48 THE CHEMISTRY OF THE PRIMEVAL EARTH. [IV. 



or rarity of high mountains, contributed much towards the mild 

 climate of later ages, when a vegetation as luxuriant as that now 

 found in the tropics flourished within the Arctic circle ; but to 

 these causes must be added the influence of a portion of carbon 

 which was afterwards condensed in the forms of coal and carbonate 

 of lime, and which then existed in the condition of a transparent 

 and permanent gas, mingled with the atmosphere, surrounding the 

 earth, and protecting it like a dome of glass. To this effect of car- 

 bonic acid it is possible that other gases may have contributed. 

 The ozone, which is mingled with the oxygen set free from grow- 

 ing plants, and the marsh gas, which is now evolved from decom- 

 posing vegetation under conditions similar to those then presented 

 by the coal fields, may, by their great absorptive power, have very 

 well aided to maintain at the earth's surface that high temperature 

 the cause of which has been one of the enigmas of geology. 



