138 LECTURES 



success, of opposing forces, aqueous and igneous, the river constantly- 

 building up, and igneous forces beneath as constantly striving to de- 

 press ; sometimes one force predominating, sometimes the other. Of 

 such contention we have many instances in existing nature. It is 

 evidently going on in the delta of the Mississippi at the present time. 



It may not be possible, in the present condition of science, to picture 

 to ourselves all the circumstances connected with this process. Per- 

 haps I have already gone too far in this attempt ; but the general 

 facts upon which the theory rest are incontestible. Coal has almost 

 certainly accumulated m situ in extensive peat swamps at the mouths 

 of large rivers, upon ground which was slowly subsiding during the 

 whole period. Under these circumstances it seems not difficult to 

 account for all the phenomena of a coal basin. All we have to do in 

 future is by study of the peat swamp of the Mississippi and the phe- 

 nomena of delta deposit to discover the details of the process, to fill 

 up the outline of the picture. 



There is a fact noticed by Mr. Lyell, which is strongly confirmatory 

 of this theory. In the sandstone of the coal measures it is common 

 to find trunks of trees, but only trunks — no small branches, leaves, or 

 tender jjarts. Moreover these trunks are observed to be mostly pines, 

 highland trees, while the trunks in the coal seam proper are sigilla- 

 ria, lepidodendron, calamites — swamp trees. Now, when we recollect 

 that coarse sandstone is the deposit of rapid current, does it not seem 

 evident that the sandstone was deposited by the freshet which over- 

 whelmed the peat swamp, and that the pine trunks are the remains 

 of drift timber brought from the highlands. Here, then, we have 

 ancient drift timber, but how different from a coal seam ! 



Let us now attempt to estimate approximatively the time necessary 

 to bring about these stupendous results. I believe we should never 

 neglect an opportunity of this kind, because the popular mind has not 

 yet grasped the idea of illimitable time required by geology to the 

 same extent as it has the idea of illimitable space required by 

 astronomy; and, as I believe, this is one of the greatest difficulties 

 with which geology has to contend. 



According to Boussingault luxuriant vegetation at the present day 

 takes from the atmosphere about a half ton of carbon per acre annu- 

 ally, or 50 tons per acre in a century. Fifty tons of carbon of the 

 specific gravity of coal, about 1.50, spread evenly over the surface of 

 an acre, would make a layer of less than \ of an inch. Humboldt 

 makes the estimate a little higher, viz : \ an inch. We are willing 

 to take the higher estimate. It appears, then, that if all the carbon 

 taken from the air was preserved in the form of coal, our most luxu- 

 riant vegetation would make but a \ inch of coal in a century. But 

 in the coal measures the aggregate thickness of the coal seams in the 

 same basin is sometimes 150 feet or more. In 150 feet there are 

 1,800 inches, or 3,600 half inches. At the present rate of vegetation, 

 then, it would take 3,600 centuries, or 360,000 years^ to accumulate 

 this amount. But it will be objected that the vegetation of the coal 

 period was probably m jch more luxuriant than the present, and the 

 tendency of this difference would be to shorten the time. True ; but 

 it will be recollected that this estimate was made upon the ground 



