THE PLEISTOCENE OR GLACIAL PERIOD. 433 



thus supposed to have been supplied, conditions without which all 

 more special and local causes would be inoperative. 



Two serious problems, however, remain: (1) the localization of 

 the Pleistocene glaciation, which, though not so remarkable as that 

 of the Permian period, was yet very extraordinary, and (2) the period- 

 icity expressed in a succession of glacial and interglacial epochs which 

 formed a declining series of very unequal lengths. 



i. Localization. — The localization l is assigned to the two great 

 areas of permanent atmospheric depression that have their present 

 centers near Greenland and the Aleutian Islands respectively (Figs. 530 

 and 531). It is within these permanent cyclonic areas that the excep- 

 tional glaciations of Greenland and Alaska occur at present. There 

 is also a remarkable correspondence between the border of the ice- 

 sheets and the courses of the moving storms on the borders of these 

 permanent cyclonic areas. It is also notable that the great ice-lobes 

 converged toward the area where the storm-frequency is now greatest. 

 It is not a little remarkable that the ice-sheets after their several retreats, 

 and perhaps entire disappearances, should have advanced repeatedly 

 in nearly the same forms and to nearly the same extents, though in 

 some particulars their habits otherwise were noticeably unlike. All 

 these and many minor facts are associated in theory with these per-i 

 manent " lows " and the related storm-tracks. These features are 

 presumed to have been extended and intensified during the glacial 

 stages, but to have retained the general relations and configurations 

 they now possess. The basal cause of these features is probably to 

 be found in the configuration of the land and water of the northern 

 hemisphere. 



2. Periodicity. — The periodicity of glaciation under this hypoth- 

 esis is assigned to a rather complex interaction of a combination of 

 agencies which is not susceptible of brief statement without more 

 qualification than our limits will permit, if it is to be wholly accu- 

 rate and fully protected against misinterpretation; but the leading 

 features may be sketched and the necessary qualifications must be 

 taken for granted. 



The basal conception is that, under general conditions favorable 



1 An Attempt to Frame a Working Hypothesis of the Cause of Glacial Periods on 

 an Atmospheric Basis. Jour. Geol., Vol. VII, 1899, pp 752-771. See also discussion 

 of localization under Permian, Vol. II, p. 674. 



