Chapter III (1)

THE ERIAN OF DEVONIAN FORESTS--ORIGIN OF PETROLEUM--THE AGE OF ACROGENS AND GYMNOSPERMS.

In the last chapter we were occupied with the comparatively few and obscure remains of plants entombed in the oldest geological formations. We now ascend to a higher plane, that of the Erian or Devonian period, in which, for the first time, we find varied and widely distributed forests.

The growth of knowledge with respect to this flora has been somewhat rapid, and it may be interesting to note its principal stages, as an encouragement to the hope that we may yet learn something more satisfactory respecting the older floras we have just discussed.

In Goeppert's memoir on the flora of the Silurian, Devonian, and Lower Carboniferous rocks, published in 1860,[AI] he enumerates twenty species as Silurian, but these are all admitted to be Algæ, and several of them are remains which may be fairly claimed by the zoologists as zoophytes, or trails of worms and mollusks. In the Lower Devonian he knows but six species, five of which are Algæ, and the remaining one a _Sigillaria_, but this is of very doubtful nature. In the Middle Devonian he gives but one species, a land-plant of the genus _Lepidodendron_. In the Upper Devonian the number rises to fifty-seven, of which all but seven are terrestrial plants, representing a large number of the genera occurring in the succeeding Carboniferous system.

[AI] Jena, 1860.

Goeppert does not include in his enumeration the plants from the Devonian of Gaspé, described by the author in 1859,[AJ] having seen only an abstract of the paper at the time of writing his memoir, nor does he appear to have any knowledge of the plants of this age described by Lesquereux in Roger's "Pennsylvania." These might have added ten or twelve species to his list, some of them probably from the Lower Devonian. It is further to be observed that a few additional species had also been recognised by Peach in the Old Red Sandstone of Scotland.

[AJ] "Journal of the Geological Society of London," also "Canadian Naturalist."

But from 1860 to the present time a rich harvest of specimens has been gathered from the Gaspé sandstones, from the shales of southern New Brunswick, from the sandstones of Perry in Maine, and from the wide-spread Erian areas of New York, Pennsylvania, and Ohio. Nearly all these specimens have passed through my hands, and I am now able to catalogue about a hundred species, representing more than thirty genera, and including all the great types of vascular Cryptogams, the Gymnosperms, and even one (still doubtful) Angiosperm. Many new forms have also been described from the Devonian of Scotland and of the Continent of Europe.

Before describing these plants in detail, we may refer to North America for illustration of the physical conditions of the time. In a physical point of view the northern hemisphere presented a great change in the Erian period. There were vast foldings of the crust of the earth, and great emissions of volcanic rock on both sides of the Atlantic. In North America, while at one time the whole interior area of the continent, as far north as the Great Lakes, was occupied by a vast inland sea, studded with coral islands, the long Appalachian ridge had begun to assume, along with the old Laurentian land, something of the form of our present continent, and on the margins of this Appalachian belt there were wide, swampy flats and shallow-water areas, which, under the mild climate that seems to have characterised this period, were admirably suited to nourish a luxuriant vegetation. Under this mild climate, also, it would seem that new forms of plants were first introduced in the far north, where the long continuance of summer sunlight, along with great warm th, seems to have aided in their introduction and early extension, and thence made their way to the southward, a process which, as Gray and others have shown, has also occurred in later geological times.

The America of this Erian age consisted during the greater part of the period of a more or less extensive belt of land in the north with two long tongues descending from it, one along the Appalachian line in the east, the other in the region west of the Rocky Mountains. On the seaward sides of these there were low lands covered with vegetation, while on the inland side the great interior sea, with its verdant and wooded islands, realised, though probably with shallower water, the conditions of the modern archipelagoes of the Pacific.

Europe presented conditions somewhat similar, having in the earlier and middle portions of the period great sea areas with insular patches of land, and later wide tracts of shallow and in part enclosed water areas, swarming with fishes, and having an abundant vegetation on their shores. These were the conditions of the Eifel and Devonshire limestones, and of the Old Red Sandstone of Scotland, and the Kiltorcan beds of Ireland. In Europe also, as in America, there were in the Erian age great ejections of igneous rock. On both sides of the Atlantic there were somewhat varied and changing conditions of land and water, and a mild and equable climate, permitting the existence of a rich vegetation in high northern latitudes. Of this latter fact a remarkable example is afforded by the beds holding plants of this age in Spitzbergen and Bear Island, in its vicinity. Here there seem to be two series of plant-bearing strata, one with the vegetation of the Upper Erian, the other with that of the Lower Carboniferous, though both have been united by Heer under his so-called "Ursa Stage" in which he has grouped the characteristic plants of two distinct periods. This has recently been fully established by the researches of Nathorst, though the author had already suggested it as the probable explanation of the strange union of species in the Ursa group of Heer.

In studying the vegetation of this remarkable period, we must take merely some of the more important forms as examples, since it would be impossible to notice all the species, and some of them may be better treated in the Carboniferous, where they have their headquarters. (Fig. 15.)

I may first refer to a family which seems to have culminated in the Erian age, and ever since to have occupied a less important place. It is that of the curious aquatic plants known as Rhizocarps,[AK] and referred to in the last chapter.

[AK] Or, as they have recently been named by some botanists, "Heterosporous Filices," though they are certainly not ferns in any ordinary sense of that term.

My attention was first directed to these organisms by the late Sir W. E. Logan in 1869. He had obtained from the Upper Erian shale of Kettle Point, Lake Huron, specimens filled with minute circular discs, to which he referred, in his report of 1863, as "microscopic orbicular bodies." Recognising them to be macrospores, or spore-cases, I introduced them into the report on the Erian flora, which I was then preparing, and which was published in 1871, under the name _Sporangites Huronensis_.

