1864.] ADDRESS BY SIR CHARLES LYELL. 401 



cided ill time with the submergence of the Sahara, namely, the 

 diversion of the Gulf-stream from its present course. The shape 

 of Europe and North America, or the boundaries of sea and land, 

 departed so widely in the glacial period from those now established, 

 that we cannot suppose the Gulf-stream to have taken at that 

 period its present north-western course across the Atlantic. If it 

 took some other direction, the climate of the north of Scotland 

 would, according to the calculations of Mr. Hopkins, suffer a 

 diminution in its average annual temperature of 12° F., while that 

 of the Alps would lose 2° F, A combination of all the conditions 

 above enumerated would certainly be attended with so great a 

 revolution in climate as miiiht o;o far to account for the excessive 

 cold which was developed at so modern a period in the earth's 

 history. But even when we assume all three of them to have 

 been simultaneous in action, we have by no means exhausted all 

 the resources which a diiference in the geographical condition of 

 the globe might supply. Thus, for example, to name only one of 

 them, we might suppose that the height and quantity of land near 

 the north pole was greater at the era in question than it is now. 



The vast mechanical force that ice exerted in the glacial period 

 has been thought by some to demonstrate a want of uniformity in 

 the amount of energy which the same natural cause may put forth 

 at two successive epochs. But we must be careful, when thus 

 reasoning, to bear in mind that the power of ice is here substituted 

 for that of running water. The one becomes a mighty agent in 

 transporting huge erratics, and in scoring, abrading, and polishing 

 rocks ; but meanwhile the other is in abeyance. When, for 

 example, the ancient llhone glacier conveyed its moraines from the 

 upper to the lower end of the Lake of Geneva, there was no great 

 river, as there now is, forming a delta many miles in extent, and 

 several hundred feet in depth, at the upper end of the lake. 



The more we study and comprehend the geographical changes 

 of the glacial period, and the migrations of animals and plants to 

 which it gave rise, the higher our conceptions are raised of the 

 duration of that subdivision of time, which, though vast when 

 measured by the succession of events comprised in it, wms brief, 

 if estimated by the ordinary rules of geological classification. The 

 glacial period was, in fact, a mere episode in one of the great 

 epochs of the earth's history ; for the inhabitants of the lands 

 and seas, before and after the grand development of snow and ice, 

 were nearly the same. As yet we have no satisfactory proof that 



