548 WESTERN COAL MEASURES AND INDIANA COAL. 
From this, we may readily infer that the North American lakes 
communicated at one time with the ocean, and that their fauna and 
flora, were to a certain extent brought to accommodate themselves 
to the gradual change from salt to fresh water. 
The position of the oceans, relative to the land, and the great 
preponderance of water on the American continent, during the 
Carboniferous epoch, must have had a decided influence in modify- 
ing the temperature and increasing the humidity of the atmos- 
phere, thereby rendering it in every way adapted to the luxuriant 
growth of the tropical plants which furnished the carbon so prov- 
identially stored away in the fossil fuel, for we find that many of 
these coal producing plants, whose dwarfed prototypes are now 
confined to the tropics, flourished then as far north as the arctic 
zone. j 
There could have been no necessity for any increase of carbonic 
acid or other material change, as some have supposed, in the 
composition of the atmosphere beyond a slight increase in its 
humidity, and the probability is that none existed. 
The two great coal fields being separated from each other from 
the very beginning, as I have endeavored to show, by a barrier of 
rocks, which present no evidence of any subsequent submergence, 
and which long antedate the carboniferous era, renders it difficult 
to comprehend how an equivalency in the coal beds of the Appa- 
lachian field, can be found in .those of the West, as many of our 
eminent geologists have maintained. 
It is true that the fluctuations in level which served to build up 
the various strata, may have been, and in all probability were, 
synchronous over the two basins, but the special requirements for _ 
the productioh of coal beds could hardly have proved uniform over 
istricts so widely separated. 
Though once a firm believer in the magai valency of coal seams 
throughout the Western coal measures, I have seen much of late 
to shake my faith in the possibility of determining an entire agree- 
ment in the coal beds, even in the limited area of the coal fields 
of Indiana. 
From a marked irregularity in the thickness of the Carbon- 
iferous beds over any great extent of territory, we have good 
reason to believe that these inland seas, like all other great 
bodies of water, were of unequal depth, and consequently, did 
not present at all times, over their entire area, the conditions 
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