In the Baltic area we know that the great change of level was due largely to a subsidence of the land and only to a small extent to a rise of the sea. But in other parts of the world the amount of submergence was remarkably uniform at places in the same latitude, and decreased steadily from the polar regions to about latitude 40-50°, where it became zero. Now such a general change suggests that it was the sea which rose rather than the land which sank, and points to some general cause which piled up the waters of the oceans in the higher latitudes. A possible cause of this nature has been adduced by O. Pettersson, which he terms the “tide-generating force,” which reached one of its maxima in an 1800-year cycle about 3500 B.C. This possibility will be dealt with more fully in Chapter XVII. BIBLIOGRAPHY _Bericht Internat. Geologenkongress_, Stockholm, 1910. “Die Veränderungen des Klimas seit der Maximum des letzten Eiszeits.” Numerous papers ranging from the Arctic to the Antarctic. Brooks, C. E. P. “The evolution of climate in north-west Europe.” London, _Q. J. R. Meteor. Soc._, 47, 1921, p. 173. Praeger, R. Ll. “Report on the estuarine clays of the north-east of Ireland.” _Proc. R. Irish Acad._, ser. 3, Vol. 2, 1892, pp. 212-89. Goldthwait, J. W. “The twenty-foot terrace and sea-cliff of the lower St. Lawrence.” _Amer. J. Science_, ser. 4, Vol. 32, 1911, pp. 291-317. Cowles, H. C. “A remarkable colony of northern plants along the Apalachicola River, Florida, and its significance.” _Rep. 8 Internal. Geogr. Congress_, 1904, p. 599. Shimer, H. W. “Post-glacial history of Boston.” _Amer. J. Science_, ser. 4, Vol. 40, 1915, pp. 437-42. Halle, T. G. “On Quaternary deposits and changes of level in Patagonia and Tierra del Fuego.” _Bull. Geol. Inst._, Upsala, 9, 1908-9, pp. 93-117. Süssmilch, C. A. “An introduction to the geology of New South Wales,” Sydney, 1914. Marshall, P. “New Zealand.” _Handbuch regional Geologie_, Heft 5, 1911. CHAPTER XV THE FOREST PERIOD OF WESTERN EUROPE