Chapter 6

[BO]“Journal of the Geological Society,” London, 1859.

[BP]Ibid., 1877.

The first point to which I shall refer, and which will lead to the other matters to be discussed, is the relation of the characteristicLepidodendronof the Devonian of eastern America,L. Gaspianum, toL. nothumof Unger and of Salter. At the time when I described this species I had not access to Scottish specimens ofLepidodendronfrom the Devonian, but these had been well figured and described by Salter, and had been identified withL. nothumof Unger, a species evidently distinct from mine, as was also that figured and described by Salter, whether identical or not with Unger’s species. In 1870 I had for the first time an opportunity to study Scottish specimens in the collection of Mr. Peach; and on the evidence thus afforded I stated confidently that these specimens represented a species distinct fromL. Gaspianum, perhaps even generically so.[BQ]It differs fromL. Gaspianumin its habit of growth by developing small lateral branches instead of bifurcating, and in its foliage by the absence or obsolete character of the leaf-bases and the closely placed and somewhat appressed leaves. If an appearance of swelling at the end of a lateral branch in one specimen indicates a strobile of fructification, then its fruit was not dissimilar from that of the Canadian species in its position and general form, though it may have differed in details. On these grounds I declined to identify the Scottish species withL. Gaspianum. The Lepidodendron from the Devonian of Belgium described and figured by Crepin,[BR]has a better claim to such identification, and would seem to prove that this species existed in Europe as well as in America. I also saw in Mr. Peach’s collection in 1870 some fragments which seemed to me distinct from Salter’s species, and possibly belonging toL. Gaspianum.[BS]

[BQ]“Report on Devonian Plants of Canada,” 1871.

[BR]“Observations sur quelques Plantes Fossiles des dépôts Devoniens.”

[BS]“Proceedings of the Geological Society of London,” March, 1871.

In the earliest description ofPsilophytonI recognised its probable generic affinity with Miller’s “dichotomous plants,” with Salter’s “rootlets,” and with Goeppert’sHaliserites Dechenianus, and stated that I had “little doubt that materials exist in the Old Red Sandstone of Scotland for the reconstruction of at least one species of this genus.” Since, however, Miller’s plants had been referred to coniferous roots, and to fucoids, and Goeppert’sHaliseriteswas a name applicable only to fucoids, and since the structure and fruit of my plants placed them near to Lycopods, I was under the necessity of giving them a special generic name, nor could I with certainty affirm their specific identity with any European species. The comparison of the Scottish specimens with woody rootlets, though incorrect, is in one respect creditable to the acumen of Salter, as in almost any state of preservation an experienced eye can readily perceive that branchlets ofPsilophytonmust have been woody ratherthan herbaceous, and their appearance is quite different from that of any true Algæ.

The type ofPsilophytonis myP. princeps, of which the whole of the parts and structures are well known, the entire plant being furnished in abundance and in situ in the rich plant-beds of Gaspé. A second species,P. robustius, has also afforded well-characterised fructification.P. elegans, whose fruit appears as “oval scales,” no doubt bore sac-like spore-cases resembling those of the other species, but in a different position, and perfectly flattened in the specimens procured. The only other Canadian species,P. glabrum, being somewhat different in appearance from the others, and not having afforded any fructification, must be regarded as uncertain.

The generic characters of the first three species may be stated as follows:

Stems dichotomous, with rudimentary subulate leaves, sometimes obsolete in terminal branchlets and fertile branches; and in decorticated specimens represented only by punctiform scars. Young branches circinate. Rhizomata cylindrical, with circular root-areoles. Internal structure of stem, an axis of scalariform vessels enclosed in a sheath of imperfect woody tissue and covered with a cellular bark more dense externally. Fruit, naked sac-like spore-cases, in pairs or clusters, terminal or lateral.

The Scottish specimens conform to these characters in so far as they are known, but not having as yet afforded fruit or internal structure, they cannot be specifically determined with certainty. More complete specimens should be carefully searched for, and will no doubt be found.

