218 Geological Society. 



diameter var)'ing from two to six inches. Their long slender pro- 

 cesses, covered with a pellicle of carbonaceous matter, form an en- 

 tangled mass, and traverse the beds in every direction, vertically, 

 horizontally, and obliquely ; but Mr. Logan has never been able to 

 trace them to their termination, though he has followed single pro- 

 cesses for considerable distances. Portions of the stem of the Stig- 

 maria are found in other parts of the coal measures, but it is only 

 in the underclay that the fibrous processes are attached to the stem 

 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 pha?nomena, 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 tM'o 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 



