HYPOTHESIS OF CAUSE OF GLACIAL PERIODS 775 
bowlder beds. From this and other paleontological evidence it 
would seem that the lowest of the g 
dated the disappearance of the Permian faunas from the seas.” 
glacial deposits at least ante- 
It is unfortunate that the peculiar nature of the formation, and 
the not less peculiar nature of the associated flora, make it 
impossible at present to fix its exact horizon in terms of the 
European and American standards, so that the contemporaneous 
conditions in these regions could be determined. However, for 
the application of the atmospheric hypothesis the foremost ques- 
tion is the relationship of glaciation to the great agencies that 
affected the constitution of the atmosphere. These agencies 
were (1) the formation of coal, and (2) chemical reaction 
between the air and the earth’s surface. 
1. Relative to the first, it is certain that the glaciation 
closely followed the great coal-depositing period, and indeed 
fell in with the latest stages. In Australia there are consider- 
able deposits of coal (the Gretna Coal Measures, embracing 
twenty to forty feet of coal) deposited ‘between the erratic- 
bearing horizon of the lower marine series and the similar 
horizon of the upper marine series” (David)? -1f, theretore, 
we look to the deposition of coal as the agency of atmospheric 
exhaustion, the relationship is nearly ideal. 
2. If we look to elevation, and consequent large earth- 
contact, the relationship does not appear to be what theory 
would demand. It can scarcely be questioned that at the close 
of the Paleozoic period there was a very unusual surface move- 
ment, affecting great areas of the earth’s surface and increasing 
largely the exposure of the land. The period appears to have 
been altogether comparable to that at the close of the Tertiary 
period. If we were seeking the cause of the glaciation assigned 
to the Triassic, or the occasion of the salt and gypsum deposits 
«There is some evidence of another glacial horizon at or about the base of the 
Triassic (well indicated in White’s excellent synopsis, Am. Geol., May 18809, table, 
p- 315). The present discussion will, however, be confined to the lower horizon, that 
of the Talchirs and their equivalents. 
2 Quarterly Jour. Geol. Soc., Vol. LII, May 1896, p. 300. 
