CHAP. XXXIV.] FLOORS OF BLUE CLAY. 245 



after an unusually dry season, pits are found burnt 

 into the ground as far as the fire can descend with 

 out coming in 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 horizon 

 tally far and wide, those of one tree interlacing 

 with another, such root-bearino; beds of argillaceous 



o o 



loam must be very analogous to what are called 

 fire-clays, so well known to the geologist as oc 

 curring 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 accumulating in the valley-plain and 

 delta of the Mississippi, and those of the ancient 

 carboniferous rocks. &quot;When, for example, depressions 

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

 before described, certain wooded areas being sub 

 merged, the lower parts of the erect trees become 

 enveloped with sand and mud, the upper portions 



* Sillimau s Journal, Second Series, vol. v. p. 17. January, 

 1848. 



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

 with Stigmaria (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. 



M 3 



