COAL PLANTS. 



COAL PLANTS. 



30 



single individuals. Of flowers there is no trace that can be satis- 

 factorily identified ; for Antholithes Pitcairnue, the most perfect that 

 has yet been discovered, is altogether of a doubtful nature. 



It will at once be seen that the investigation of plants in such a 

 condition is very much more difficult than that which is presented by 

 a recent Flora. The nature of the inquiries, and the difficulties 

 presented to an investigator of the plants of the coal formation, have 

 been well described by Dr. Joseph D. Hooker, in a paper ' On the 

 Vegetation of the Carboniferous Period as compared with that of the 

 Present Day," in the second volume of the ' Memoirs of the 

 Geological Survey of Great Britain.' His remarks are arranged under 

 four heads the nature of the plants, their geographical distribution, 

 relation to the soil, and the reciprocal influence of the whole mass of 

 the vegetation on the surface it covers : 



" 1. Of the mutual affinities of the groups under which the 

 majority of the genera of coal-plants arrange themselves little more 

 can be said but that the ferns occupy the lower end of the series and 

 the C'onifera possibly the highest ; but this depends upon the view 

 taken of the affinities of SigiUaria, the most important group. 

 These are classed by some observers amongst Ferns, by others with 

 Cunifrra, another considers them as linking these two widely-different 

 families, whilst a fourth ranks them much higher than either. The 

 affinities of another group, Calamitet, are entirely unascertained. 

 Of the whole amount of species in each no conjecture can be formed, 

 or any but a very rough one, of the number into which those with 

 which we are familiar as of common occurrence should be divided. 

 The Ferns far outnumber probably all the others ; but this again 

 materially depends on the value according to the markings of 

 Xi-/i'//<triie, as means of dividing that genus; for if the slight 

 differences hitherto employed be insisted upon, the number of the 

 so-called species may be unlimitedly increased. 



" 2. With regard to the geographical distribution of the species, &c., 

 it appears that a uniformity once existed in the vegetation through- 

 out the extra-tropical countries of the Northern Hemisphere, to 

 which there is now no parallel ; and this was so whether we consider 

 the coal-plants as representing all the flora of the period, or a part 

 only, consisting of some widely-distributed forms that characterised 

 certain local conditions. Nor is this uniformity less conspicuous in 

 what may be called the vertical distribution ; the fossils in the 

 lowest coal-beds of one field very frequently pervading all the 

 succeeding beds, though so many as thirty may be interposed 

 between the highest and the lowest. 



" 3. Of the relations between the soil and the plants nourished by it, 

 little more is recognisable than that the Sigillarva have been particu- 

 larly abundant on the under clay, which,.judging from the absence of 

 any other fossils but Sigiliaria roots (Stigmaria), seems to have been 

 either in itself unfriendly to vegetation, or so placed (perhaps from 

 being submerged) as to be incapable of supporting any other. The 

 latter is the most probable, because both Sigillarice and their 

 Xtujmnria roots occur in other soils, besides under clay, and are there 

 accompanied by Catamites, Ferns, &c. The Cuniferce again are chiefly 

 found in the sandstone*, and their remains being exceedingly rare in 

 the clays, shales, or ironstones, it may be concluded that they never 

 were associated with the KigiUai-iit and other plants which abound 

 in the coal-seams, but that they flourished in the neighbourhood, and 

 were at times transported to these localities. The quantity of 

 moisture to which these plants were subjected must remain a 

 question so long as some authors insist upon the Sigilfarite being 

 allied to plants now characteristic of deserts, and others to such as 

 are the inhabitants of moist and insular climates. The singular 

 succulent texture and extraordinary size of both the vascular and 

 cellular tissues of many, possibly indicate a great amount of humidity. 

 The question of light and heat involves a yet more important 

 question, some of the coal-plants of the arctic regions being considered 

 identical with those of Britain. How these can have existed in that 

 latitude under the now prevailing distribution of light and heat has 

 not been hitherto explained ; they are too bulky for comparison with 

 any vegetables inhabiting those regions at the present time, and of 

 too lax a tissue to admit of a prolonged withdrawal of the stimulus 

 of light, or of their being subjected to continued frosts. 



