BITUMINOUS AND ANTHRACITIC. 453 



light is to be derived both as to the statical conditions, and the 

 order of the physical events which attended the production of the 

 whole coal-formation. There is an interesting and characteristic 

 difference, in point of composition and structure, between the 

 beds bounding the upper and lower surfaces of every coal-seam. 

 This, though of great significance in its bearings on the theory 

 of the formation of coal, has never been distinctly examined with 

 that view. 



Of the material underlying the coal-beds. The deposit, upon 

 which each seam of coal immediately rests, and which I shall 

 call the floor, is, \vi\\\ a few rare exceptions, wholly distinct in 

 its composition from the roof, or that which reposes directly upon 

 the bed. To Mr. Logan we are indebted for having ascertained 

 the highly important fact, that the floor of every coal-seam in 

 South Wales is composed of a peculiar variety of more or less 

 sandy clay, distinguished by its containing the Stigmaria ficoides. 

 " Portions of the stem of the Stigmaria are found in other parts of 

 the coal-measures, but it is only in the under clay, that the fibrous 

 processes are attached to the stem, or associated with it." * Since 

 the publication of his Observations on the Sfigmaria Beds of 

 South Wales, the same gentleman has extended his researches to 

 the United States, and has found our own coal-seams in Penn- 

 sylvania to be similarly accompanied.! JMr. Lyell has also 

 shown, that this peculiar stratum underlies the bituminous coal- 

 beds at Blossburg, in Pennsylvania. I subsequently visited, with 

 that eminent geologist, the anthracite beds of the Pottsville and 

 the Beaver Meadow basins in Pennsylvania, where we found the 

 Stigmaria bed, in the same position, below those seams. StiU 

 more recently, I have ascertained from my own notes on the geo- 

 logical survey of Pennsylvania, and from those of my brother in 

 relation to Virginia, that this deposit accompanies nearly every 

 coal-bed in the great bituminous region west of the Allegheny 

 mountain. I shall take occasion presenfly, however, to point out 

 some peculiar exceptions to its general prevalence. The theoret- 



* Logan, Proceedings Geological Society, No. 69. 



t Logan, Proceedings Geological Society, for April, 1S42. 



