CONCLUSIONS 363 



ancient mountain building and eruptive activity, and also the usual con- 

 ception of the original of the schistose rocks of the 'T)asal complex." 



We can no longer account for the supposed uniformity of past climates 

 from the equator to the poles by assuming a supply of heat from beneath. 

 Instead we must adjust our theories to an earth cold on the surface from 

 the beginning, perhaps even warming up by internal work and the action 

 of radiant matter, but more probably undergoing comparatively little 

 change in temperature. 



Certain other climatic theories are ruled out also. No merely local 

 cause or probable combination of local causes, such as those sMllfully sug- 

 gested by Professor Gregory,®^ seems capable of covering the ground in a 

 great ice age like that of the Pleistocene or the Permo-Carboniferous. 

 Local elevations of thousands of feet can hardly be conceived as taking 

 place at the same time over most of North America, the whole length of 

 the Andes and Patagonia, all northern Europe, the Alps, the mountains 

 of Turkestan, the Himalayas and Altai mountains, the Atlas region, 

 Euwenzori, Kenia and Kilimanjaro, the New Zealand Alps, and Kosci- 

 usko in Australia, not to mention other localities glaciated in Pleistocene 

 times. The theory breaks down of its own weight. It should be added, 

 however, that elevation in some cases is hostile to the formation of ice- 

 sheets, as Scott has shown on the Antarctic tableland 9,000 feet above the 

 sea, where the ice is dwindling and the glaciers are retreating.^* There 

 depression might extend glaciation by increasing precipitation. 



Again, the mere enumeration of points where glaciation took place in 

 the Pleistocene — some on broad low plains, others on mountains, some on 

 isolated islands or the seacoast, others, like Euwenzori and the Keewatin 

 ice-sheet, in the heart of a continent — seems enough to discredit theories 

 in which an adjustment of 'Tiighs" and "lows," of winds and precipita- 

 tion, is brought forward to account for ice ages. Could any imaginable 

 shifting of barometric pressures produce the enormous Permo-Carbonif- 

 eious ice-sheets of India, Australia, and South Africa, all touching the 

 tropics on plains reaching to sealevel ? 



Some general cause, applicable to the whole world, must be assumed, 

 such as a change in the sun's heat or in the composition of the atmos- 

 phere, to account for periods of universal refrigeration, separated by long 

 periods of mild climate reaching even to the polar regions. But the 

 cause must permit also of comparatively rapid oscillations to account for 

 warm interglacial episodes. The astronomical theories founded on the 



" Climatic variations. Mexican Geological Congress, 1906. 

 " Voyage of the Discovery. 



XXXV — Bull. Geol. Soc. Am., Vol. 19, 1907 