In Belgium, M. Crepin has described a new species from the Upper Devonian of Condroz under the nameP. Condrusianum(1875). It wants, however, some of the more important characters of the genus, and differs in having a pinnate ramification, giving it the aspect of a fern. In a later paper (1876) the author considers this species distinct fromPsilophyton, and proposes for it a new generic nameRhacophyton.

The characters given by Mr. Carruthers, in his paper of 1873, for the speciesP. Dechenianum, are very few and general: “Lower branches short and frequently branching, giving the plant an oblong circumscription.” Yet even these characters do not apply, so far as known, to Miller’s fucoids or Salter’s rootlets or Goeppert’sHaliserites. They merely express the peculiar mode of branching already referred to in Salter’sLepidodendron nothum. The identification of the former plants with theLepidodendronandLycopodites, indeed,rests only on mere juxtaposition of fragments, and on the slight resemblance of the decorticated ends of the branches of the latter plants toPsilophyton. It is contradicted by the obtuse ends of the branches of theLepidodendronandLycopodites, and by the apparently strobilaceous termination of some of them.

Salter’s description of hisLepidodendron nothumis quite definite, and accords with specimens placed in my hands by Mr. Peach: “Stems half an inch broad, tapering little, branches short; set on at an acute angle, blunt at their terminations. Leaves in seven to ten rows, very short, not a line long, and rather spreading than closely imbricate.” These characters, however, in so far as they go, are rather those of the genusLycopoditesthan ofLepidodendron, from which this plant differs in wanting any distinct leaf-bases, and in its short, crowded leaves. It is to be observed that they apply also to Salter’sLycopodites Milleri, and that the difference of the foliage of that species may be a result merely of different state of preservation. For these reasons I am disposed to place these two supposed species together, and to retain for the species the nameLycopodites Milleri. It may be characterised by the description above given, with merely the modification that the leaves are sometimes nearly one-third of an inch long and secund (Fig. 17,supra, lower figure).

Decorticated branches of the above species may no doubt be mistaken forPsilophyton, but are nevertheless quite distinct from it, and the slender branching dichotomous stems, with terminations which, as Miller graphically states, are “like the tendrils of a pea,” are too characteristic to be easily mistaken, even when neither fruit nor leaves appear. With reference to fructification, the form ofL. Millerirenders it certain that it must have borne strobiles at the ends of its branchlets, or some substitute for these, and not naked spore-cases like those ofPsilophyton.

The remarkable fragment communicated by Sir Philip Egerton to Mr. Carruthers,[BT]belongs to a third group, and has, I think, been quite misunderstood. I am enabled to make this statement with some confidence, from the fact that the reverse or counterpart of Sir Philip’s specimen was in the collection of Sir Wyville Thomson, and was placed by him in my hands in 1870. It was noticed in my paper on “New Devonian Plants,” in the “Journal of the Geological Society of London,” and referred to my genusPtilophyton, as stated above under Section II.,page 86et seq.

[BT]“Journal of Botany,” 1873.

Mr. Salter described, in 1857,[BU]fragments of fossil wood from the Scottish Devonian, having the structure of Dadoxylon, though very imperfectly preserved; and Prof. McNab has proposed[BV]the generic namePalæopitysfor another specimen of coniferous wood collected by Hugh Miller, and referred to by him in the “Testimony of the Rocks.” From Prof. McNab’s description, I should infer that this wood may, after all, be generically identical with the woods usually referred to Dadoxylon of Unger (Araucarioxylonof Kraus). The description, however, does not mention the number and disposition of the rows of pores, nor the structure of the medullary rays, and I have not been able to obtain access to the specimens themselves. I have described five species of Dadoxylon from the Middle and Upper Erian of America, all quite distinct from the Lower Carboniferous species. There is also one species of an allied genus, Ormoxylon. All these have been carefully figured, and it is much to be desired that the Scottish specimens should be re-examined and compared with them.

