CHAP. XXXIV.] FLOORS OF BLUE CLAY. 18! 



ations of Messrs. Dickeson and Brown, who state, &quot; that wher 

 the woods are burning 1 , after an unusually dry season, pits ar% 

 found burnt into the ground as far as the fire can descend Avithout 

 coming into contact with water, and scarcely any residuum or 

 earthy matter is left.&quot;* They also state that at the bottom of 

 all the cypress swamps or brakes, there is found a peculiar layer 

 of tenacious blue clay, which forms the foundation, or floor, on 

 which the vegetable matter accumulates. We may conclude, 

 therefore, that as the roots of the cypress penetrate far beneath 

 the soil, and project horizontally far and wide, those of one tree 

 interlacing with another, such root-bearing beds of argillaceous 

 loam must be very analogous to what are called fire-clays, so well 

 known to the geologist as occurring underneath almost every seam 

 of coal in the ancient carboniferous rocks. f 



Other points of analogy might also be indicated between the 

 deposits, whether of organic or inorganic matter, now accumulat 

 ing in the valley-plain and delta of the Mississippi, and those of 

 the ancient carboniferous rocks. When, for example, depressions 

 are suddenly caused, as in the &quot; sunk country&quot; before described, 

 certain wooded areas being submerged, the lower parts of the 

 erect trees become enveloped with sand arid mud, the upper por 

 tions rotting away, as must have happened in the case of the 

 celebrated fossil forest of Dixon-fold, in Lancashire, belonging to 

 the ancient coal-measures. $ In the modern alluvial plain, also, 

 river-sand will be often thrown down, as the Mississippi shifts its 

 course over spaces on which pure vegetable matter had been pre 

 viously accumulating for hundreds or thousands of years, just as 

 we find sandstone sometimes resting immediately upon the old 

 coal-seams ; and, if there be a long succession of downward move 

 ments, the thickness of strata, all formed in shallow water or in 

 swamps, may be indefinitely great. Should the hilly country, 

 moreover, be distant, pebbles will no more be seen in the modern 



* Silliman s Journal, Second Series, vol. v. p. 17, January, ] 848. 



t In my former &quot;Travels,&quot; I have alluded to the fire-stones with Stig- 

 maria (now acknowledged to be the root of Sigillaria), underlying the 

 American coal-seams, as they do those of South Wales, 3000 miles distant. 

 &quot; Travels in North America,&quot; vol. i. p. 62. 



t Proceedings of Geol. Society, 1839. p. 139. 