In 1871, having occasion to write a communication to the "American Journal of Science" on the question then raised as to the share of spores and spore-cases in the accumulation of coal, a question to be discussed in a subsequent chapter, these curious little bodies were again reviewed, and were described in substance as follows:

"The oldest bed of spore-cases known to me is that at Kettle Point, Lake Huron. It is a bed of brown bituminous shale, burning with much flame, and under a lens is seen to be studded with flattened disc-like bodies, scarcely more than a hundredth of an inch in diameter, which under the microscope are found to be spore-cases (or macrospores) slightly papillate externally (or more properly marked with dark pores), and sometimes showing a point of attachment on one side and a slit more or less elongated and gaping on the other. When slices of the rock are made, its substance is seen to be filled with these bodies, which, viewed as transparent objects, appear yellow like amber, and show little structure, except that the walls can be distinguished from the internal cavity, which may sometimes be seen to enclose patches of granular matter. In the shale containing them are also vast numbers of rounded, translucent granules, which may be escaped spores (microspores)." The bed containing these spores at Kettle Point was stated, in the reports of the "Geological Survey of Canada," to be twelve or fourteen feet in thickness, and besides these specimens it contained fossil plants referable to the species _Calamites inornatus_ and _Lepidodendron primævum_, and I not unnaturally supposed that the Sporangites might be the fruit of the latter plant. I also noticed their resemblance to the spore-cases of _L. corrugatum_ of the Lower Carboniferous (a Lepidodendron allied to _L. primævum_), and to those from Brazil described by Carruthers under the name _Flemingites_, as well as to those described by Huxley from certain English coals, and to those of the Tasmanite or white coal of Australia. The bed at Kettle Point is shown to be marine by its holding the sea-weed known as _Spirophyton_, and shells of _Lingula_.

The subject did not again come under my notice till 1882, when Prof. Orton, of Columbus, Ohio, sent me some specimens from the Erian shales of that State, which on comparison seemed undistinguishable from _Sporangites Huronensis_.[AL] Prof. Orton read an interesting paper on these bodies, at the meeting of the American Association in Montreal, in which were some new and striking facts. One of these was the occurrence of such bodies throughout the black shales of Ohio, extending "from the Huron River, on the shore of Lake Brie, to the mouth of the Scioto, in the Ohio Valley, with an extent varying from ten to twenty miles in breadth," and estimated to be three hundred and fifty feet in thickness. I have since been informed by my friend Mr. Thomas, of Chicago, that its thickness, in some places at least, must be three times that amount. About the same time. Prof. Williams, of Cornell, and Prof. Clarke, of Northampton, announced similar discoveries in the State of New York, so that it would appear that beds of vast area and of great thickness are replete with these little vegetable discs, usually converted into a highly bituminous, amber-like substance, giving a more or less inflammable character to the containing rock.

[AL] These shales have been described, as to their chemical and geological relations, by Dr. T. Sterry Hunt, "American Journal of Science," 1863, and by Dr. Newberry, in the "Reports of the Geological Survey of Ohio," vol. i., 1863, and vol. iii., 1878.

Another fact insisted on by Prof. Orton was the absence of Lepidodendroid cones, and the occurrence of filamentous vegetable matter, to which the Sporangites seemed to be in some cases attached in groups. Prof. Orton also noticed the absence of the trigonal form, which belongs to the spores of many Lepidodendra, though this is not a constant character. In the discussion on Prof. Orton's paper, I admitted that the facts detailed by him shook my previous belief of the lycopodiaceous character of these bodies, and induced me to suspect, with Prof. Orton, that they might have belonged to some group of aquatic plants lower than the Lycopods.

Since the publication of my paper on Rhizocarps in the Palæozoic period above referred to, I have received two papers from Mr. Edward Wethered, F. G. S., in one of which he describes spores of plants found in the lower limestone shales of the Forest of Dean, and in the other discusses more generally the structure and origin of Carboniferous coal-beds.[AM] In both papers he refers to the occurrence in these coals and shales of organisms essentially similar to the Erian spores.

[AM] "Cotteswold Naturalists' Field Club," 1884; "Journal of the Royal Microscopical Society," 1885.

In the "Bulletin of the Chicago Academy of Science," January, 1884, Dr. Johnson and Mr. Thomas, in their paper on the "Microscopic Organisms of the Boulder Clay of Chicago and Vicinity," notice _Sporangites Huronensis_ as among these organisms, and have discovered them also in large numbers in the precipitate from Chicago city water-supply. They refer them to the decomposition of the Erian shales, of which boulders filled with these organisms are of frequent occurrence in the Chicago clays. The Sporangites and their accompaniments in the boulder clay are noticed in a paper by Dr. G. M. Dawson, in the "Bulletin of the Chicago Academy," June, 1885.

Prof. Clarke has also described, in the "American Journal of Science" for April, 1885, the forms already alluded to, and which he finds to consist of macrospores enclosed in sporocarps. He compares these with my _Sporangites Huronensis_ and _Protosalvinia bilobata_, but I think it is likely that one of them at least is a distinct species.

I may add that in the "Geological Magazine" for 1875, Mr. Newton, F. G. S., of the Geological Survey of England, published a description of the Tasmanite and Australian white coal, in which he shows that the organisms in these deposits are similar to my _Sporangites Huronensis_, and to the macrospores previously described by Prof. Huxley, from the Better-bed coal. Mr. Newton does not seem to have been aware of my previous description of _Sporangites_, and proposes the name _Tasmanites punctatus_ for the Australian form.

