CAUSE OF THE ICE AGE. 519 



were still sufficient io keep the earth's defarmation less than is required to 

 produce glaciation; for no evidences of intense and widely extended glacial 

 conditions are found in the great series of Tertiary and Mesozoic forma- 

 tions, representing the earth's history tlu'ough jirobably ten to fifteen million 

 years. And, indeed, these conclusions, drawn from the Pleistocene period 

 and the absence of glaciation through vast eras preceding, accord well with 

 the known age and stages of growth of mountain ranges that have been 

 formed during these eras. No j)eriod since the close of Paleozoic time has 

 been more characterized by mountain-building than the comparatively short 

 Pleistocene, whose duration may probably be included within 100,000 or 

 150,000 years. 



Elevation of broad areas, as half of North America and half of Europe, 

 either synchronous!}" or, less probably for these companion continental 

 regions adjoining the North Atlantic, in alternation, to such heights that 

 their precipitation of moisture throughout the year was nearly all snow, 

 gradually forming ice-sheets of great thickness, seems consistent with 

 the conditions of the earth's crust and interior which are indicated liy the 

 changes in the levels of the beaches of Lake Agassiz. A plastic interior 

 or molten magma beneath the solid crust accounts for the uplift of the area 

 of this glacial lake, with its gradual increase from south to north, and also 

 appears, in connection with contraction of the earth and the formation of 

 mountain ranges, to afford an adequate explanation of glaciation. It is 

 probable that the great uplifts which are thus supposed to have caiised ice 

 accumulation were very slow in their progress, and that their effect upon 

 extensive continental areas was so disti'ibuted that the maximum changes 

 in slope on their borders would nowhere exceed 20 or 30 feet or at the 

 most 50 or 75 feet per mile, while perhaps some portions of the uplifted 

 region would receive no change of slope. And the subsidence beneath the 

 weight of accumulated ice was probably slow, though apparently much 

 faster than the processes of pi'eglacial and interglacial elevation, and was 

 similarly distributed, no limited district being greatly chang-ed. Excepting 

 the areas where disturbances of mountain-building or extraordinary rising 

 or sinking of mountain ranges were associated with these movements, the 

 contour of the country, with its valleys, hills, and mountains, remained in 



