. Elements of geology : a text-book for colleges and for the general reader. Geology. s i Fig. 963.—Diagram showing effect of Precession: A, condition of things now; B, as it will be 10,500 years hence. The eccentricity is of course greatly exaggerated. seen that winter (north pole turned away from the sun) will be in aphelion, and summer (north pole turned toward the sun) in peri- helion. Now, according to Croll, the coincidence of aphelion winter, with a period of greatest eccentricity, produces a glacial climate. The cycle of aphelion winter, as already said, is 21,000 years, that of great-
. Elements of geology : a text-book for colleges and for the general reader. Geology. s i Fig. 963.—Diagram showing effect of Precession: A, condition of things now; B, as it will be 10,500 years hence. The eccentricity is of course greatly exaggerated. seen that winter (north pole turned away from the sun) will be in aphelion, and summer (north pole turned toward the sun) in peri- helion. Now, according to Croll, the coincidence of aphelion winter, with a period of greatest eccentricity, produces a glacial climate. The cycle of aphelion winter, as already said, is 21,000 years, that of great- est eccentricity is much longer and far less regular. Again, these effects, Croll thinks, would be still further increased by changes in the direction of oceanic currents. During aphelion win- ter in the northern hemisphere the equator of heat would be south of the geographical equator instead of north of it, as at present. The equa- torial current of the Atlantic (p. 40), instead of turning northward to form the Gulf Stream, would be turned southward by the wedge- shaped eastern point of South America, and the northern hemisphere would be still further chilled by the withdrawal of this great moderator of northern climates. Finally, to all these effects must now be added the important effect of the extreme range of temperature during an aphelion-winter period, especially at a time of maximum eccentricity. McGee f has shown that the effect of increased range is to diminish the mean temperature ; while Hill J has shown that it increases the mean evaporation, and therefore the mean precipitation, as snow. If this theory be true, one corollary is the recurrence of Glacial * The cycle of precession is 26,000 years, but the advance movement of the major axis of the orbit makes the cycle of aphelion winter 21,000 years, f American Journal, vol. xxii, p. 437, 1881. % Geological Magazine, vol. viii, p. 481, Please note that these images are extracted from scanned page images t
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