Here we have the remarkable fact that the waste macrospores, or larger spores of a species of Cryptogamous plant, occur dispersed in countless millions of tons through the shales of the Erian in Canada and the United States.

No certain clue seemed to be afforded by all these observations as to the precise affinities of these widely distributed bodies; but this was furnished shortly after from an unexpected quarter. In March, 1883, Mr. Orville Derby, of the Geological Survey of Brazil, sent me specimens found in the Erian of that country, which seemed to throw a new light on the whole subject. These I described and pointed out their connection with _Sporangites_ at the meeting of the American Association at Minneapolis, in 1883, and subsequently published my notes respecting them in its proceedings, and in the "Canadian Record of Science."

Mr. Derby's specimens contained the curious spiral sea-weed known as _Spirophyton_, and also minute rounded Sporangites like those obtained in the Erian of Ohio, and of which specimens had been sent to me some years before by the late Prof. Hartt. But they differed in showing the remarkable fact that these rounded bodies are enclosed in considerable numbers in spherical and oval sacs, the walls of which are composed of a tissue of hexagonal cells, and which resemble in every respect the involucres or spore-sacs of the little group of modern acrogens known as Rhizocarps, and living in shallow water. More especially they resemble the sporocarps of the genus _Salvinia_. This fact opened up an entirely new field of investigation, and I at once proceeded to compare the specimens with the fructification of modern Rhizocarps, and found that substantially these multitudinous spores embedded in the Erie shales may be regarded as perfectly analogous to the larger spores of the modern _Salvinia natans_ of Europe, as may be seen by the representation of them in Fig. 16.

The typical macrospores from the Erian shales are perfectly circular in outline, and in the flattened state appear as discs with rounded edges, their ordinary diameter being from one seventy-fifth to one one-hundredth of an inch, though they vary considerably in size. This, however, I do not regard as an essential character. The edges, as seen in profile, are smooth, but the flat surface often presents minute dark spots, which at first I mistook for papillæ, but now agree with Mr. Thomas in recognising them as minute pores traversing the wall of the disc, and similar to those which Mr. Newton has described in Tasmanite, and which Mr. Wethered has also recognised in the similar spores of the Forest of Dean shales. The walls also sometimes show faint indications of concentric lamination, as if they had been thickened by successive deposits.

As seen by transmitted light, and either in front or in profile, the discs are of a rich amber colour, translucent and structureless, except the pores above referred to. The walls are somewhat thick, or from one-tenth to one-twentieth the diameter of the disc in thickness. They never exhibit the triradiate marking seen in spores of Lycopods, nor any definite point of attachment, though they sometimes show a minute elongated spot which may be of this nature, and they are occasionally seen to have opened by slits on the edge or front, where there would seem to have been a natural line of dehiscence. The interior is usually quite vacant or structureless, but in some cases there are curved internal markings which may indicate a shrunken lining membrane, or the remains of a prothallus or embryo. Occasionally a fine granular substance appears in the interior, possibly remains of microspores.

The discs are usually detached and destitute of any envelope, but fragments of flocculent cellular matter are associated with them, and in one specimen from the corniferous limestone of Ohio, in Mr. Thomas's collection, I have found a group of eight or more discs partly enclosed in a cellular sac-like membrane of similar character to that enclosing the Brazilian specimens already referred to.

The characters of all the specimens are essentially similar, and there is a remarkable absence of other organisms in the shale. In one instance only, I have observed a somewhat smaller round body with a dark centre or nucleus, and a wide translucent margin, marked by a slight granulation. Even this, however, may indicate nothing more than a different state of preservation.

It is proper to observe here that the wall or enclosing sac of these macrospores must have been of very dense consistency, and now appears as a highly bituminous substance, in this agreeing with that of the spores of Lycopods, and, like them, having been when recent of a highly carbonaceous and hydrogenous quality, very combustible and readily admitting of change into bituminous matter. In the paper already referred to, on spore-cases in coals, I have noticed that the relative composition of lycopodium and cellulose is as follows:

Cellulose, C{24}H{20}O{20}.

Lycopodium, C{42}H{19-4/12}NO{5-6/10}.

Thus, such spores are admirably suited for the production of highly carbonaceous or bituminous coals, etc.

Nothing is more remarkable in connection with these bodies than their uniformity of structure and form over so great areas and throughout so great thickness of rock, and the absence of any other kind of spore-case. This is more especially noteworthy in contrast with the coarse coals and bituminous shales of the Carboniferous, which usually contain a great variety of spores and sporangia, indicating the presence of many species of acrogenous plants, while the Erian shales, on the contrary, indicate the almost exclusive predominance of one form. This contrast is well seen in the Bedford shales overlying these beds, and I believe Lower Carboniferous.[AN] Specimens of these have been kindly communicated to me by Prof. Orton, and have been prepared by Mr. Thomas. In these we see the familiar Carboniferous spores with triradiate markings called _Triletes_ by Reinsch, and which are similar to those of Lycopodiaceous plants. Still more abundant are those spinous and hooked spores or sporangia, to which the names _Sporocarpon_, _Zygosporites_, and _Traquaria_ have been given, and some of which Williamson has shown to be spores of Lycopodiaceous plants.[AO]

[AN] According to Newberry, lower part of Waverly group.

[AO] _Traquaria_ is to be distinguished from the calcareous bodies found in the corniferous limestone of Kelly's Island, which I have described in the "Canadian Naturalist" as _Saccamina Eriana_, and believe to be Foraminiferal tests. They have since been described by Ulrich under a different name (_Moellerina_: contribution to "American Palæontology," 1886). See Dr. Williamson's papers in "Transactions of Royal Society of London."