[BU]“Journal of the London Geological Society.”

[BV]“Transactions of the Edinburgh Botanical Society,” 1870.

Messrs. Jack and Etheridge have given an excellent summary of our present knowledge of the Devonian flora of Scotland, in the Journal of the London Geological Society (1877). From this it would appear that species referable to the generaCalamities,Lepidodendron,Lycopodites,Psilophyton,Arthrostigma,Archæopteris,Caulopteris,Palæopitys,Araucarioxylon, andStigmariahave been recognised.

The plants described by these gentlemen from the Old Red Sandstone of Callender, I should suppose, from their figures and descriptions, to belong to the genusArthrostigma, rather than to Psilophyton. I do not attach any importance to the suggestions referred to by them, that the apparent leaves may be leaf-bases. Long leaf-bases, like those characteristic ofLepidofloyos, do not occur in these humbler plants of the Devonian. The stems with delicate “horizontal processes” to which they refer may belong toPtilophytonor toPinnularia.

In conclusion, I need scarcely say that I do not share in the doubts expressed by some British palæontologists as to the distinctness of the Devonian and Carboniferous floras. In eastern America, where these formations are mutually unconformable, there is, of course, less room for doubt than in Ireland and in western America, where they are stratigraphically continuous. Still, in passingfrom the one to the other, the species are for the most part different, and new generic forms are met with, and, as I have elsewhere shown, the physical conditions of the two periods were essentially different.[BW]

[BW]“Reports on Devonian Plants and Lower Carboniferous Plants of Canada.”

It is, however, to be observed that since—as Stur and others have shown—Calamities radiatus, and other forms distinctively Devonian in America, occur in Europe in the Lower Carboniferous, it is not unlikely that the Devonian flora, like that of the Tertiary, appeared earlier in America. It is also probable, as I have shown in the “Reports” already referred to, that it appeared earlier in the Arctic than in the temperate zone. Hence an Arctic or American flora, really Devonian, may readily be mistaken for Lower Carboniferous by a botanist basing his calculations on the fossils of temperate Europe. Even in America itself, it would appear, from recent discoveries in Virginia and Ohio, that certain Devonian forms lingered longer in those regions than farther to the northeast;[BX]and it would not be surprising if similar plants occurred in later beds in Devonshire or in the south of Europe than in Scotland. Still, these facts, properly understood, do not invalidate the evidence of fossil plants as to geological age, though errors arising from the neglect of them are still current.

[BX]Andrews, “Palæontology of Ohio,” vol. ii.; Meek, “Fossil Plants from Western Virginia,” Philosophical Society, Washington, 1875.

VI.—Geological Relations of some Plant-bearing Beds of Eastern Canada.(“Report on Erian Plants,” 1871.)

The Gaspé sandstones have been fully described by Sir W. E. Logan, in his “Report on the Geology of Canada,” 1863. He there assigns to them a thickness of seven thousand and thirty-six feet, and shows that they rest conformably on the Upper Silurian limestones of the Lower Helderberg group (Ludlow), and are in their turn overlaid unconformably by the conglomerates which form the base of the Carboniferous rocks of New Brunswick. I shall add here merely a few remarks on points in their physical character connected with the occurrence of plants in them.

Prototaxites(Nematophyton)Loganiand other characteristic Lower Erian plants occur in the base of the sandstones at Little Gaspé. This fact, along with the occurrence, as stated in my paper of 1863, of rhizomes ofPsilophytonpreserving their scalariformstructure, in the upper part of the marine Upper Silurian limestones,[BY]proves the flora of the Devonian rocks to have had its beginning at least in the previous geological period, and to characterise the lower as well as the upper beds of the Devonian series. In this connection I may state that, from their marine fossils, as well as their stratigraphical arrangement, Sir W. E. Logan and Mr. Billings regard the lower portions of the Gaspé sandstones as the equivalents of the Oriskany sandstone of New York. On the other hand, the great thickness of this formation, the absence of Lower Devonian fossils from its upper part, and the resemblance of the upper beds to those of the newer members of the Devonian elsewhere, render it probable that the Gaspé sandstones, though deficient in the calcareous members of the system, seen farther to the westward, represent the whole of the Devonian period.

