THE CAUSE OF THE GLACIAL PERIOD. 311 



in the light of the general considerations hereafter to be 

 adduced. 



The aid of theoretical consequent changes in the vol- 

 ume of the Gulf Stream, and in the area of the trade- 

 winds, has also to be invoked by Mr. Croll. The theory 

 likewise receives supposed confirmation from facts alleged 

 concerning the present climate of the southern hemisphere 

 which is passing through the astronomical conditions 

 thought to be favourable to its glaciation. The antarctic 

 continent is completely enveloped in ice, even down to 

 the sixty-seventh degree of latitude. A few degrees nearer 

 the pole Sir J. C. Boss describes the ice as rising from 

 the water in a precipitous wall one hundred and eighty 

 feet high. In front of such a wall, and nearly twenty 

 degrees from the south pole, this navigator sailed four 

 hundred and fifty miles ! Voyagers, in general, are said 

 to agree that the summers of the antarctic zone are 

 much more foggy and cold than they are in corresponding 

 latitudes in the northern hemisphere ; and this, even 

 though the sun is 3,000,000 miles nearer the earth during 

 the southern summer than it is during the northern. 



Another direction from which evidence is invoked in 

 confirmation of Mr. Croll's theory is the geological indi- 

 cations of successive Glacial epochs in times past. If 

 there be a recurring astronomical cause sufficient of itself 

 to produce Glacial periods, such periods should recur as 

 often as the cause exists ; but glaciation upon the scale of 

 that which immediately preceded the historic era could 

 hardly have occurred in early geological time without leav- 

 ing marks which geologists would have discovered. Were 

 the " till " now covering the glaciated region to be con- 

 verted into rock, its character would be unmistakable, and 

 the deposit is so extensive that it could not escape notice. 



In his inaugural address before the British Association 

 in 1880, Professor Eamsey, Director-General of the Geo- 

 logical Survey of Great Britain, presented a formidable 