The true "Sporangites," on the contrary, are round and smooth, with thick bituminous walls, which are punctured with minute transverse pores. In these respects, as already stated, they closely resemble the bodies found in the Australian white coal and Tasmanite. The precise geological age of this last material is not known with certainty, but it is believed to be Palæozoic.

With reference to the mode of occurrence of these bodies, we may note first their great abundance and wide distribution. The horizontal range of the bed at Kettle Point is not certainly known, but it is merely a northern outlier of the great belt of Erian shales referred to by Prof. Orton, and which extends, with a breadth of ten to twenty miles, and of great thickness, across the State of Ohio, for nearly two hundred miles. This Ohio black shale, which lies at the top of the Erian or the base of the Carboniferous, though probably mainly of Erian age, appears to abound throughout in these organisms, and in some beds to be replete with them. In like manner, in Brazil, according to Mr. Derby, these organisms are distributed over a wide area and throughout a great thickness of shale holding _Spirophyton_, and apparently belonging to the Upper Erian. The recurrence of similar forms in the Tasmanite and white coal of Tasmania and Australia is another important fact of distribution. To this we may add the appearance of these macrospores in coals and shales of the Carboniferous period, though there in association with other forms.

It is also to be observed that the Erian shales, and the Forest of Dean beds described by Wethered, are marine, as shown by their contained fossils; and, though I have no certain information as to the Tasmanite and Australian white coal, they would seem, from the description of Milligan, to occur in distinctly aqueous, possibly estuarine, deposits. Wethered has shown that the discs described by Huxley and Newton in the Better-bed coal occur in the earthy or fragmentary layers, as distinguished from the pure coal. Those occurring in cannel coal are in the same case, so that the general mode of occurrence implies water-driftage, since, in the case of bodies so large and dense, wind-driftage to great distances would be impossible.

These facts, taken in connection with the differences between these macrospores and those of any known land-plant of the Palæozoic, would lead to the inference that they belonged to aquatic plants, and these vastly abundant in the waters of the Erian and Carboniferous periods.

It is still further to be observed that they are not, in the Erian beds, accompanied with any remains of woody or scalariform tissues, such as might be expected in connection with the _débris_ of terrestrial acrogens, and that, on the other hand, we find them enclosed in cellular sporocarps, though in the majority of cases these have been removed by dehiscence or decay.

These considerations, I think, all point to the probability which I have suggested in my papers on this subject referred to above, that we have in these objects the organs of fructification of plants belonging to the order _Rhizocarpeæ_, or akin to it. The comparisons which I have instituted with the sporocarps and macrospores of these plants confirm this suggestion. Of the modern species which I have had an opportunity to examine, _Salvinia natans_ of Europe perhaps presents the closest resemblance. In this plant groups of round cellular sporocarps appear at the bases of the floating fronds. They are about a line in diameter when mature, and are of two kinds, one containing macrospores, the other microspores or antheridia. The first, when mature, hold a number of closely packed globular or oval sporangia of loose cellular tissue, attached to a central placenta. Each of these sporangia contains a single macrospore, perfectly globular and smooth, with a dense outer membrane (exhibiting traces of lamination, and showing within an irregularly vacuolated or cellular structure, probably a prothallus). I cannot detect in it the peculiar pores which appear in the fossil specimens. Each macrospore is about one-seventieth of an inch in diameter when mature. The sporocarps of the microspores contain a vastly greater number of minute sporangia, about one two-hundredths of an inch in diameter. These contain disc-like antheridia, or microspores of very minute size.

The discs from Kettle Point and from the Ohio black shale, and from the shale boulders of the Chicago clays, are similar to the macrospores of _Salvinia_, except that they have a thicker wall and are a little less in diameter, being about one-eightieth of an inch. The Brazilian sporocarps are considerably larger than those of the modern _Salvinia_, and the macrospores approach in size to those of the modern species, being one seventy-fifth of an inch in diameter. They also seem, like the modern species, to have thinner walls than those from Canada, Ohio, and Chicago. No distinct indication has been observed in the fossil species of the inner Sporangium of _Salvinia_. Possibly it was altogether absent, but more probably it is not preserved as a distinct structure.

With reference to the microspores of _Salvinia_, it is to be observed that the sporocarps, and the contained spores or antheridia, are very delicate and destitute of the dense outer wall of the macrospores. Hence such parts are little likely to have been preserved in a fossil state; and in the Erian shales, if present, they probably appear merely as flocculent carbonaceous matter not distinctly marked, or as minute granules not well defined, of which there are great quantities in some of the shales.

The vegetation appertaining to the Sporangites has not been distinctly recognised. I have, however, found in one of the Brazilian specimens two sporocarps attached to what seems a fragment of a cellular frond, and numerous specimens of the supposed Algæ, named _Spirophyton_, are found in the shales, but there is no evidence of any connection of this plant with the _Protosalvinia_.

Modern Rhizocarps present considerable differences as to their vegetative parts. Some, like _Pilularia_, have simple linear leaves; others, like _Marsilea_, have leaves in whorls, and cuneate in form; while others, like _Azolla_ and _Salvinia_, have frondose leaves, more or less pinnate in their arrangement. If we inquire as to fossils representing these forms of vegetation, we shall find that some of the plants to be noticed in the immediate sequel may have been nearly allied to the Rhizocarps. In the mean time I may state that I have proposed the generic name _Protosalvinia_ for these curious macrospores and their coverings, and have described in the paper in the "Bulletin of the Chicago Academy of Sciences," already quoted, five species which may be referred to this genus.