[BY]The marine fossils of these beds have been determined by Mr. Billings. They are Upper Silurian, with an intermixture of Lower Devonian in the upper part. Fragments ofNematophytonoccur in beds of the same age in the Bay des Chaleurs, at Cape Bon Ami.

The Gaspé sandstones, as their name imports, are predominantly arenaceous, and often coarsely so, the sandstones being frequently composed of large grains and studded with quartz-pebbles. Grey and buff are prevalent colours, but red beds also occur, more especially in the upper portion. There are also interstratified shaly beds, sometimes occurring in groups of considerable thickness, and associated with fine-grained and laminated argillaceous sandstone, the whole having in many places the lithological aspect of the coal-measures. At one place, near the middle of the series, there is a bed of coal from one inch to three inches in thickness, associated with highly bituminous shales abounding in remains of plants, and also containing fragments of crustaceans and fishes (Pterygotus,Ctenacanthus ?&c). The beds connected with this coal are grey sandstones and grey and dark shales, much resembling those of the ordinary coal formation. The coal is shining and laminated, and both its roof and floor consist of laminated bituminous shale with fragments ofPsilophyton. It has no true under-clay, and has been, I believe, a peaty mass of rhizomes ofPsilophyton. It occurs near Tar Point, on the south side of Gaspé Bay, a place so named from the occurrence of a thick dyke of trap holding petroleum in its cavities. The coal is of considerable horizontal extent, as in its line of strike a similar bed has been discovered on the Douglas River, about four miles distant. It has not been recognised on the northside of the bay, though we find there beds, probably on very nearly the same horizon, holdingPsilophytonin situ.

As an illustration of one of the groups of shaly beds, and of the occurrence of roots ofPsilophyton, I may give the following sectional list of beds seen near “Watering Brook,” on the north shore of the bay. The order is descending:

Groups of beds similar to the above, but frequently much more rich in fossils, occur in many parts of the section, and evidently include fossil soils of the nature of under-clays, on which little else appears to have grown than a dense herbage ofPsilophyton, along with plants of the genusArthrostigma.

In addition to these shaly groups, there are numerous examples of beds of shale of small thickness included in coarse sandstones, and these beds often occur in detached fragments, as if the remnants of more continuous layers partially removed by currents of water. It is deserving of notice that nearly all these patches of shale are interlaced with roots or stems ofPsilophyton, which sometimes project beyond their limits into the sandstone, as if the vegetable fibres had preserved the clay from removal. In short, these lines of patches of shale seem to be remnants of soils on whichPsilophytonhas flourished abundantly, and which have been partially swept away by the currents which deposited the sand. Some of the smaller patches may even be fragments of tough swamp soils interwoven with roots, drifted by the agency of the waves or possibly by ice; such masses are often moved in this way on the borders of modern swamps on the sea-coast.

The only remaining point connected with local geology to which I shall allude is the admirable facilities afforded by the Gaspé coast both for ascertaining the true geological relations of the beds, and for studying the Devonian plants, as distinctly exposed on large surfacesof rock. On the coast of the river St. Lawrence, at Cape Rozier and its vicinity, the Lower Silurian rocks of the Quebec group are well exposed, and are overlaid unconformably by the massive Upper Silurian limestones of Cape Gaspé, which rise into cliffs six hundred feet in height, and can be seen filled with their characteristic fossils on both sides of the cape. Resting upon these, and dipping at high angles toward Gaspé Bay, are the Devonian sandstones, which are exposed in rugged cliffs slightly oblique to their line of strike, along a coast-line of ten miles in length, to the head of the bay. On the opposite side of the bay they reappear; and, thrown into slight undulations by three anticlinal curves, occupy a line of coast fifteen miles in length. The perfect manner in which the plant-bearing beds are exposed in these fine natural sections may serve to account for the completeness with which the forms and habits of growth of the more abundant species can be described.