"4. The consequence of the existence of the coal-plants has been 

 the formation of coal; but how this operation was conducted is a 

 question yet unsolved. The under-clay or soil upon which the coal 

 rests, and upon which some of the plants grew, seems in general to 

 have Buffered little change thereby, further than what was effected by 

 the intrusion of a vast number of roots throughout the mass. The 

 hales on the other hand are composed of inorganic matter, materially 

 altered by the presence of the vegetable matter they contain. The 

 iron-clays again present a third modification of this mixture of 

 organic and inorganic matter, often occurring in the form of nodules. 

 These nodules seem to be the result of a peculiar action of vegetable 

 matter upon water, charged with soil and a salt of iron ; the iron- 

 stone nodules of existing peat-bogs appearing altogether analogous 

 t-. those of the carboniferous period, whether in form or in chemical 

 ituents. Il.-r- tli'-n the botanist recognises 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 

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

 may have grown amongst them or at a distance, and have been after- 

 wards 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 matters, 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 con- 

 tained 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 innumerable stumps of 

 SiffOloria, 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 scattered 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 Siyillariie roots (Stigmaria) 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 ironstone (or black band) are the last modifications 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 ironstone 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 

 aggregated round bits of vegetable matter (as fern leaves, stems, or 

 cones) which served as nuclei. 



"Now, though each of these points admits of some explanation 

 when taken separately, and some illustration from the action of an 

 existing vegetation on the soil, Ac., it is very difficult to understand 

 their combined operation over so enormous a surface for instance, 

 as one of the American coal-fields and even more to account for 

 their regular recurrence according to some fixed law in every succes- 

 sive coal-seam throughout the whole carboniferous formation." 



Coal-plants may be divided for practical purposes into three classes : 

 1, those of which only wood still containing organic structure has 

 been found ; 2, those which have an obvious analogy with receut 

 plants ; 3, those with which no existing analogy has been traced. 



1. Coal-Planti of which Wood only containing Organic Structure 



hat been found. 



The existence of wood in the coal formation with its texture still 

 preserved, is a discovery of very modern date. Mr. Nicol, of Edin- 

 burgh, claims the credit of having first invented the art of preparing 

 fossil wood so as to show its structure microscopically ; Mr. Withain 

 has investigated the subject extensively, and he has been followed by 

 Messrs. Lindley, Hutton, and others. The result of these inquiries 

 has been, that wood still preserving its texture exists in a mineral 

 state extensively throughout the coal-mines of the north of England ; 

 that it in most cases has a structure analogous to, although not 

 identical with, that of recent coniferous wood ; and that in those 

 cases in which its structure is not coniferous it is unlike that of any 

 existing trees. 



Coniferous wood is known amongst other things by the presence of 

 small discs upon the sides of its woody tubes ; differences in the 

 arrangements of these discs have given rise to the formation of the 

 genera Pence and Pinites, to one or other of which all the coniferous 

 coal-wood seems referrible. Mr. Nicol believes that it may all be 

 referred to either the existing genera Pinus or Araucaria. Specimens 

 of this kind of wood occur sometimes of considerable size. A trunk 

 of Pinitet Brandlingi has been found 72 feet long, and another of 

 Pinitet Withami 36 feet long. 



The wood to which Mr. Witham's genus Anabathra apparently 

 belongs is known by its longitudinal section representing tubes 

 marked by parallel transverse lines resembling the steps of a ladder. 

 This is very uncommon, and is stated by Messrs. Lindley and Hutton 

 to belong to the genus Stigmaria, mentioned hereafter. 



Specimens in ironstone also have occurred of the wood of the 

 genus Lepidodendron. It consists principally of loose cellular tissue, 

 having near the centre a zone of spiral vessels, connected with the 

 bases of the leaves by arcs of spiral vessels, and having rudiments of 

 wood on the outside of the zone. 



2. Coal-Planti which hare an obviout Analogy with Recent Planti. 



Coniferous plants have but few remains, except wood, by 

 which they can be recognised. A cone of Pinvs anthracina has been 

 met with, and there is reason to believe that certain stems called 

 Botkrodendron, having numerous minute dots upon their surface, and 

 deep circular oblique concavities 4 or 5 inches across, at intervals of 

 10 or 11 inches, are also remains of trees of this description. It is 

 probable moreover that some of the fossils referred to the genua 



