CAUSES OF GLACIAL CLIMATE. Various theories have been proposed to account for the cold climate of the Glacial period. A sufficient cause may be found in terrestrial changes, such, for instance, as would lead to a variation in the distribution of land and water. The formation of glaciers is influenced by precipitation and thus by the proximity of warm waters to areas of cold land. It is conceivable that the poles may have been surrounded by a large land area which would exert a cooling effect upon the climate, and that the flow of ocean currents may have been so directed as to increase precipitation, but such a view is unsupported by geological evi dence. A second theory. based upon terrestrial changes, ascribes the cold climate to a general elevation of the land surface in the north tem perate zone, possibly accompanied by a diver sion of the Gulf Stream across the present Isth mus of Central America into the Pacific. This theory fails in the same particular as the first, i.e. there are no evidences of such great vicissi tudes. While either of these theories would ac count for the cold, it is also difficult to bring them into consonance with the view now com monly accepted by geologists, that the Glacial pe riod was marked by periodical variations in the climate. One of the most ingenious explanations that have yet been proposed. is based upon the rela tive positions of the earth and sun at distant periods of time. It is known that the eccentricity of the earth's orbit is subject to secular varia tions. With a maximum of eccentricity the earth is 14,000,000 miles nearer the sun during perihe lion than in aphelion, and the difference in the amount of direct heat received from the sun be tween these positions is about one-fifth. If now,
by precession of the equinoxes, winter in the Northern Hemisphere should occur when the earth is in aphelion, the effect would be to length en this season by twenty-two days, and to shorten the summer by an equal period. This coincidence of maximum eccentricity with aphelion winter would undoubtedly result in the refrigeration of the climate in the Northern Hemisphere. This theory, developed by Dr. James Croll, is accepted by many eminent geologists of the present day as the most satisfactory explanation of the Glacial climate.
BIBLIOGRAPHY. Geikie, Great Ice Age and Its Bibliography. Geikie, Great Ice Age and Its Relation to the Antiquity of Man (New York, 1895) ; Bonney, Ice Work Present and Past (New York, 1896) ; Herrmann, Glacialerscheinungen in der geologischen Vergangenheit (Hamburg, 1896) ; Wright, Unity of the Glacial Epoch (n.p., 1892) ; Wright, Ice Age in North America (New York, 1891) ; Dawson, Canadian Ice Age (New York, 1894) ; Lewis, Papers and Notes on the Glacial Geology of Great Britain and Ireland (London, 1894) ; Heim and Penck, On the Dis trict of the Ancient Glaciers of the Isar and Linth (London, 1886) ; Penck, Die Vergletsche rung der deutschen Alpen (Leipzig, 1882) ; Lyell, Geological Evidences of the Antiquity of Man (4th ed., London, 1873) ; Croll, Climate and Time (Edinburgh, 1885). See GEOLOGY; GLACIER; PLEISTOCENE PERIOD.