MODERN SCIENCE. 169 



course. The first view is entertained by French geologists, 

 the second by James Croll and Sir R. Ball, the third by 

 Lyell ; but it is possible that this wonderful phenomenon 

 may have been brought about by the concurrence of the 

 three causes obtaining at the same time, otherwise a glacial 

 era would recur periodically. Another hypothesis is that 

 our siderial system moving towards Hercules as it does- 

 at the rate of 150 millions of miles per annum, passed 

 through intensely cold regions of space a supposition of 

 which we have no experience. The phenomenon, whatever 

 the cause, and whether it occurred once or several times 

 within a certain range of time for there are traces of two 

 Glacial periods in the Carboniferous and Permian Ages, 

 and undoubted evidences of inter-Glacial period within the 

 time-limits of the Ice Age teaches us one thing, viz., that 

 the uniformity of geological laws is not absolute. For here 

 we see, at an advanced stage of the terrestrial history 

 that is, during the Quaternary period, to speak of one 

 period only a phenomenon overwhelming the earth with 

 comparative suddenness, and this suddenness, so far, baffles 

 every explanation. The knowledge of this great Ice Age 

 is the work of Agassiz. 



We may point out that careful computation has enabled 

 us to approximately assign the time of the Glacial period : 

 reckoning backwards, the present or post-Glacial epoch 

 cannot be estimated to have covered less than 6o,coo years; 

 the retreat of ice-fields cannot have covered less than. 

 80,000 years ; the duration of the Glacial noontide in- 

 cluding in it the inter-Glacial eras may have covered any 

 number of ages, for we have no evidence for stating whether 

 it declined as soon as it reached its maximum, or whether 

 it remained stationary for ages and ages ; but as there is 

 evidence of several partial advances and retreats, and of 

 prolonged inter-Glacial periods, the total duration of this 

 stage cannot have covered less than 80,000 years, taking 

 the very lowest possible estimate. Before that noontide, 

 the advance of ice southward may have covered from 5,000- 

 to 80,000 years. All these estimates are far below those 

 given by some geologists, and especially by Lyell, who has 



