860 T. 8. Hunt—Chemical and Geological 
readily accounted for by changed geographical conditions. 
Such changes of sea and land are, however, inadequate to ex- 
plain the elevated temperature which, according to the observa- 
tions of Nordenskidld, prevailed in the Carboniferous age, 
when the arctic climate permitted the development, overa 
area of land, of a vegetation not unlike the Carboniferous 
flora of the inter-tropical regions. It is not easy to conceive 
that, with an atmosphere like that of the present time, any 
geographical conditions could maintain during the long polar 
winter the mild climate required for such a vegetation, even In 
insular regions, and still less over a continental area within the 
polar circle. 
Weare thus led to the conclusion that geographical changes, 
though adequate to explain the greater refrigeration of certain 
areas since the beginning of Pliocene time, are not sufficient to 
account for the warmer climates of previous ages, and to find 
the explanation of these in the different relations of the earlier 
epend upon any diminution in the earth’s mean agit tem- 
re, in this view, 
as has been well said by J. F. Campbell, not celestial, but 
In a note in the Comptes Rendus of the French Academy of 
Sciences, for Oct. 7, 1878, criticising my previous one 0 Sept. 
23 “Sur les relations géologiques de l’atmosphere,” already 
referred to at the beginning of this paper, Mr. Stanislas Meu- 
nier has argued in favor of the terrestrial origin of the atmos- 
pheric carbonic dioxide, the source of which he supposes to be a 
subterranean oxidation of a primitive store of carbon, a view 
which seems unsupported by any facts or analogies in nature. 
He opposes to the bpndthedia which I have advocated, the fact 
of the absence of an atmosphere from the moon, while he 
asserts the existence of an abundant one around both Mercury 
and Venus. The evidences of such an atmosphere around 
the latter planet are well known, but the observations of recent 
* Campbell on Glacial Periods; Quart. Jour. Geol. Soc., 1879, vol. xxxV; P- 98. 
