276 
or associated with it. Mr. Logan, however, states, that if such 
specimens exist in other strata, they are not so likely to be ex- 
posed, as those beds are less worked than the underclay. 
In some instances, the Stigmaria, with its processes, is found 
equally abundant in the roof as in the floor of a coal pit, but in 
such cases the roof has been ascertained to be the underclay of an 
immediately overlying bed of coal. 
Mr. Logan then quotes at length, Steinhauer’s account of the 
Stigmaria, as it gives the best explanation he has seen of the ex- 
ternal botanical character of the plant, as well as of its position in 
the beds in which it occurs; the only point in which his experience 
induces him to differ from Steinhauer, being the vertical extent to 
which the fibres range. Mr. Logan has never traced them in that 
direction more than seven or eight feet from the stem, though he 
admits they may have an horizontal range of twenty or more feet. 
(American Phil. Trans. New Series, vol. i. p. 265, 1818.) 
When it is considered, that over so considerable an area as the 
coal field of South Wales, not a seam has been discovered without 
an underclay, abounding in Stigmaria, Mr. Logan says, it is impos- 
sible to avoid the inference, that there is some essential and neces- 
sary connexion between the existence of the Stigmaria and the pro- 
duction of the coal. ‘To account for their unfailing combination by 
drift, seems to him unsatisfactory; but whatever may be the mu- 
tual dependence of the phenomena, he is of opinion, that it affords 
reasonable grounds to suppose, that the Stigmaria ficoides is the 
plant to which may be mainly ascribed the vast stores of fossil fuel. 
In the second part of the paper, Mr. Logan gives an account of 
boulders or rounded fragments of coal, contained in the coal mea- 
sures themselves. 
The thickness of the coal deposit of South Wales, he says is 
equal in the deepest part to 12,000 feet, and that consequently a 
long period must have been required for the accumulation of the 
materials, and that any fact which may assist in ascertaining its 
length, cannot fail to possess some interest. ‘The occurrence of 
these boulders he is of opinion bears upon the subject. 
From a layer of indurated clay, two inches thick, lying on the 
top of a seam of common bituminous coal, and covered by hard sand- 
stone, at Penclawdd on the Bury river, he obtained. in the spring of 
1839, a worn, rounded mass of cannel coal, six inches long, four 
inches wide, and two inches thick. The discovery of this singular 
specimen having excited attention to the subject, it was ascertained 
that in the quarries of the enormous mass of sandstone forming 
Cilfay hill and the Town-hill range from Swansea to the Bury river, 
there occur many irregular conglomerate beds, formed of innume- 
rable pebbles and small boulders of coal, sometimes four inches in 
diameter, mingled with sand and pebbles of ironstone; and there 
have been also found in them small boulders of granite and mica- 
slate. Many impressions, coated with coal, of Sigillarize and other 
plants, occur in the mass ; and the difference of age between this coal 
and that of pebbles, he says, is beautifully illustrated in numerous 
