PERMO-CARBONIFEROUS CLIMATIC CHANGES 629 
the land route. The absence of the northern flora from the series 
above the glacial deposits can therefore be due only to the uncon- 
geniality of the province to that flora. 
CAUSES OF THE PERMO-CARBONIFEROUS GLACIATION 
So varied as well as great are the geological changes since Paleozoic 
time that the exposure, at this late date, of an ancient glaciated floor 
at even a few points is fortuitous. Yet proofs of Permo-Carbon- 
iferous land ice movement have been observed over an area more than 
800 miles in length and 4oo miles in breadth in South Africa,* while 
glaciated material is seen here and there over a very much larger 
territory. In Victoria the till and bowlder beds compose the greater 
part of a section about 1,700 feet in thickness. Bedrock striation, 
the work of grounded icebergs, if not of subaerial ice, is seen in 
India, and several provinces of Australia as well as in Tasmania. 
The GANGAMOPTERIS province undoubtedly witnessed Permo- 
Carboniferous glacial action many times greater than that which 
occurred in the Northern Hemisphere during Pleistocene time. 
However complete the unanimity as to the fact of Permo-Carbon- 
iferous glaciation in the GANGAMOPTERIS province, there is little 
agreement as to the causes of that remarkable episode. The informa- 
tion obtained from South America both extends and defines more 
clearly the problem, though in the solution of the latter the testimony 
of the new data is perhaps chiefly negative. 
So long as the glaciation was supposed to have been confined to 
the Indo-Africo-Australian quarter of the earth a shifting of the axis 
so as to place the pole in the Indian Ocean was urged in explanation 
of the regional refrigeration. The recognition of the glaciation in 
South America and as far north as latitude 28° in that continent 
seriously modifies if it does not completely destroy this hypothesis. 
Depletion of atmospheric carbonic-acid gas.—Of the other hypoth- 
eses that which seems by far most nearly to meet the situation has for 
its essential condition loss of heat on account of a depletion of the 
atmosphere in carbonic dioxide consequent to great coal and limestone 
deposition during the Carboniferous epoch. Co-operative and 
t Wm. M. Davis, Bulletin of the Geological Society of America, Vol. XVII, 1906, 
P: 377- 
