X 







f 



** 



4 



«8 



X 



5k 





w 



«t 



natit 



: 



arttet 



* sap. 



«g is 



that in 



1 exist 



at in. 



de tk 

 ed tkt 

 er that 

 iwering 

 g vege- 



to com- 

 mselves 



jnria 



same 

 ether 



ie 





a 



iferons 

 r * 



. ,a 



a 



i#' e 



« 



it# c: ' 







Ch. XL] 



EXCESS OF CARBONIC ACID IN THE AIR. 



227 



only flourish, therefore, in an atmosphere highly charged 

 with aqueous vapour, and such an atmosphere must have 



been warm 



Yet we must not suppose the heat to have been 

 tropical, for hot sunshine, by promoting the decomposition of 



\ 



matter 



to that of peat. 



>/ 



That the air 



was charged with, an excess of carbonic acid in the Coal period 

 has long been a favourite theory with many geologists who 

 have attributed partly to that cause an exuberant growth of 

 plants. It has been said that there is ten times more carbon 

 locked up in a solid form in the ancient coal measures than 



ranting 





mo 



all that is now contained in the atmosphere ; but 

 the truth of this estimate, which is probably far below the 

 mark, the inference deduced from it has always appeared to 

 me most delusive. The atmosphere now receives large sup- 

 plies of carbonic acid by gaseous emanations from the interior 



of th 



regions, and by volcanos during eruptions. Carburetted hy- 



ipes from beds of coal and lignite and other 

 fossiliferous strata in which organic matter is decomposing ; 

 the same gas evidently rising from great depths is also evolved 

 from rents in the granitic and other crystalline rocks in which 

 there are no organic remains. But it does not follow that the 



" more and more loaded with carbonic acid, for 

 there are causes in action which prevent such a change in the 

 constitution of the atmosphere. Wherever drift-timber is 



buried in the delta of a river, sea, or lake, or wherever peat 

 is forming, we behold the process by which carbon is first ex- 

 tracted by the powers of vegetation from the atmosphere, and 

 then locked up permanently, or for ages, in the earth's crust. 

 As to the volume of carbonaceous matter which may thus be 

 accumulated, it is a mere question of the time for which 

 certain species of plants, together with the conditions fit for 

 making peat and for burying drift-timber, may endure.* 



Some botanists are of opinion that the Sigillaria in the 

 Carboniferous period played the same part which is now 

 performed by the Sphagnum in Europe, both of them tending 



* See below, Chap. XVII, 



Q 2 