In the Bay des Chaleurs, similar rocks exist with some local variations. In the vicinity of Campbellton are calcareous and magnesian breccia or agglomerate, hard shales, conglomerates and sandstones of Lower Devonian age. The agglomerate and lower shales contain abundant remains of fishes of the generaCephalaspis,Coccosteus,Ctenacanthus, andHomacanthus, and also fragments ofPterygotus. The shales and sandstones abound in remains ofPsilophyton, with which areNematophyton,Arthrostigma, andLeptophleumof the same species found in the Lower Devonian of Gaspé Bay. These beds near Campbellton dip to the northward, and the Restigouche River here occupies a synclinal, for on the opposite side, at Bordeaux Quarry, there are thick beds of grey sandstone dipping to the southward, and containing large silicified trunks of Prototaxites, in addition toPsilophyton. These beds are all undoubtedly Lower Erian, but farther to the eastward, on the north side of the river, there are newer and overlying strata. These are best seen at Scaumenac Bay, opposite Dalhousie, between Cape Florissant and Maguacha Point, where they consist of laminated and fine-grained sandstone, with shales of grey colours, but holding some reddish beds at top, and overlaid unconformably by a great thickness of Lower Carboniferous red conglomerate and sandstone. In these beds numerous fossil fishes have been found, among which Mr. Whiteaves recognises species ofPterichthys,Glyptolepis,Cheirolepis, &c. With these are found somewhat plentifully four species of fossil ferns, all of Upper Erian types, of which one is peculiar to this locality; but the others are found in the Upper Erian of Perry, in Maine, or in the Cat skill group of New York.

In order that distinct notions may be conveyed as to the geological horizons of the species, I may state that the typical Devonian or Erian series of Canada and New York may be divided in descending order into—1. The Chemung group, including the Chemung and Portage sandstones and shales. 2. The Hamilton group, including the Genesee, Hamilton, and Marcellus shales. 3. The Corniferous limestone and its associated beds. 4 The Oriskany sandstone. As the Corniferous limestone, which is the equivalent of the Lower Carboniferous limestone in the Carboniferous period, is marine, and affords scarcely any plants, we may, as is usually done for like purposes in the Carboniferous, group it with the Oriskany under the name Lower Erian. The Hamilton rocks will then be Middle Erian, and the Chemung group Upper Erian. In the present state of our knowledge, the series may be co-ordinated with the rocks of Gaspé, New Brunswick, and Maine, as in the following table:

It may be proper, before closing this note, to state the reasons which have induced me to suggest in the following pages the use of the term “Erian,” as equivalent to “Devonian,” for the great system of formations intervening between the Upper Silurian and the Lower Carboniferous in America. I have been induced to adopt this course by the following considerations: 1. The great area ofundisturbed and unaltered rocks of this age, including a thickness in some places of eighteen thousand feet, and extending from east to west through the Northern States of the Union and western Canada for nearly seven hundred miles, while it spreads from north to south from the northern part of Michigan far into the Middle States, is undoubtedly the most important Devonian area now known to geologists. 2. This area has been taken by all American geologists as their typical Devonian region. It is rich in fossils, and these have been thoroughly studied and admirably illustrated by the New York and Canadian Surveys. 3. The rocks of this area surround the basin of Lake Erie, and were named, in the original reports of the New York Survey, the “Erie Division” 4. Great difficulties have been experienced in the classification of the European Devonian, and the uncertainties thus arising have tended to throw doubt on the results obtained in America in circumstances in which such difficulties do not occur.

