1870.] 71 [Perry. 



conditions suggested by facts, and needful to the glaciation of the 

 country. Whatever theory, in short, we may adopt, in regard to the 

 ice period, the facts of astronomy compel us to admit changes of 

 temperature — great aeonian summers and winters — in the progress 

 of the globe. And geology tells the same story ; for instance, the 

 carboniferous period followed by the permian, the rniocene tertiary 

 by the period of drift. 



Again, however, it is said that, even if ice were formed, its motion 

 would be impossible; there being no great elevation of the conti- 

 nent, an inclined surface like that of Switzerland must have been 

 wanting, and thus the necessary condition of motion. Let us sup- 

 pose North America a level plane, and that, vapors condensing, a vast 

 amount of moisture is deposited upon it. What would be the result ? 

 This accumulation of water cannot remain heaped up. even on a 

 dead level, to say nothing of an inclined surface ; it must flow off. 

 The case is not different, even if it be congealed. Ice, as influenced 

 by gravity, would have the same tendency to motion as water. If, 

 now, the cold on the extreme north be greater than elsewhere, there 

 would be a barrier to motion in that direction. If melting take 

 place to some extent on the upper surface and the southern side, if 

 moisture from the snows melting at midday percolate the ice which 

 beneath the surface was much below the point of freezing, if gravita- 

 tion does its legitimate work on a mass live or six, not to say ten 

 thousand feet in thickness, surely some elements of motion are fur- 

 nished, in case the ice-sheet were resting on a plane, and still more, 

 if there were, as was no doubt the case, on the whole a gentle inclina- 

 tion toward the Gulf of Mexico. Indeed, under such conditions, 

 motion southward would be inevitable. 



But it is also objected that, such cold prevailing, the rocks would 

 absorb all moisture, and the surface of the earth be left, like that of 

 the moon, without air or water. It should be remembered that the 

 cold of the glacial period was not necessarily so intense as has been 

 sometimes asserted, very extreme cold not being the condition most 

 favorable, all things considered, to the production of glaciers. So it 

 should be borne in mind, that as soon as snows and ice began to man- 

 tle large tracts of land, they would be largely proof against external 

 cold. Meantime, the internal heat being as intense as ever, the tem- 

 perature of the water that penetrated the rocks would be raised, and 

 thus an extreme absorption of moisture prevented. 



Again it is asked, by way of objection, how, in case intense cold 



