288 



TEMPERATURE OF THE GLACIAL PERIOD. [On. XIII, 



When we endeavour to estimate tlie mean thickness of the 

 ice at present, we must allow in the north for large spaces 

 where, instead of ice, there is a deep open sea ; and even in 

 regard to a continent like Greenland, we must remember that 

 it is only at the termination of valleys or straths that Rink 

 found great vomitaries discharging ice 2,000 feet thick into 

 Baffin's Bay. He counted twenty-two of these ice-streams ; 

 but we may presume that on the intervening higher lands, 

 parting the valleys which disgorge the ice, snow would never 

 accumulate to more than a few hundreds of feet. If we were 

 to imagine an average sheet of ice 2,000 feet thick, extending 

 from the north pole to lat. 77°, and another 2,500 feet thick 

 stretching from the south pole to lat. 70°, each ice-cap being 

 continuous over the whole space now occupied by sea and 

 land, this thickness would probably be nearer the truth than 

 the extravagant estimates indulged in by some who have 

 speculated on this subject. 



Several writers have fallen into the strange error of sup- 

 posing that the Glacial Period must have been one of a higher 

 mean temperature than usual, because an excess of snow 

 implies an excess of evaporation, and consequently of heat. 

 This fallacy has arisen from omitting the element of time 

 from the calculation. No doubt the formation of so much 

 snow requires that a corresponding quantity of water should 

 be converted by heat into vapours; but we must tak6 into 

 account the number of years over which the process of dis- 

 tillation was spread. If the summer's warmth cannot get 

 rid of all the winter's snow, even by a few feet in a century, 

 there will, in the course of thousands of years, be as large a 

 store of ice formed as geologists may require. 



That in the Glacial Period there was a larger volume of ice 

 in high latitudes than now exists there, we are, I think, war- 

 ranted in concluding from geological evidence, even admitting 

 that the extreme growth of the northern and southern ice- 

 caps would never occur simultaneously. But although we 

 find the monuments of extinct glaciers in the southern parts 

 of New Zealand and South Australia, as well as in some cor- 

 responding low latitudes in the northern hemisphere, espe- 

 cially in North America, we can by no means infer that the 



