50 PRELIMINARY INQUIRIES. 



existed, but to advance the margfin towards the equator ; and all that the margin would gain in 

 increase would be the few years' accumulation which might have fallen before it was left behind 

 in the interior by the general advance of the margin towards the tropics. The glacial ice, according 

 to my view therefore, never was thicker than it is now in Greenland and the Polar Seas. 

 Of course, if I am wrong in my reasoning as to the deposit of snow in the Polar regions, and if 

 the analogy of what is now to be seen in the Polar regions, can be disregarded or explained away, 

 I then must abandon my position and acknowledge that here is no limiting power but time to 

 the thickness which the sheet ice may have attained during the glacial epoch. I would only 

 say in that case, that I am astonished at the moderation of Agassiz and his followers in limiting it 

 to a mile. The rate at which the glaciers of the Alps move (from several inches to a foot or two in 

 the twenty-four hours) indicates a rate of increase at the upper end of many feet during the year ; 

 for although they are, as it were, the outlets of large lakes of ice, and consequently their rate of 

 movement is no guide as to the amount of snow which may have fallen on every square foot : still 

 considering how much is lost by melting, the rate of movement shows that the increase is very 

 great ; or, if we merely reckon all the rain that falls during the year in our own country, which 

 would then of course all be snow, and estimate the depth of ice as cqiial to that of the rainfall 

 it will be a very low estimate to take that at a foot in the year ; and if we then take Mr. 

 Croll's reduced datmn of only 10,000 years' continuance of cold without a break, we shoiJd on that 

 ratio have a thickness about two miles in height. Or if the alternative proposition of no breaks 

 of warmth be adopted, and his 100,000 years be accepted as the limit of time, the thickness on 

 the same ratio would reach twenty miles in height. 



Again, as to the repeated or alternate submergencies and elevations during the continuance of 

 the glacial epoch, this, no doubt, may have been, but it can scarcely be called more than a conjecture. 

 All that can be said of the facts to which Mr. CroU alludes as in some degree supporting this 

 idea is, that they are not irreconcilable with it. They are as consistent with the subsidence 

 (which all admit) haraig taken place subsequent to the retreat of the ice as during its sub- 

 sistence. The evidence of subsidence, such as that of beds containing shells being found overlying 

 the drift, jDoints to a date subsequent to the cessation of the chief rigour of the glacial epoch, 

 and some of them indicate the lapse of long periods of time between its close and the submer- 

 gence. The old watercourses and striated pavements found by Mr. Geikie in the drift, speak 

 neither for nor against submergence, but are so far in favour of a break in the intensity of the 

 cold, although they do not necessarily prove this. They may have arisen while the ground 

 where they occur formed part of the outer margin of ice, and vibrated between advance and 

 retreat. In our own times, without any apparent alteration in our climate, an immense barrier of 

 ice, which had surrounded the east coast of Greenland for fom- centuries, broke up in the year 

 1816, and in that and the following year disappeared from the coast. Its disruption and re- 

 growth might simulate some of the i^henomena referred to by Mr. CroU. 



Lastly, if the elevation or transference of mountain chains and vast continents from one 

 hemisphere to another, failed to disturb the centre of gravity of the earth, the existence of such 

 a quantity of ice as it seems reasonable to admit the existence of, could have had still less influence 

 especially if the balance of the earth were preserved by both Poles being refrigerated at the same 

 time. 



