Miscellaneous. 



433 



And the luxuriant vegetation which covered the sea as well as the land, 

 would constantly be abstracting this carbonic acid for the purposes of 

 food, from the surrounding water. The carbonate of lime being thus 

 rendered insoluble, would, be tranquilly deposited. And thus two sub- 

 stances being removed, which in excess are fatal to certain animals, 

 other forms of organic life would spring into being. Here, then, we 

 are furnished with an explanation why the limestone does not occur in 

 continuous beds in former strata; for however great the quantity of 

 limestone in solution might have been, it could not have been depo- 

 sited, were there no means of removing the carbonic acid which retain- 

 ed it in solution. Only in particular localities, where adventitious cir- 

 cumstances occasioned the expulsion of carbonic acid, the limestone 

 would be deposited. 



And now we come to the consideration of those vast deposits of coal 

 which form such striking monuments of a primeval vegetation. The 

 entire coal series often attains a thickness of 1,000 yards; the beds of 

 coal occurring in it are occasionally three or four feet thick, and not 

 unfrequently several yards. The thickness of all the coal beds taken 

 together may average forty or fifty feet in the English and Scotch coal 

 fields. Now when we consider the vast area covered by the coal 

 series, we must feel convinced that during its formation, peculiar 

 causes were in operation, which occasioned a great luxuriance in vege- 

 tation. It is true that such rivers as the Oronoko and Mississippi roll 

 down to the ocean vast quantities of vegetable matter; but great as 

 these are, they do not even furnish us with a faint conception of the 

 manner in which the great carboniferous deposits have been formed. 

 The wonderful luxuriance of vegetation during the carboniferous era is 

 doubtless attributable to the amount of carbonic acid in the air. The 

 remains of plants which constitute the various seams of coal, shew that 

 they were principally terrestrial. Many of these beds of coal appear 

 to have been formed of drift vegetation ; but others shew every evi- 

 dence of the plants having lived and died on the spot. This is the case 

 with the North of England coal field ; and most of the North American 

 coal fields shew similar evidences ; the Devonshire coal fields, or culm 

 measures are, on the other hand, I believe, frequently composed of 

 drift matter. 



Now in these coal fields which shew evidences of having been 

 formed in situ, a bed of fire clay is almost invariably found immedi- 

 ately below the coal. In this are present large quantities of stems 

 and leaves of stigmaria, ficoides, &c. The constant occurrence of 

 this underclay, evidently indicates some general cause. Now, when 



3 K 



