489 
Mr. W. E. Logan has also communicated to usa series of minute 
results of extensive examinations made by himself, and in many 
cases confirmed by Mr. De la Beche, on the character of the beds 
of clay immediately below the coal seams in South Wales, from 
which it appears that immediately beneath every bed of coal in 
that extensive district is a substratum, called the wnderclay, varying 
in thickness from six inches to more than ten feet; and that this un- 
derclay so universally and inseparably accompanies nearly a hundred 
seams of coal throughout South Wales, that the collier seldom finds 
coal where this substratum.is wanting: it is usually a fire-clay, con- 
taining sometimes an admixture of sand, and near Swansea passes 
into a hard, fine-grained, siliceous sandstone. This never-failing sub- 
stratum of the coal is everywhere characterized by the exclusive 
presence of innumerable remains of Stigmaria,ficoides, the stems of 
which are often of great length, and usually parallel to the plane 
of the bed, and more abundant near the top than the bottom of 
the underclay. From each of these stems there proceeds a series 
of very long and narrow leaves, forming an entangled mass, which 
traverses the fire-clay in| every direction and to great distances ; 
fragments of the stems of Stigmaria occur in other parts of the coal 
formation, but in the underclay alone are the long thin leaves at- 
tached to them. In 1818 the Rev. H. Steinhauer published in the 
American Philosophical Transactions, vol. i. p. 273, a similar ac- 
count of the occurrence in the English coal. formation near Brad- 
ford in Yorkshire, of continuous stems and leayes of Stigmarie, dif- 
fering from those lately observed by Mr. Logan only as to the greater 
vertical range to which the leaves extended. Mr. Logan has traced 
them in a vertical direction seven or eight feet from the stem, and . 
more than twenty feet horizontally *, and concludes that it is im- 
possible to account for these phenomena by any theory of drift. 
He further supposes the Stigmaria to be the plant of which fossil 
coal is mainly composed. 
I think we may derive, from the important facts above quoted, a 
the spot it now occupies, has been entertained by DeLuc, Macculloch, 
‘Jameson, Brongniart, Lindley, and other writers, but I have nowhere be- 
fore seen such convincing proofs of this hypothesis as are furnished by the 
facts advanced by Mr. Hawkshaw, Mr. Bowman, and Mr. Logan, taken in 
connexion with one another. 
* Mr. John Craig, of Glasgow, in an excellent paper on the coal forma- 
tion of the West of Scotland read to the British Association at Glasgow, 
1840, remarks that “‘ the Stigmaria jicoides is frequently found in the shales, 
with the leaves attached to the stem and spread out laterally, in a manner 
which never could have occurred had the plant been drifted from a distance. 
The ripple-marks also (he adds), which are observable on almost all the 
shales and laminated sandstones throughout the whole carboniferous forma- 
tion, show that these portions of the coal strata were deposited in very 
shallow water.” 
I learn from Mr. Binney that stems andleaves of Stigmaria abound in 
the beds of clay or fine sand that lie immediately below many beds of coal 
in the district of Manchester. 
