514 Eev. E. KUl— Ice-Age Theories. 



Any explanation ought to account not only for cold greater than 

 the present, but for accumulations of snow and ice. A kindred 

 phenomenon is the greater size of the Antarctic ice-cap. The sup- 

 posed interglacial warm periods, and the vmquestioned luxuriance of 

 Miocene vegetation in Greenland, ought also to find their causes in 

 any thoroughly satisfactory theory. 



The theories which have been propounded fall into three groups, 

 as Cosmical, Terrestrial, and Astronomical (or Periodical). 



The Cosmical theories are Poisson's Cold-Space theory — incom- 

 prehensible ; and the Cold-Sun theory of S. V. Wood and others 

 — lacking any evidence. 



The Terrestrial theories are numerous. Lyell's suggestion of 

 Polar-continent and Equatorial-ocean is opposed by evidence that 

 continents and oceans lay on much the same areas as now. The 

 contrary view, Polar-ocean and Equatorial-land, would deserve con- 

 sideration but for the same opposing evidence. The elevation view 

 (Dana, Wallace), which alleges greater altitude of mountain-chains, 

 disagrees with the strong evidence for land-depression during the 

 period. The submergence view of Dr. Dawson agrees with this 

 evidence, but requires elucidation. Alteration of ocean-currents 

 (Grunn, J. S. Gardner) is a most powerful agency, but would act 

 locally rather than universally, round the pole. Alteration of pre- 

 valent winds, hitherto woi'ked out by no one, deserves attentive 

 consideration. Conditions are conceivable which would produce 

 over an ai'ea winds from cold quarters almost permanently. How- 

 ever, this seems open to the same objection as the preceding theory. 



Last come the Astronomical or Periodical theories. A tilt of the 

 earth's axis was suggested by Belt, but suggested as owing to causes 

 which ai'e wholly insufficient. Tilting from astronomical agencies 

 is slight, though its action would be in the direction required. 

 Herschel suggested the Eccentricity theory, but abandoned it. 

 Adhemar's Precession theory, as explained by himself, involved an 

 absolute fallacy. The celebrated view of Dr. Croll combines the 

 Precession and Eccentricity theories into one. It exactly agrees 

 with the Antarctic greater extension of ice, and provides an expla- 

 nation of interglacial warm periods. The great difficulty in its way 

 is to see how a mere difference in distribution through the year of 

 an unchanged total heat-receipt can produce consequences so vast. 

 The laws of radiation explain but a very minute part, the laws of 

 evaporation perhaps rather more ; but, so far as can at present be 

 seen, both together are inadequate. Another serious objection is 

 that the theory seems to require the climate of the northern hemi- 

 sphere to be now in a state of change for the better, of which at pre- 

 sent there appears no evidence. 



Dr. Croll's elaborate explanations of the reaction of one effect upon 

 another — fogs, deflection of currents, and the like — have no special 

 connection with his own theory. They would act in all cases, and 

 support all theories equally. The arguments, if admitted, would 

 only prove that the earth's climates are in a state of highly unstable 

 equilibrium, in which a slight cause may produce an enormous 

 change. Nor are his arguments universally admitted. 



