324 A. P. COLEMAN 
year, and the difficulty of accounting for such tremendous climatic 
changes is by no means lessening. The fact that the most extensive 
ice-sheets were in the Southern Hemisphere and that India only in 
the Northern Hemisphere shows important glaciation at the end 
of the Carboniferous forms one of the puzzling features of the 
problem. The idea that a change in the position of the poles could 
account for Permo-Carboniferous ice-sheets has been completely 
set aside by the discoveries in South America, since with the South 
Pole planted in the middle of the Indian Ocean southern Brazil 
would have been within the tropics. 
The theory of glaciation due to elevation is disproved also by 
the evidence from Australia and South America, showing that the 
_ice-sheets reached sea-level; and in any case it is inconceivable 
that such vast areas could all be elevated the necessary thousands 
of feet at the same time. Even if they were sufficiently elevated 
to give the required temperature, a large enough supply of moisture 
could hardly be arranged for on the greatly enlarged continents 
which this implies. The high tableland of the Andes is arid or semi- 
arid at present, and even the loftier peaks usually show little snow 
and few and small glaciers. It is evident that elevation alone will 
not account for the millions of square miles of mevé and ice-fields 
which must have covered much of India, Africa, Australia, and 
South America. 
The most satisfactory theory is that of refrigeration due to 
changes in the earth’s atmosphere; but even this fails to explain 
why Europe, Northern Asia, and North America should have been 
so little affected when great regions in other parts of the world were 
powerfully glaciated. One would expect that the change of climate 
affecting the tropics in India, Australia, and South America, and 
probably also in Africa would have been felt everywhere. It may 
be, however, that while the refrigeration was universal the supply 
of moisture necessary to form glaciers was lacking in Northern 
Asia, Europe, and North America. They may have had no ice- 
sheets for the same reason that Siberia was left uncovered with ice 
in the Pleistocene Ice Age; because the position of the open seas 
and the direction of the atmospheric circulation made them rela- 
‘ tively dry regions with little snowfall. 
