418 W. M. DAVIS OBSERVATIONS IN SOUTH AFRICA 



cold current running equatorward along the east coast of Permian Africa 

 in lower latitudes than 40 degrees is hardly conceivable. It may be 

 barely possible that such a current existed in Permian times, but it has 

 no analogue in the present arrangement of ocean currents. 



The subtropical belt. — The occurrence at present of winter droughts 

 and summer rains over the Dwyka area is so unfavorable a condition for 

 glaciation that it is necessary to inquire whether winter precipitation 

 might occur there under any admissible conditions. The only reasonable 

 scheme for producing such a result involves an increased migration of the 

 subtropical belt, such as may be brought about by increasing the contrast 

 of equatorial and polar temperatures in the winter hemisphere. There 

 are two unlike conditions from which such an increased contrast might 

 follow: One involves a persistently greater warming of the equator by 

 sunshine or a greater cooling of the poles by radiation. As a conse- 

 quence, the general circulation would be strengthened all the year round, 

 and the subtropical belt in the winter hemisphere would be driven farther 

 toward the equator ; and in this way the winter precipitation, which now 

 reaches only the southern extremity of Africa, might advance over the 

 interior. Yet even under extreme conditions of this kind it is doubtful 

 whether the subtropical belt could be thus driven ten degrees of latitude 

 nearer the equator than it now reaches; and furthermore it is manifest 

 that, if it were so driven, there would still be the warmth of summer and 

 its rainy precipitation to melt the winter snows; hence it is not likely 

 that effective assistance in the production of the Dwyka ice-sheet can be 

 thus afforded. 



The second possible condition involves an increased eccentricity of 

 the earth's orbit, with the southern winter in aphelion. An increased 

 contrast between equatorial and south polar temperatures in winter would 

 then be brought about by a fall in the temperature of the Antarctic re- 

 gions. The general circumpolar circulation of the atmosphere in the 

 southern hemisphere would consequently be strengthened during the long 

 severe winter, the subtropical belt would be thereby driven farther toward 

 the equator, and the precipitation from it would be more largely in the 

 form of snoAv than it is at present. It is difficult to believe, however, that 

 a change of this kind would suffice to pro dace enough saow in latitude 

 25 degrees for the formation of an extensive ice-sheet, unless a decided 

 reduction of mean annual temperature accompanied it. It may be 

 pointed out that the aphelion winter theory involves a contemporaneous 

 mild winter in the northern hemisphere; it is therefore to that exteat 

 inconsistent with the supposed equivalence of the Dwyka glacial forma- 

 tion with the Talchir glacial formation of northwestern India. 



