CARBONIFEROUS AGE. 353 



(3.) Impurity of the Atmosphere. — In the present era, the atmos- 

 phere consists essentially of oxygen and nitrogen, in the proportion of 

 23 to 77 parts by volume. Along with these constituents, there are 

 about four parts by volume of carbonic acid, in 10,000 parts of air. 

 Much more carbonic acid would be injurious to animal life. To vege- 

 table life, on the contrary, it would be, within certain limits, promotive 

 of growth ; for plants live mainly by means of the carbonic acid they 

 receive through their leaves. The carbon they contain comes princi- 

 pally from the air. 



This being so, it follows, as has been well argued, that the carbon 

 which is now coal, and was once in plants of different kinds, has come 

 from the atmosphere, and, therefore, that the atmosphere now contains 

 less carbonic acid than it did at the beginning of the Carboniferous 

 period, by the amount stowed away in the coal of the globe. 



Volcanoes contribute at the present time a little to the carbonic acid of the atmos- 

 phere; and it may be that some of the carbon in coal is from this source. But this car- 

 bonic acid is given out only where the heat of the volcanic vent has limestone to act 

 upon; and, if this is a rare case now, it was even less common in Paleozoic time, when 

 volcanoes were probably far less numerous. Moreover, the carbon in the limestone 

 (carbonate of lime) of the globe, while it was taken directly from the earth's waters 

 (p. 130), came in part from the atmosphere, the rains carrying it down to the ocean. 

 If, then, the limestones robbed the atmosphere, as well as the coal, the amount of Car- 

 boniferous coal in the earth's rocks does not probably represent more carbonic acid than 

 the atmosphere of the Carboniferous age lost. 



Such an atmosphere, containing an excess of carbonic acid as well 

 as of moisture, would have had greater density than the present ; con- 

 sequently, as urged by E. B. Hunt, it would have occasioned increased 

 heat at the earth's surface, and this would have been one cause of a 

 higher temperature over the globe than the present. 



During the progress of the Carboniferous period there was, then, 

 (1) a using up and storing away of the carbon of the superfluous car- 

 bonic acid, and, thereby, (2) a more or less perfect purification of the 

 atmosphere, and a diminution of its density. In early time, there was 

 no aerial animal life on the earth ; and, so late as the Carboniferous 

 period, there were only Reptiles, Myriapods, Spiders, Insects, and pul- 

 monate Mollusks. The cold-blooded Reptiles, of low order of vital 

 activity, correspond with these conditions of the atmosphere. The 

 after-ages show an increasing elevation of grade and variety of type 

 in the living species of the land. 



(4.) Influence of the Climate on the Growth of Plants. — A moist 

 warm climate produces exuberant growth in plants that are fitted for 

 it. The plants of the Coal period were made for the period. The 

 Sigillarids and Calamites manifest, by their characters and mode of 

 occurrence, that they could nourish only in a moist region ; and the 

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