426 GEOLOGY. 



simple. To escape the growing force of the evidence of frequent and 

 important interglacial intervals, the older- phase of the hypothesis 

 has been amended by adding to elevation the main features of the 

 Crollian hypothesis next to be sketched, which carries a postulate that 

 involves climatic oscillations. The periods of these oscillations, however, 

 are equal, while the observed oscillations seem to be notably unequal. 

 The elevation hypothesis also encounters grave difficulties when 

 applied to the Permian glaciation of India, Australia, and South Africa, 

 because of their low latitudes, because of the great height apparently 

 required to furnish the necessary conditions for plateau glaciation, 

 and because of the great oscillations necessary to account for the marine 

 beds between the glacial beds. If the plateaus of Tibet and the Pamir, 

 ranging from 15,000 to 18,000 feet above the sea, are not glaciated 

 under present conditions, one cannot but wonder what elevation the 

 southern peninsula of India would have required in the Permian period 

 if elevation were the essential factor. No plateau outside the polar 

 circles is now glaciated, except as the ice is derived from adjacent 

 mountains, no matter what its relations to sea or land, to winds or 

 currents, to moisture or aridity or other conditions. The observa- 

 tional basis for assigning the glaciation of a half of the North Ameri- 

 can continent to any elevation that can fairly be assigned to it, during 

 either the Permian or Pleistocene period, is thus not as broad and 

 firm as could be desired for a satisfactory working hypothesis. 



Astronomic Hypotheses. 



CrolFs hypothesis. 1 — A semi-astronomical hypothesis was advanced 

 by James Croll in the latter part of the last century, and for a time 

 gained very wide acceptance in Europe, and found not a few adherents 

 in America. The hypothesis is founded on variations in the eccen- 

 tricity of the earth's orbit, combined with the precession of the equi- 

 noxes, together with the effects of meteorological and geographical 

 influences, particularly the configuration of the Atlantic Ocean. 



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

 subject to variations on account of the varying positions of the planets, 



1 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 Cosmol- 

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



