the centre of the Scandinavian glaciation, appears to have reached its greatest depth not more than 6000 years after the maximum of glaciation, indicating a lag of this period. The subsidence of the land due to the weight of the ice-sheet may have commenced some time before the maximum of glaciation, but the duration of the subsidence can hardly have been more than 10,000 years, and this is the limit for the second half of the Wurm glacial period. Further, we know that during the growth of the ice-sheets there was comparatively little melting, for the rivers then had little power of carrying debris. Recent measurements in Greenland give the rate of ice-growth on the surface of the ice-sheet as 40 cm., or 15 inches a year; let us say a foot, and assume a marginal loss equivalent to half this amount over the whole ice-sheet. This gives us an average increase of six inches a year, or 10,000 years for growth to a maximum thickness of 5000 feet. On these grounds the estimated duration of the Rissian glacial period has been reduced to 30,000 years, and that of the Wurm period to 22,000 years. Only in the case of the long and complicated Mindelian period, which, as will be seen later, was virtually a series of overlapping glaciations from various centres, has the figure of 60,000 years been accepted. In the present state of our knowledge no estimate of the duration of the Gunz-Mindel interglacial can have any value, and the dates are accordingly carried back only to the Mindelian. In this way we obtain the time-scale given on page 48.