These facts lead to inquiries as to the origin of the bituminous matter which naturally escapes from the rocks of the earth as petroleum and inflammable gas, or which may be obtained from certain shales in these forms by distillation. These products are compounds of carbon and hydrogen, and may be procured from recent vegetable substances by destructive distillation. Some vegetable matters, also, are much richer in carbon and hydrogen than others, and it is a remarkable fact that the spores of certain cryptogamous plants are of this kind, as we see in the inflammable character of the dry spores of Lycopodium; and we know that the slow putrefaction of such material underground effects chemical changes by which bituminous matter can be produced. There is, therefore, nothing unreasonable in the supposition advanced by Prof. Orton, that the spores so abundantly contained in the Ohio black shales are important or principal sources of the bituminous matter which they contain. Microscopic sections of this shale show that much of its material consists of the rich bituminous matter of these spores (Fig. 16). At the same time, while we may trace the bitumen of these shales, and of some beds of coal, to this cause, we must bear in mind that there are other kinds of bituminous rocks which show no such structures, and may have derived their combustible material from other kinds of vegetable matter, whether of marine or of land plants. We shall better understand this when we have considered the origin of coal.

The macrospores above referred to may have belonged to humble aquatic plants mantling the surfaces of water or growing up from the bottom, and presenting little aërial vegetation. But there are other Erian plants, as already mentioned, which, while of higher structure, may be of Rhizocarpean affinities.

One of these is the beautiful plant with whorls of wedge-shaped leaves, to which the name _Sphenophyllum_ (see Fig. 20) has been given. Plants referred to this genus have been described by Lesquereux from the upper part of the Siluro-Cambrian,[AP] and a beautiful little species occurs in the Erian shales of St. John, New Brunswick.[AQ] The genus is also continued, and is still more abundant, in the Carboniferous. Many years ago I observed, in a beautiful specimen collected by Sir W. E. Logan, in New Brunswick, that the stem of this plant had an axis of reticulated and scalariform vessels, and an outer bark.[AR] Renault and Williamson have more recently obtained more perfect specimens, and the former has figured a remarkably complex triangular axis, containing punctate and barred vessels, and larger punctate vessels filling in its angles. Outside of this there is a cellular inner bark, and this is surrounded by a thick fibrous envelope. That a structure so complex should belong to a plant so humble in its affinities is one of the strange anomalies presented by the old world, and of which we shall find many similar instances. The fruit of _Sphenophyllum_ was borne in spikes, with little whorls of bracts or rudimentary leaves bearing round sporocarps.

[AP] "American Journal of Science."

[AQ] Dawson, "Report on Devonian Plants," 1870.

[AR] "Journal of the Geological Society," 1865.

A second type of plant, which may have been Rhizocarpean in its affinities, is that to which I have given the name _Ptilophyton_.[AS] It consists of beautiful feathery fronds, apparently bearing on parts of the main stem or petiole small rounded sporocarps. They are found abundantly in the Middle Erian of the State of New York, and also occur in Scotland, while one species appears to occur in Nova Scotia, as high as the Lower Carboniferous (Figs. 17, 18).

[AS] _Plumalina_ of Hall.

These organisms have been variously referred to Lycopods, to Algæ, or to Zoöphytes, but an extended comparison of American and Scottish specimens has led me to the belief that they were aquatic plants, more likely to have been allied to Rhizocarps than to any other group. Some evidence of this will be given in a note appended to this chapter.

Another genus, which I have named _Psilophyton_[AT] (Figs. 19, 21), may be regarded as a connecting link between the Rhizocarps and the Lycopods. It is so named from its resemblance, in some respects, to the curious parasitic Lycopods placed in the modern genus _Psilotum_. Several species have been described, and they are eminently characteristic of the Lower Erian, in which they were first discovered in Gaspé. The typical species, _Psilophyton princeps_, which fills many beds of shale and sandstone in Gaspé Bay and the head of the neighbouring Bay des Chaleurs with its slender stems and creeping, cord-like rhizomes, may be thus described:

[AT] "Journal of the Geological Society," vols, xv., xviii., and xix., "Report on Devonian Plants of Canada," 1871.

Stems branching dichotomously, and covered with interrupted ridges. Leaves rudimentary, or short, rigid, and pointed; in barren stems, numerous and spirally arranged; in fertile stems and branchlets, sparsely scattered or absent; in decorticated specimens, represented by a minute punctate scars. Young branches circinate; rhizomata cylindrical, covered with hairs or ramenta, and having circular areoles irregularly disposed, giving origin to slender cylindrical rootlets. Internal structure--an axis of scalariform vessels, surrounded by a cylinder of parenchymatous cells, and by an outer cylinder of elongated woody cells. Fructification consisting of naked oval spore-cases, borne usually in pairs on slender, curved pedicels, either lateral or terminal.

This species was fully described by me in the papers referred to above, from specimens obtained from the rich exposures at Gaspé Bay, and which enabled me to illustrate its parts more fully, perhaps, than those of any other species of so great antiquity. In the specimens I had obtained I was able to recognise the forms of the rhizomata, stems, branches, and rudimentary leaves, and also the internal structure of the stems and rhizomata, and to illustrate the remarkable resemblance of the forms and structures to those of the modern _Psilotum_. The fructification was, however, altogether peculiar, consisting of narrowly ovate sporangia, borne usually in pairs, on curved and apparently rigid petioles. Under the microscope these sporangia show indications of cellular structure, and appear to have been membranous in character. In some specimens dehiscence appears to have taken place by a slit in one side, and, clay having entered into the interior, both walls of the spore-case can be seen. In other instances, being flattened, they might be mistaken for scales. No spores could be observed in any of the specimens, though in some the surface was marked by slight, rounded prominences, possibly the impressions of the spores within. This peculiar and very simple style of spore-case is also characteristic of other species, and gives to _Psilophyton_ a very distinct generic character. These naked spore-cases may be compared to those of such lycopodiaceous plants as _Psilotum_, in which the scales are rudimentary. They also bear some resemblance, though on a much larger scale, to the spore-cases of some Erian ferns (_Archæopteris_), to be mentioned in the sequel. On the whole, however, they seem most nearly related to the sporocarps of the Rhizocarpeæ.

