CLIMATE IN DWYKA TIME 419 



General refrigeration. — Eefrigeration alone — either by a decrease of 

 solar radiation or by a change in the constitution of our atmosphere — 

 without any significant shifting of the wind belts, would have to be ex- 

 treme in order to produce an ice-sheet in. latitude 25 degrees ; for it would 

 be necessary in such a case to reduce the summer temperature of South 

 Africa so that the summer rains should be changed to summer snows, 

 the winters remaining dry, but becoming extremely cold. Such a refrig- 

 eration would freeze up all the temperate lands ; and there does yet appear 

 to be sufficient reason for supposing that so general a Permian refrigera- 

 tion has taken place. A special study of the Permian formations of - the 

 whole world in this connection would be instructive. 



Shifting of the poles. — An altogether different explanation for the cli- 

 mate of the Dwyka ice-sheet is found in the frequently suggested change 

 in the attitude of the earth's axis, so that the poles with their normally 

 low temperatures might be placed nearer the present position of the 

 equator. It is well known that a change of the north pole to a position 

 near Iceland would favor the production of the Pleistocene ice-sheets in 

 northeastern America and northwestern Europe; for it would not only 

 decrease the mean annual temperature of the glaciated areas, but it would, 

 by setting the equator in the Atlantic south of cape San Eoque, prevent 

 the deflection of a large body of warmed water from the South Atlantic 

 to the North Atlantic, and thus decrease the volume of abnormally warm 

 water which now, under the popular but inappropriate name of the Gulf 

 stream, flows past Norway. 



If a change in the position of the axis took place in Permian time, it 

 would seem easy thus to account not only for the Dwyka glacial formation 

 of South Africa, but also for the Talchir glacial formation of northwest- 

 ern India, and for the Muree glacial formation of southeastern Australia, 

 as various geologists have pointed out. It should be recalled that, in 

 favor of the hypothesis, the movement of the Talchir ice-sheet in India 

 was, like that of the Dwyka ice-sheet in South Africa, away from the 

 equator. But there are various embarrassments connected with the ac- 

 ceptance of this daring hypothesis. There is no efficient cause known by 

 which so great a change in the position of the earth's axis as is here 

 needed can be accounted for; there appears to be a less frequent occur- 

 rence and a smaller extension of glacial formations in the whole geological 

 series than one might fairly expect there would be if the earth's axis 

 were in the habit of varying its position ; and there are so many areas of 

 undisturbed Paleozoic strata that it is unreasonable to admit the occur- 

 rence in later time of the various deformations of the earth's crust that 

 might have to follow a shift in the location of the equatorial bulge 



