THE PLEISTOCENE OR GLACIAL PERIOD. 429 



of glaciation within a given period of high eccentricity, while none 

 of them could be more recent than 60,000 years; indeed, Croll consist- 

 ently placed the close of the glacial epoch 80,000 years ago. 



The extended and critical glacial studies of recent years seem to 

 show that the intervals between the different invasions are very un- 

 equal in time relations, and that the most recent is relatively young. 

 It has also been found that glaciation was notably extended beyond 

 its present limits on the lofty mountains of the equatorial regions. 

 The progress of inquiry seems, therefore, to have weakened, rather 

 than strengthened, the grounds of presumption in favor of this attrac- 

 tive hypothesis. 



To appreciate the difficulties that arise from the shortness of the 

 epochs of the Crollian hypothesis, it is to be observed that the Labra- 

 dorean and Keewatin ice-sheets pushed out from what appear to have 

 been their centers about 1600 and 1500 miles respectively. In making 

 this estimate the centers are placed as far south as a fair interpreta- 

 tion will permit. If for a generous safety margin we place these centers 

 of the initial snow-fields 500 miles farther to the southward, the edge of 

 the ice-sheets had still to creep 1000 miles during the advancing stage of 

 glaciation. To this is to be added its haltings and its retreating stages. 

 It is to be noted that the advance of the frontal border of this ice- 

 sheet is radically different from the movement of the ice itself, since 

 the advance of the margin is only the difference between the rate of 

 the ice movement and the melting of the margin. If one foot per 

 day be allowed for the advance of the margin — an estimate much 

 beyond the probabilities — it would take more than 14,000 years for the 

 ice-edge to reach the extension observed. This is two thirds of the 

 whole precessional period. If the safety margin of 500 miles be included, 

 as it perhaps should be, and it be assumed that the accumulation of 

 the central portion to a thickness sufficient to give effectual motion 

 required as long a time per mile as its subsequent extension (since 

 it took place in the initial stages of the glacial winter when its effective- 

 ness was doubtless relatively small), the whole precessional period 

 or more would be occupied in extending the ice the required distance. 

 Nor is the difficulty essentially escaped by assuming that the snow- 

 field grew up simultaneously over the whole area, or some large part 

 of it, for numerous bowlders are found 600 or 700 miles from their 

 nearest assignable sources, and 800 to 1000 miles or more from their 



