§ 42. FORMATION OF COAL-MEASURES. 729 



change were induced in the constitution of the atmosphere 

 by such an agency, it would be the reverse of that assumed ; 

 for an excess of vegetation would tend to diminish the 

 average amount of carbonic acid, and consequently the air 

 must have been purer than in the succeeding epochs.* It 

 is therefore clear that the absence of herbivorous animals 

 cannot be explained by unfavourable atmospheric conditions : 

 and again we may point to New Zealand as a country 

 having a luxuriant vegetation, yet without herbivorous 

 quadrupeds. | 



42. Formation of Coal-measures. — From the facts 

 which have passed under our examination, we may now 

 advantageously consider what were the circumstances which 

 gave rise to these prodigious layers of carbonized matter 

 unmixed with other materials — these immense beds of 

 vegetables, from which animal remains are almost wholly 

 excluded; — and whether accumulations of trees and plants, 

 which in after ages shall present phenomena of a like nature, 

 are in progress at the present time ? 



The manner in which the carboniferous strata have been 

 deposited, has been a fruitful source of discussion among 

 geologists. Some contend that the coal-measures were 

 originally peat-bogs, and that the successive layers were 

 occasioned by repeated subsidences of the land (ante, 

 p. 670) ; others, that the vegetable matter originated from 

 rafts, or drifted forests, like those of the Mississippi, which 

 floated out to sea, and there became engulfed ; — others sup- 

 pose that they were formed in vast inland seas or lakes, the 

 materials of the successive beds being brought down by 

 periodical land-floods : and the supporters of each hypo- 

 thesis adduce numerous facts in corroboration of their 

 respective opinions. There can be no doubt that coal may 



* Mr. Lyell's Travels in America, vol. i. p. 152. 

 f The only indigenous mammalia in New Zealand is one species of 

 Rat, and Bat ; and there are no large reptiles whatever. 



