THE QUATERNARY PERIOD 361 



grees lower than at present over the glaciated area in order to 

 have brought on the Ice age. 



A Geologic (elevation) Hypothesis. — As we have already- 

 pointed out, the evidence, chiefly from the submerged river chan- 

 nels along the Atlantic Coast, clearly indicates greater altitude of 

 northeastern North America late in the Tertiary and probably 

 also in the early Quaternary. An altitude of from 4000 to 5000 

 feet greater than now has been claimed for this region. Since it is 

 well known that the temperature becomes lower with increasing 

 altitude (one degree for about 300 feet), it has been argued that 

 the greater altitude of the glaciated area was in itself sufficient 

 cause for the glaciation. " Northern elevation produced ice- 

 accumulation; ice-accumulation by weight produced subsidence; 

 subsidence produced moderation of temperature and melting of 

 ice; and this last by lightening of load produced re-elevation" 

 (J. Le Conte). It is not necessary to assume that maximum eleva- 

 tion and ice-accumulation were coincident, because an effect often 

 lags behind its cause. This northern elevation also is believed to 

 have sufficiently upraised the northern ocean basins to cut off 

 warm currents, like the Gulf Stream, thereby depriving the north- 

 ern lands of such warming influences. 



It has been urged against this hypothesis that there is no posi- 

 tive evidence for nearly as much as 4000 to 5000 feet of elevation 

 of the glaciated region ; that it is not at all proved that the northern 

 elevation occurred at the proper time to produce glaciation; and 

 that the only way glacial and interglacial stages could be accounted 

 for would be by the unreasonable assumption of repeated elevation 

 and subsidence corresponding to each advance and retreat of the 

 ice. 



Croll's Astronomic Hypothesis. 1 — According to Croll, as 

 excellently interpreted by Le Conte, the glaciation was caused by 

 "the combined influence of precession of the equinoxes and secular 

 changes in the eccentricity of the earth's orbit. By the former 

 — viz., precession — winter, which in the northern hemisphere 

 occurs now when the earth is nearest the sun (perihelion), is 

 gradually in 10,500 years brought round so as to occur when the 

 earth is farthest off from the sun (aphelion) (Fig. 226). The effect 

 of this, it is claimed, would be to make longer and colder winters, 



1 For a fuller statement of this hypothesis see Croll's Climate and Time 

 in their Geological Relations, 1890. 