These reasons are, I think, sufficient to warrant me in holding the greatErie Divisionof the New York geologists as the typical representative of the rocks deposited between the close of the Upper Silurian and the beginning of the Carboniferous period, and to use the term Erian as the designation of this great series of deposits as developed in America, in so far at least as their flora is concerned. In doing so, I do not wish to introduce a new name merely for the sake of novelty; but I hope to keep before the minds of geologists the caution that they should not measure the Erian formations of America, or the fossils which they contain, by the comparatively depauperated representatives of this portion of the geological scale in the Devonian of western Europe.

VII.—On the Relations of the so-called “Ursa Stage” of Bear Islandwith the Palæozoic Flora of North America.

The following note is a verbatim copy of that published by me in 1873, and the accuracy of which has now been vindicated by the recent observations of Nathorst:

The plants catalogued by Dr. Heer, and characterising what he calls the “Ursa Stage,” are in part representatives of those of the American flora which I have described as the “Lower Carboniferous Coal-Measures” (Subcarboniferous of Dana), and whose characteristic species, as developed in Nova Scotia, I noticed in the “Journal of the Geological Society” in 1858 (vol. xv.). Dr. Heer’s list, however, includes some Upper Devonian forms; and I would suggest thateither the plants of two distinct beds, one Lower Carboniferous and the other Upper Devonian, have been near to or in contact with each other and have been intermixed, or else that in this high northern latitude, in which (for reasons stated in my “Report on the Devonian Flora”[BZ]) I believe the Devonian plants to have originated, there was an actual intermixture of the two floras. In America, at the base of the Carboniferous of Ohio, a transition of this kind seems to occur; but elsewhere in northeastern America the Lower Carboniferous plants are usually unmixed with the Devonian.

[BZ]“Geological Survey of Canada,” 1871.

Dr. Heer, however, proceeds to identify these plants with those of the American Chemung, and even with those of the Middle Devonian of New Brunswick, as described by me—a conclusion from which I must altogether dissent, inasmuch as the latter belong to beds which were disturbed and partially metamorphosed before the deposition of the lowest Carboniferous or “Subcarboniferous” beds.

Dr. Heer’s error seems to have arisen from want of acquaintance with the rich flora of the Middle Devonian, which, while differing in species, has much resemblance in its general facies, and especially in its richness in ferns, to that of the coal-formation.

To geologists acquainted with the stratigraphy and the accompanying animal fossils, Dr. Heer’s conclusions will of course appear untenable; but they may regard them as invalidating the evidence of fossil plants; and for this reason it is, I think, desirable to give publicity to the above statements.

I consider the British equivalent of the lower coal-measures of eastern America to be the lower limestone shales, theTweedian groupof Mr. Tate (1858), but which have sometimes been called the “Calciferous Sandstone” (a name preoccupied for a Cambrian group in America). This group does not constitute “beds of passage” to the Devonian, more especially in eastern America, where the lower coal-formation rests unconformably on the Devonian, and is broadly distinguished by its fossils.

The above notes would not have been extended to so great length, but for the importance of the Erian flora as the precursor of that of the Carboniferous, and the small amount of attention hitherto given to it by geologists and botanists.

CHAPTER IV.

THE CARBONIFEROUS FLORA—CULMINATION OF THE ACROGENS—FORMATION OF COAL.

Ascendingfrom the Erian to the Carboniferous system, so called because it contains the greatest deposits of anthracite and bituminous coal, we are still within the limits of the Palæozoic period. We are still within the reign of the gigantic club-mosses, cordaites, and taxine pines. At the close of the Erian there had been over the whole northern hemisphere great changes of level, accompanied by active volcanic phenomena, and under these influences the land flora seems to have much diminished. At length all the old Erian species had become extinct, and their place was supplied by a meagre group of lycopods, ferns, and pines of different species from those of the preceding Erian. This is the flora of the Lower Carboniferous series, the Tweedian of England, the Horton series of Nova Scotia, the lower coal-measures of Virginia, the culm of Germany. But the land again subsided, and the period of the marine limestone of the Lower Carboniferous was introduced. In this the older flora disappeared, and when the land emerged we find it covered with the rich flora of the coal-formation proper, in which the great tribes of the lycopods and cordaites attained their maxima, and the ferns were continued as before, though under new generic and specific forms.

