AS COMPARED WITH THAT OF THE PRESENT DAY. 397 



Here, then, the botanist recognizes in one coal-seam a vegetable 

 detritus under three distinct phases, and which has been acted upon in 

 each by very different causes. In the under-clay there are roots only :* 

 these permeate its mass, as those of the water-lily and other aquatic 

 plants do the silt at the bottom of still waters. 



The coal is the detritus either of those plants whose roots are pre- 

 served in the under-clay, or of those, together with others which may 

 have grown amongst them, or at a distance, and have been afterwards 

 drifted to the same position. 



Above the coal is the third soil, bearing evidence of the action of a 

 vigorous vegetation ; this is the shale, which has all the appearance of a 

 quiet deposit from water charged with mineral matter, and into which 

 broken pieces of plants have fallen. Here there is so clear a divisional 

 line between the coal and shale, that it is still a disputed point whether 

 the plants, contained in the latter, actually grew upon the former, or 

 were drifted to that position in the fluid which deposited the mineral 

 matter. Amongst the shales are also interspersed, in many cases, in- 

 numerable stumps of Sigillarue, similar to those whose roots occur in 

 the under-clay, and which are themselves found attached to those roots 

 in soils similar to the under-clays, but unconnected with any seam of 

 coal. These stumps are almost universally erect, are uniformly scat- 

 tered over the seams, and otherwise appear to have decidedly grown on 

 the surface of the coal : the shales likewise seem deposited between these 

 stumps. The rarity of Sigillarice roots (Stigmarice) in this position, is 

 probably due to their being incorporated with the coal itself, though 

 they sometimes occur above that mineral and between the layers of 

 shale. The seams of iron-stone (or black-band) are the last modifica- 

 tions of soil by vegetable matter, to which allusion has been made: when 

 these are uniform beds or layers, they may be supposed to be the 

 deposit from water charged with iron and soil which has percolated 

 through the peat, and in so doing absorbed a great deal of vegetable 

 matter. The layers of nodular iron-stone are simple modifications of 

 these, and may be caused by the sedimentary particles contained in the 

 fluid, which, instead of being deposited in a uniform stratum, are aggre- 



* The absence of other parts of plants, and indeed of any plant but the roots of Sigil- 

 larice, in the under-clay, appears a fact of considerable importance. In the first place, it 

 indicates that that soil was in a condition unsuited to the growth of other vegetables (as 

 mentioned above), whose seeds might have accompanied those of that genus on its pre- 

 viously-naked surface. In the second place, this absence of other fossils in the under- 

 clay is opposed to the theory of the drift-origin of the vegetable matter comprising 

 coal ; for there is no interst'ratification of coal with this subjacent deposit, which might 

 have been expected to occur over some portion of an extensive coal-field ; whereas, the 

 gradual decay of these plants, whose roots struck into the under-clay, would produce a 

 uniform bed of peat, perhaps adapted to the growth of those ferns and other plants which 

 are fossilized in the superincumbent shales. 