_Arthrostigma_, which is found in the same beds with Psilophyton, was a plant of more robust growth, with better-developed, narrow, and pointed leaves, borne in a verticillate or spiral manner, and bearing at the ends of its branches spikes of naked sporocarps, apparently similar to those of _Psilophyton_ but more rounded in form. The two genera must have been nearly related, and the slender branchlets of _Arthrostigma_ are, unless well preserved, scarcely distinguishable from the stems of _Psilophyton_.[AU]

[AU] Reports of the author on "Devonian Plants," "Geological Survey of Canada," which see for details as to Erian Flora of northeastern America.

If, now, we compare the vegetation of these and similar ancient plants with that of modern Rhizocarps, we shall find that the latter still present, though in a depauperated and diminished form, some of the characteristics of their predecessors. Some, like _Pilularia_, have simple linear leaves; others, like _Marsilea_, have leaves in verticils and cuneate in form; while others, like Azolla and Salvinia, have frondose leaves, more or less pinnate in their arrangement. The first type presents little that is characteristic, but there are in the Erian sandstones and shales great quantities of filamentous and linear objects which it has been impossible to refer to any genus, and which might have belonged to plants of the type of _Pilularia_. It is quite possible, also, that such plants as _Psilophyton glabrum_ and _Cordaites angustifolia_, of which the fructification is quite unknown, may have been allied to Rhizocarps. With regard to the verticillate type, we are at once reminded of _Sphenophyllum_ (Fig. 20), which many palæobotanists have referred to the _Marsiliacæ_, though, like other Palæozoic Acrogens, it presents complexities not seen in its modern representatives. _S. primævum_ of Lesquereux is found in the Hudson River group, and my _S. antiquum_ in the Middle Erian. Besides these, there are in the Silurian and Erian beds plants with verticillate leaves which have been placed with the Annulariæ, but which may have differed from them in fructification. _Annularia laxa_, of the Erian, and _Protannularia Harknessii_, of the Siluro-Cambrian, may be given as examples, and must have been aquatic plants, probably allied to Rhizocarps. It is deserving of notice, also, that the two best-known species of _Psilophyton_ (_P. princeps_ and _P. robustius_), while allied to Lycopods by the structure of the stem and such rudimentary foliage as they possess, are also allied, by the form of their fructification, to the Rhizocarps, and not to ferns, as some palæobotanists have incorrectly supposed. A similar remark applies to _Arthrostigma_; and the beautiful pinnately leaved _Ptilophyton_ may be taken to represent that type of foliage as seen in modern Rhizocarps, while the allied forms of the Carboniferous which Lesquereux has named _Trochophyllum_, seem to have had sporocarps attached to the stem in the manner of _Azolla_.

The whole of this evidence, I think, goes to show that in the Erian period there were vast quantities of aquatic plants, allied to the modern Rhizocarps, and that the so-called _Sporangites_ referred to in this paper were probably the drifted sporocarps and macrospores of some of these plants, or of plants allied to them in structure and habit, of which the vegetative organs have perished. I have shown that in the Erian period there were vast swampy flats covered with _Psilophyton_, and in similar submerged tracts near to the sea the _Protosalvinia_ may have filled the waters and have given off the vast multitudes of macrospores which, drifted by currents, have settled in the mud of the black shales. We have thus a remarkable example of a group of plants reduced in modern times to a few insignificant forms, but which played a great role in the ancient Palæozoic world.

Leaving the Rhizocarps, we may now turn to certain other families of Erian plants. The first to attract our attention in this age would naturally be the Lycopods, the club-mosses or ground-pines, which in Canada and the Eastern States carpet the ground in many parts of our woods, and are so available for the winter decoration of our houses and public buildings. If we fancy one of these humble but graceful plants enlarged to the dimensions of a tree, we shall have an idea of a _Lepidodendron_, or of any of its allies (Figs. 15, 21). These large lycopodiaceous trees, which in different specific and generic forms were probably dominant in the Erian woods, resembled in general those of modern times in their fruit and foliage, except that their cones were large, and probably in most cases with two kinds of spores, and their leaves were also often very long, thus bearing a due proportion to the trees which they clothed. Their thick stems required, however, more strength than is necessary in their diminutive successors, and to meet this want some remarkable structures were introduced similar to those now found only in the stems of plants of higher rank. The cells and vessels of all plants consist of thin walls of woody matter, enclosing the sap and other contents of these sacs and tubes, and when strength is required it is obtained by lining their interior with successive coats of the hardest form of woody matter, usually known as lignin. But while the walls remain thin, they afford free passage to the sap to nourish every part. If thickened all over, they would become impervious to sap, and therefore unsuited to one of their most important functions. These two ends of strength and permeability are secured by partial linings of lignin, leaving portions of the original wall uncovered. But this may be done in a great variety of ways.

