ON COAL. 



137 



Let us now suppose that 

 there existed during the 

 carboniferous period a large 

 river, perhaps less than the 

 Mississippi, but with enor- 

 mous swamps and delta, 

 overgrown with rank vege- 

 tation far surpassing in 

 luxuriance anything we 

 know at the present day. 

 In the midst of such swamps 

 there would evidently occur 

 spots of great extent, the 

 waters of which, for the 

 reasons already mentioned, would never be contaminated with sedi- 

 ment, as at (a) fig. 13. Here for untold ages pure carbonaceous matter 

 would accumulate undisturbed. In the course of time the surround- 

 ing portions of the swamps (b) where the mud is detained would rise 

 by deposit of sediment, while the peat swamp (a) would remain as a 

 sunken country, such as exist now in the swamps of the Mississippi. 

 Finally, at uncertain intervals, a more than usually large freshet, or 

 perhaps some change in the level of the land, would deluge the swamp 

 with mud and bury the peat. Gradually the vegetation would re- 

 turn, and the former condition of things be restored, to pass again 

 through the same changes. We have but one other supposition to 

 make, viz : that the whole river swamp and delta were gradually sub- 

 siding during the whole carboniferous period. This is by no means 

 a violent supposition, but one which we have a right in this case to 

 make for two good reasons : 1st. We have the best evidence that 

 many of the large deltas of the present day are thus subsiding. This 

 is proved in the case of the Mississippi delta by cypress stumps in situ 

 below the level of the sea. 2d. The coal strata themselves give indu- 

 bitable evidence of gradual subsidence during the period of their de- 

 posit. The character of these strata and their fossils shows that they 

 were deposited in shallow water, but their enormous thickness (nearly 

 three miles in Nova Scotia) renders this clearly impossible^ unless we 

 suppose such subsidence ; for, if the bottom was stationary, it must 

 have been three miles below the surface of the water when the lowest 

 stratum was deposited. Now, if such subsidence went on constantly, 

 but slowly, so that, under ordinary circumstances, the delta could be 

 maintained by deposit from the river^ but at uncertain intervals, more 

 rapidly than the river could build up, so that the sea would again 

 usurp possession and make its deposit of limestone, and again more 

 slowly^ so that the area might again be reclaimed by the river, and 

 become a peat swamp, and so on alternately, we should easily, with- 

 out any violent hypothesis, account for all the phenomena of a coal 

 basin. 



It will be observed that by this hypothesis the area of a coal basin 

 has, indeed, been successively above and below the sea-surface, but 

 not by successive upheaval and depression, as it has been supposed 

 necessary on the peat bog theory, but by the contention, with various 



