84 



yet those magnificent trees above alluded to grow there ; but 

 the soil is strongly impregnated with carbonic acid. Tt is more 

 or less saturated in all soils, and without it vegetation cannot 

 thrive, as it feeds the plants by means of the roots, and not by 

 the leaves. The function of the leaves is to evolve the gases, 

 i. e. they form the negative plates to the currents of sap in the 

 living plants. Had there not been a provision to take the car- 

 bonic acid away from the atmosphere we could not exist ; in- 

 deed it is the absence of it from the air that preserves the vital 

 principle. The purer the air is at the leaves, with a due pro- 

 portion of moisture, the more healthy is vegetable life *• 



Probably our coal-beds may be as much due to the combina- 

 tion of carbonic acid and hydrogen forming bitumen, as they 

 are to decomposed vegetation. We have abundant facts to prove 

 that plants are as liable to become silicified as to be converted 

 into coal ; and there are also proofs of beds of lignites forming 

 from bituminous substances, near the roots of living plants, 

 within the tropics. 



We shall not enter into a description of the probable mode 

 of deposition of the carboniferous series, because this has been 

 well explained in all geological works ; but we may note in con- 

 clusion, that the coal basins appear to be principally lacustrine 

 deposits, where bitumen, mud, sand, and decomposed vegetable 

 matter may have accumulated, such as we now see forming 

 within the tropics and in the southern hemisphere. 



3. South Tropical Zone. The system of rocks between the car- 

 boniferous strata and the chalk, containing a large propor- 

 tion of gigantic amphibious Saurians. 



The oolitic group may be considered as representing this di- 

 vision of the series. The remains of plants, Zoophyta, Mollusca, 

 Articulosa, and vertebral animals belonging to the oolitic system, 



* The respiration which takes place at the leaves has been found to vitiate 

 the atmosphere, and more especially at night ; the sun's rays appear to restore 

 the purity of the air. An atmosphere containing 60 per cent, of carbonic acid 

 gas has been found destructive of vegetable life ; the doses become less preju- 

 dicial as they are diminished. Any addition, however small, of this gas to 

 common air, if placed in the shade so as not to be neutralized by the sun's 

 rays, has been found prejudicial to plants. 