The most ancient of these contrivances, and one still continued in the world of plants, is that of the barred or scalariform vessel. This may be either square or hexagonal, so as to admit of being packed without leaving vacancies. It is strengthened by a thick bar of ligneous matter up each angle, and these are connected by cross-bars so as to form a framework resembling several ladders fastened together. Hence the name _scalariform_, or ladder-like. Now, in a modern Lycopod there is a central axis of such barred vessels associated with simpler fibres or elongated cells. Even in _Sphenophyllum_ and _Psilophyton_, already referred to as allied to Rhizocarps,[AV] there is such a central axis, and in the former rigidity is given to this by the vascular and woody elements being arranged in the form of a three-sided prism or three-rayed star. But such arrangements would not suffice for a tree, and hence in the arboreal Lycopods of the Erian age a more complex structure is introduced. The barred vessels were expanded in the first instance into a hollow cylinder filled in with pith or cellular tissue, and the outer rind was strengthened with greatly thickened cells. But even this was not sufficient, and in the older stems wedge-shaped bundles of barred tissue were run out from the interior, forming an external woody cylinder, and inside of the rind were placed bundles of tough bast fibres. Thus, a stem was constructed having pith, wood, and bark, and capable of additions to the exterior of the woody wedges by a true exogenous growth. The plan is, in short, the same with that of the stems of the exogenous trees of modern times, except that the tissues employed are less complicated. The structures of these remarkable trees, and the manner in which they anticipate those of the true exogens of modern times, have been admirably illustrated by Dr. Williamson, of Manchester. His papers, it is true, refer to these plants as existing in the Carboniferous age, but there is every reason to believe that they were of the same character in the Erian. The plan is the same with that now seen in the stems of exogenous phænogams, and which has long ceased to be used in those of the Lycopods. In this way, however, large and graceful lycopodiaceous trees were constructed in the Erian period, and constituted the staple of its forests.

[AV] First noticed by the author, "Journal of Geological Society," 1865; but more completely by Renault, "Comptes Rendus," 1870.

The roots of these trees were equally remarkable with their stems, and so dissimilar to any now existing that botanists were long disposed to regard them as independent plants rather than roots. They were similar in general structure to the stems to which they belonged, but are remarkable for branching in a very regular manner by bifurcation like the stems above, and for the fact that their long, cylindrical rootlets were arranged in a spiral manner and distinctly articulated to the root after the manner of leaves rather than of rootlets, and fitting them for growing in homogeneous mud or vegetable muck. They are the so-called _Stigmaria_ roots, which, though found in the Erian and belonging to its lycopodiaceous plants, attained to far greater importance in the Carboniferous period, where we shall meet with them again.

There were different types of lycopodiaceous plants in the Erian. In addition to humble Lycopods like those of our modern woods and great Lepidodendra, which were exaggerated Lycopods, there were thick-stemmed and less graceful species with broad rhombic scars (_Leptophleum_), and others with the leaf-scars in vertical rows (_Sigillaria_), and others, again, with rounded leaf-scars, looking like the marks on Stigmaria, and belonging to the genus _Cyclostigma_. Thus some variety was given to the arboreal club-mosses of these early forests. (See Fig. 15.)

Another group of plants which attained to great development in the Erian age is that of the Ferns or Brackens. The oldest of these yet known are found in the Middle Erian. The _Eopteris_ of Saporta, from the Silurian, at one time supposed to carry this type much further back, has unfortunately been found to be a mere imitative form, consisting of films of pyrites of leaf-like shapes, and produced by crystallisation. In the Middle Erian, however, more especially in North America, many species have been found (Figs. 22 to 24).[AW] I have myself recorded more than thirty species from the Middle Erian of Canada, and these belong to several of the genera found in the Carboniferous, though some are peculiar to the Erian. Of the latter, the best known are perhaps those of the genus _Archæopteris_ (Fig. 24), so abundant in the plant-beds of Kiltorcan in Ireland, as well as in North America. In this genus the fronds are large and luxuriant, with broad obovate pinnules decurrent, on the leaf-stalk, and with simple sac-like spore-cases borne on modified pinnæ. Another very beautiful fern found with _Archæopteris_ is that which I have named _Platyphyllum_, and which grew on a creeping stem or parasitically on stems of other plants, and had marginal fructification.[AX] Another very remarkable fern, which some botanists have supposed may belong to a higher group than the ferns, is Megalopteris (Fig. 26).

[AW] For descriptions of these ferns, see reports cited above.

[AX] "Reports on Fossil Plants of the Devonian and Upper Silurian of Canada," 1871, &c.

Some of the Erian ferns attained to the dimensions of tree-ferns. Large stems of these, which must have floated out far from land, have been found by Newberry in the marine limestone of Ohio (_Caulopteris antiqua_ and _C. peregrina_, Newberry),[AY] and Prof. Hall has found in the Upper Devonian of Gilboa, New York, the remains of a forest of tree-ferns standing _in situ_ with their great masses of aërial roots attached to the soil in which they grew (_Caulopteris Lockwoodi_, Dn.).[AZ]

[AY] "Journal of the Geological Society," 1871.

[AZ] _Ibid._

These aërial roots introduce us to a new contrivance for strengthening the stems of plants by sending out into the soil multitudes of cord-like cylindrical roots from various heights on the stem, and which form a series of stays like the cordage of a ship. This method of support still continues in the modern tree-ferns of the tropics and the southern hemisphere. In one kind of tree-fern stem from the Erian of New York, there is also a special arrangement for support, consisting of a series of peculiarly arranged radiating plates of scalariform vessels, not exactly like those of an exogenous stem, but doing duty for it (_Asteropteris_)[BA] Similar plants have been described from the Erian of Falkenberg, in Germany, and of Saalfeld, in Thuringia, by Goeppert and Unger, and are referred to ferns by the former, but treated as doubtful by the latter,[BB] This peculiar type of tree-fern is apparently a precursor of the more exogenous type of _Heterangium_, recently described and referred to ferns by Williamson. Here, again, we have a mechanical contrivance now restricted to higher plants appropriated by these old cryptogams.