Fig. 32.—Foliage from the coal-formation,a,Alethopteris lonchitica, fern (Moose River).b,Sphenophyllum Schlotheimii(Pietou).c,Lepidodendron binerve(Sydney),d,Asterophyllites foliosa(_?_) (Sydney).e,Cordaites(Joggins).f,Neuropteris rarinervis, fern (Sydney).g,Odontopteris subcuneata, fern (Sydney).

There is something very striking in this succession of a new plant world without any material advance. It is like passing in the modern world from one district to another, in which we see the same forms of life, only represented by distinct though allied species. Thus, when the voyager crosses the Atlantic from Europe to America, he meets with pines, oaks, birches, poplars, and beeches of the same genera with those he had left behind; but the species are distinct. It is something like this that meets us in our ascent into the Carboniferous world of plants. Yet we know that this is a succession in time, that all our old Erian friends are dead and buried long ago, and that these are new forms lately introduced (Fig. 32).

Conveying ourselves, then, in imagination forward to the time when our greatest accumulations of coal were formed, and fancying that we are introduced to the American or European continent of that period, we find ourselves in a new and strange world. In the Devonian age, and even in the succeeding Lower Carboniferous, there was in the interior of America a wide inland sea, with forest belts clinging to its sides or clothing its islands. But in the coal period this inland sea had givenplace to vast swampy flats, and which, instead of the oil-bearing shales of the Erian, were destined to produce those immense and wide-spread accumulations of vegetable matter which constitute our present beds of bituminous and anthracite coal. The atmosphere of these great swamps is moist and warm. Their vegetation is most exuberant, but of forms unfamiliar to modern eyes, and they swarm with insects, millipedes, and scorpions, and with batrachian reptiles large and small, among which we look in vain for representatives of the birds and beasts of the present day.

Fig. 33.—Sigillariæ, restored. A,Sigillaria Brownii. B,Sigillaria elegans.

Fig. 34.—Sigillaria Lorwayana, Dawson.a, Zones of fruit-scars.b, Leaf-scar enlarged,c, Fruit-scar enlarged. Seeappended note.

Fig. 35.—Stem ofSigillaria Brownii. reduced. Natural size.

Fig. 36.—Two ribs ofSigillaria Brownii.

Fig. 37.—Portion of lower part of stem ofS. Brownii. Natural size.

