300 Mr. MuRcmsoN on the Coal-Jicld of Brora in Sutherlandshire, 



naceous rind has yielded to some external force^ and adopted the form under 

 vvliich alone it is observed in the cla}'ey mass. 



" I should also mention that in these specimens of shale some other vegetable 

 parts are occasionally observed, in the shape of small sub-triangular or in- 

 versely-cordate carbonaceous plates, which seem to have been membra- 

 naceous involucra, connected with the inflorescence of a plant. Those bota- 

 nists who take it for granted that the composite leaves above alluded to are 

 those of ferns, and that the reed-like stems are related to Equisetum, will of 

 course be quite satisfied that the plates in question can have nothing to do 

 with either. I cannot disprove that supposition. 



"With regard to the coal from Sutherlandshire, I am (in corroboration of 

 Mr. Murchison's views respecting itsgeognostic relations) decidedly of opinion 

 that the above reed-like striated plant must have largely contributed towards 

 the formation of that variety of coal. Its original structure is indeed nearly 

 obliterated, but is nevertheless recognizable in the regular striation on the 

 rifts ; and in one specimen I have seen, it is partially exhibited with the 

 same distinctness as in the reeds themselves. 



"^■^The plant under consideration may possibly be referrible to the same na- 

 tural order with Equisetum, but it cannot be considered as a congener of the 

 striated, reed-like fossils which have been comprised under the faulty name of 

 Calamites ; it is distinct from them by its peculiar striation, by the solitary im- 

 pression above the joints, placed in such a manner as to indicate distichous 

 alternating branches, and by the very striking callosity at these joints, which 

 latter character has suggested the generic name of Oncylogonatum. 



" The following phrase may, in the present state of our knowledge of this 

 plant, serve both for its generic and specific character : — Caulis cylindricus 

 articulatus, articulis annulato-gibbosis, gibbis internodiisque longitudinaliter 

 sulcatis, sulcis acutis. — Species : O. carhonarium." 



From the coal-shale composed of the above plant, the next transition is into 

 a purer bituminous substance approaching to jet, which constitutes the great 

 bed of coal. This is from three feet three inches to three feet eight inches in 

 thickness, and is divided nearly in the middle by a thin layer of impure indu- 

 rated shale charged with pyrites, which, if not carefully excluded from the 

 mass, sometimes occasions spontaneous combustion * upon exposure to the 



* Inattention on the part of the workmen in the year 1817, in having a large quantity of this 

 pyritiferoiis shale to accumulate in the pit, occasioned a spontaneous combustion, which was only 

 extinguished by excluding the atmospheric air : the coal-pit was closed in, and remained unworked 

 during four years. I have just learned (p'eb. 1827) that fires have again broken out in the pit. 



