CLIMATE IN DWYKA TIME 417 



temperatures, nothing less than a lofty mountain range, like the Hima- 

 laya, would suffice; and even then it would be impossible to produce an 

 ice-sheet on the polar side of the range. Indeed, under existing conditions 

 as to mean annual temperatures, no imaginable mountain range in lati- 

 tude 25 degrees south could gather snowfall of sufficient amount to form 

 on its polar side a piedmont ice-sheet that would move 600 miles across a 

 lower land to latitude 33 or 34 degrees. A heavy snowfall on the polar 

 side of such a mountain range could be supplied only by the trade winds, 

 which would ascend the poleward mountain slope; but the heavy rains 

 that such winds would give forth at moderate altitudes, before their tem- 

 perature was reduced to the freezing point, would effectually melt the 

 ice that might descend from the higher levels at which snow would be 

 formed. There is little assistance gained by postulating great highlands 

 for the entire Dwyka area; for, apart from the strong improbability of 

 their occurrence, as already indicated, the precipitation on them in 

 trade wind latitudes would take place chiefly around their bordering 

 slopes, while the highland areas would be left comparatively dry, even 

 though they were cold. Changes of land area or land form therefore 

 appear to be ineffectual in producing a glacial climate in subtropical 

 South Africa. 



Ocean currents. — No conceivable arrangement of continents and ocean 

 currents could produce an abundant snowfall in latitude 25 degrees, so 

 long as the general temperature of the atmosphere preserved its present 

 values. Even if both Permian Africa and Permian Australia reached 

 farther south than present South America and diverted toward the equa- 

 tor a greater body of colder water than now flows equatorward in the 

 Peruvian or Humboldt current, such currents would not alone cause the 

 trade winds to precipitate snow over a lowland in latitude 25 degrees. It 

 must be remembered that, however cold the poles, the ocean currents 

 moving toward the equator could not be colder than 28 or 30 degrees 

 Fahrenheit at their source, and that they must under existing conditions 

 as to sunshine rise in temperature 30 or more degrees on their way to 

 the equator. So far as a cold current in the Atlantic west of Permian 

 Africa is concerned, its effect on the trade wind belt of southeastern 

 Africa would be practically nothing, because the current would be far 

 down the wind, or to leeward. So far as a cold current in the Indian 

 ocean west of Australia is concerned, it would have gained a temperature 

 of at least 60 degrees, more likely TO degrees, on reaching the equator; it 

 would be warmed to a somewhat higher temperature as it flowed west- 

 ward along the equator, and it therefore could not produce a very low 

 temperature when returning poleward along the east side of Africa. A 