Prominent among the more gigantic trees of these swampy forests are those known to us asSigillariæ(Fig. 33). They have tall, pillar-like trunks, often several feet in diameter, ribbed like fluted columns, but in the reverse way, and spreading at the top into a few thick branches, which are clothed with long, grass-like leaves. They resemble in some respects the Lepidodendra of the Erian age, but are more massive, with ribbed instead of scaly trunks, and longer leaves. If we approach one of them more closely, we are struck with the regular ribs of its trunk, dotted with rows of scars of fallen leaves, from which it receives its nameSigillaria, or seal-tree (Figs.34-37). If we cut into its stem, we find that, instead ofthe thin bark and firm wood with which we are familiar in our modern trees, it has a hard external rind, then a great thickness of cellular matter with rope-like bands of fibres, constituting an inner bark, while in the centre is a firm, woody axis of comparatively small diameter, and somewhat intermediate in its structures between that of the Lepidodendra and those of the cycads and the taxine conifers. Thus a great stem, five feet in diameter, may consist principally of cellular and bast fibres with very little true woody matter. The roots of this tree areperhaps its most singular feature. They usually start from the stem in four main branches, then regularly bifurcate several times, and then run out into great cylindrical cables, running for a long distance, and evidently intended to anchor the plant firmly in a soft and oozy soil. They were furnished with long, cylindrical rootlets placed regularly in a spiral manner, and so articulated that when they dropped off they left regular rounded scars. They are, in short, theStigmariæ, which we have already met with in the Erian (Figs.38,39). InFig. 33I have endeavoured to restore these strange trees. It is not wonderful that such plants have caused much botanical controversy. It was long before botanists could be convinced thattheir roots are properly roots at all, and not stems of some aquatic plant. Then the structure of their stems is most puzzling, and their fruit is an enigma, for while some have found connected with them cones supposed to resemble those of lycopods, others attribute to them fruits like those of yew-trees. For years I have been myself gathering materials from the rich coal-formation deposits of Nova Scotia in aid of the solution of these questions, and in the mean time Dr. Williamson, of Manchester, and Renault and other botanists in France, have been amassing and studying stores of specimens, and it is still uncertain who may finally be the fortunate discoverer to set all controversies at rest. My present belief is, that the true solution consists in the fact that there are many kinds ofSigillariæ. While in the modern forestsof America and Europe the species of any of our ordinary trees, as oaks, birches, or maples, may almost be counted on one’s fingers, Schimper in his vegetable palæontology enumerates about eighty species of CarboniferousSigillariæ; and while on the one hand many of these are so imperfectly known that they may be regarded as uncertain, on the other hand many species must yet remain to be discovered.[CA]Now, in so vast a number of species there must be a great range of organisation, and, indeed, it has already been attempted to subdivide them into several generic groups. The present state of the question appears to me to be this, that in theseSigillariæwe have a group divisible into several forms, some of which will eventually be classed with the Lepidodendra as lycopods, while others will be found to be naked-seeded phænogams, allied to the pines and cycads, and to a remarkable group of trees known asCordaites, which we must shortly notice.

[CA]In a recent memoir (Berlin, 1887) Stur has raised the number of species in one subdivision of theSigillariæ(theFavulariæ) to forty-seven!

Fig. 38.—Stigmariaroot, seen from above, showing its regular divisions.From "Acadian Geology".

Fig. 39.—Portion of bark ofStigmaria, showing scars of attachment of rootlets.

Before considering other forms of Carboniferous vegetation, let us glance at the accumulation of coal, and the agency of the forests ofSigillariætherein. Let us imagine, in the first instance, such trees as those represented in the figures, growing thickly together over vast swampy flats, with quantities of undergrowth of ferns and other plants beneath their shade, and accumulating from age to age in a moist soil and climate a vast thickness of vegetable mould and trunks of trees, and spores and spore-cases, and we have the conditions necessary for the growth of coal. Many years ago it was observed by Sir William Logan that in the coal-field of South Wales it was the rule with rare exceptions that, under every bed of coal, there is a bed of clay filled with roots of theStigmaria, already referred to as the root ofSigillaria. This discoveryhas since been extended to all the coal-fields of Europe and America, and it is a perfectly conclusive fact as regards the origin of coal. Each of these “under-clays,” as they are called, must, in fact, have been a soil on which grew, in the first instance, Sigillariæ and other trees having stigmaria-roots. Thus, the growth of a forest ofSigillariæwas the first step toward the accumulation of a bed of coal. More than this, in some of the coarser and more impure coals, where there has been sufficient earthy matter to separate and preserve impressions of vegetable forms, we can see that the mass of the coal is made up of flattenedSigillariæ, mixed with vegetabledébrisof all kinds, including sometimes vast quantities of lepidodendroid spores, and the microscopic study of the coal gives similar results (Fig. 40). Further, on the surfaces of many coals, and penetrating the shales or sandstones which form their roofs, we find erect stumps of sigillaria and other trees, showing that the accumulation of the coal terminated as it had begun, by a forest-growth. I introduce here a section of a few of the numerous beds of coal exposed in the cliffs of the South Joggins, in Nova Scotia, in illustration of these facts. We can thus see how in the slowly subsiding areas of the coal-swamps successive beds of coal were accumulated, alternating with beds of sandstone and shale (Figs.41,42). For other details of this kind I must refer to papers mentioned in the sequel.

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