THE PELISTOCENE OR GLACIAL PERIOD 895 



on changes in the constitution, movements, or cloud-content of the 

 air, the atmospheric hypotheses. 



Hypsometric Hypotheses 



The hypothesis of elevation. 1 Since the best-known glaciers 

 are in mountains, the suggestion was natural that elevation of the 

 glaciated regions was the cause of the great ice-sheets. The chief 

 evidence of the elevation postulated is the submerged valleys of the 

 sea-coasts, especially those of the northern latitudes. It has been 

 held by advocates of this hypothesis that 4,000 feet or more of eleva- 

 tion is indicated by the northern fiords, and that this elevation, to- 

 gether with accompanying geographic changes, was competent to 

 produce the Pleistocene glaciation. Those who question this view 

 doubt the fact of so great elevation, and doubt whether any eleva- 

 tion which there may have been was contemporaneous with the 

 ice-sheets. Further, they offer evidence that the land was lower than 

 now at certain important stages of the glacial period. The eleva- 

 tion hypothesis also encounters difficulty in explaining the inter- 

 glacial intervals, now well established, and in accounting for the 

 markedly mild climate of some of them. The hypothesis, in its 

 simple and popular form, would seem to require a great elevation 

 of a large part of two continents, for each ice epoch, and a great 

 depression for each interglacial epoch. This can hardly be granted. 

 On the whole, this hypothesis has lost rather than gained favor, as 

 evidence has accumulated. 



Astronomic Hypotheses 



CrolPs hypothesis. 2 A semi-astronomical hypothesis was ad- 

 vanced by Croll in the latter part of the last century. For a time 

 it was widely accepted, especially in Europe. It is founded prima- 

 rily on variations in the eccentricity of the earth's orbit, combined 

 with the precession of the equinoxes. 



The orbit of the earth is slightly elliptical, and this ellipticity is 



1 Dana, Manual of Geology, 4th ed., p. 970, and Upham, Am. Geol., Vol. 

 VI, p. 327, and Am. Jour. Sci., Vol. XII, p. 33. 



2 Climate and Time in their Geological Relations; a theory of secular 

 changes of the earth's climate, by James Croll, 1890, pp. 312-328; also Climate 

 and Cosmology, 1889, and The Cause of the Ice Age, Sir Robt. Ball, 1893. 



