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. The unit of glacial measurement chosen by Penck is the time which has elapsed since the close of the fourth and last great glaciation; this is known as the _Würm_ in the Alpine region and as the _Wisconsin_ in America. While more limited than the ice-caps of the second glaciation, those of the fourth glaciation were still of vast extent in Europe and in this country, so that an estimate of 20,000 to 34,000 years for the unit of the entire _Postglacial_ stage is not extreme. Estimating this unit at 25,000 years and accepting Reeds's(54) estimate of the relative length of time occupied by each of the preceding glacial and interglacial stages, we reach the following results (compare Fig. 14, p, 41): +-----------------------------------------------------------------------+ | | |Relative| Grand | Descent | | | |Duration| Totals|of Alpine| | | | | |Snow-Line| |--------------------------------------|-----|--------|-------|---------| | POSTGLACIAL TIME. |Units| Years | Years | Meters | | (Period of Upper Palæolithic | | | | | | culture, Crô-Magnon and | | | | | | Brünn races) | 1 | 25,000| 25,000| | | | | | | | | IV. GLACIAL STAGE (= Würm, Wisconsin)| | | | | | (Close of Lower Palæolithic | | | | | | culture, Neanderthal race) | 1 | 25,000| 50,000| 1,200 | | 3d. _Interglacial Stage._ | | | | | | (Opening period of Lower | | | | | | Palæolithic culture, | | | | | | Piltdown and | | | | | | pre-Neanderthaloid races) | 4 | 100,000|150,000| | | | | | | | |III. GLACIAL STAGE (= Riss, | | | | | | Illinoian). | 1 | 25,000|175,000| 1,250 | | 2d. _Interglacial Stage_ | | | | | | (= Mindel-Riss, Yarmouth) | 8 | 200,000|375,000| |