[BA] "Journal of the Geological Society," London, 1881.

[BB] "Sphenopteris Refracta," Goeppert; "Flora des Uebergangsgebirges." "Cladoxylon Mirabile," Unger; "Palæontologie des Thuringer Waldes."

The history of the ferns in geological time is remarkably different from that of the Lycopods; for while the latter have long ago descended from their pristine eminence to a very humble place in nature, the former still, in the southern hemisphere at least, retain their arboreal dimensions and ancient dominance.

The family of the _Equisetaceæ_, or mare's-tails, was also represented by large species of _Calamites_ and by _Asterophyllites_ in the Erian; but, as its headquarters are in the Carboniferous, we may defer its consideration till the next chapter. (Figs. 27, 28.)

Passing over these for the present, we find that the flowering plants are represented in the Erian forests by at least two types of Gymnosperms, that of _Taxineæ_ or yews, and an extinct family, that of the _Cordaites_ (Figs. 30, 31). The yew-trees are closely allied to the pines and spruces, and are often included with them in the family of _Coniferæ_. They differ, however, in the habit of producing berries or drupe-like fruits instead of cones, and there is some reason to believe that this was the habit of the Erian trees of this group, though their wood in some instances resembles rather that of the Araucaria, or Norfolk Island pine, than that of the modern yews. These trees are chiefly known to us by their mineralised trunks, which are often found like drift-wood on modern sand-banks embedded in the Erian sandstones or limestones. It often shows its structure in the most perfect manner in specimens penetrated by calcite or silica, or by pyrite, and in which the original woody matter has been resolved into anthracite or even into graphite. These trees have true woody tissues presenting that beautiful arrangement of pores or thin parts enclosed in cup-like discs, which is characteristic of the coniferous trees, and which is a great improvement on the barred tissue already referred to, affording a far more strong, tough, and durable wood, such as we have in our modern pines and yews (Fig. 29).

These primitive pines make their appearance in the Middle Erian, in various parts of America, as well as in Scotland and Germany, and they are represented by wood indicating the presence of several species. I have myself indicated and described five species from the Erian of Canada and the United States. From the fact that these trees are represented by drifted trunks embedded in sandstones and marine limestones, we may, perhaps, infer that they grew on the rising grounds of the Erian land, and that their trunks were carried by river-floods into the sea. No instance has yet certainly occurred of the discovery of their foliage or fruit, though there are some fan-shaped leaves usually regarded as ferns which may have belonged to such trees. These in that case would have resembled the modern _Gingko_ of China, and some of the fruits referred to the genus _Cardiocarpum_ may have been produced by them. Various names have been given to these trees. I have preferred that given by Unger, _Dadoxylon_, as being more non-committal as to affinities than the others.[BC] Many of these trees had very long internal pith-cylinders, with curious transverse tubulæ, and which, when preserved separately, have been named _Sternbergia_.

[BC] _Araucarites_, Goeppert; _Araucarioxylon_, Kraus.

Allied to these trees, and perhaps intermediate between them and the _Cycads_, were those known as _Cordaites_ (Fig. 30), which had trunks resembling those of _Dadoxylon_, but with still larger _Sternbergia_ piths and an internal axis of scalariform vessels, surrounded by a comparatively thin woody cylinder. Some of them have leaves over a foot in length, reminding one of the leaves of broad-leaved grasses or iridaceous plants. Yet their flowers and fruit seem to have been more nearly allied to the yews than to any other plants (Fig. 31). Their stems were less woody and their piths larger than in the true pines, and some of the larger-leaved species must have had thick, stiff branches. They are regarded as constituting a separate family, intermediate between pines and cycads, and, beginning in the Middle Devonian, they terminate in the Permian, where, however, some of the most gigantic species occur. In so far as the form and structure of the leaves, stems, and fruit are concerned, there is marvellously little difference between the species found in the Erian and the Permian. They culminated, however, in the Carboniferous period, and the coal-fields of southern France have proved so far the richest in their remains.

Lastly, a single specimen, collected by Prof. James Hall, of Albany, at Eighteen-mile Creek, Lake Erie, has the structure of an ordinary angiospermous exogen, and has been described by me as _Syringoxylon mirabile_.[BD] This unique example is sufficient to establish the fact of the existence of such plants at this early date, unless some accident may have carried a specimen from a later formation to be mixed with Erian fossils. It is to be observed, however, that the non-occurrence of any similar wood in all the formations between the Upper Erian and the Middle Cretaceous suggests very grave doubt as to the authenticity of the specimen. I record the fact, waiting further discoveries to confirm it. Of the character of the specimen which I have described I entertain no doubt.

[BD] "Journal of the Geological Society," vol. xviii.

We shall be better able to realise the significance and relations of this ancient flora when we have studied that of the succeeding Carboniferous. We may merely remark here on the fact that, in these forests of the Devonian and in the marshes on their margins, we find a wonderful expansion of the now modest groups of Rhizocarps and Lycopods, and that the flora as a whole belongs to the highest group of Cryptogams and the lowest of Phænogams, so that it has about it a remarkable aspect of mediocrity. Further, while there is evidence of some variety of station, there is also evidence of much equality of climate, and of a condition of things more resembling that of the insular climates of the temperate portions of the southern hemisphere than that of North America or Europe at present.