And lastly, the surface of these beds would be polygonally cracked and flawed, and the wider cracks filled up by the substance of the overlying strata. And these overlying strata, on the other hand,--the result of a period of quiet deposition in shallow water,--would be regularly bedded, and their ichthyic remains, consisting mainly of small littoral fishes, would be preserved in a state of comparative entireness.
For, however, such numerous repet.i.tions of alternately _felted_ and fissile ripple-marked strata as we find in the neighborhood of Thurso,--repet.i.tions carried on for hundreds of feet in vertical extent,--we require yet another condition,--that condition of gradual subsidence in the general crust which can alone account for the fact so often pressed upon the geologist in exploring the Coal Measures, that in deposits thousands of feet in thickness, each stratum in succession had been laid down in a shallow sea.]
It is a curious circ.u.mstance, that the Old Red flagstones which lie along the southern flanks of the Grampians, and are represented by the gray stone known in commerce as the Arbroath Pavement, have not, so far as is yet known, an organism in common with the Old Red flagstones of the north. I at one time supposed that the rectilinear, smooth-stemmed fucoid, already described, occurred in both series, as the gray stones have also their smooth-stemmed, rectilinear, tape-like organism; but the points of resemblance were too few and simple to justify the conclusion that they were identical, and I have since ascertained that they were entirely different plants. The fucoid of the Caithness flagstones threw off, as I have shown, in the alternate order, numerous ribbon-like branches or fronds; whereas the ribbon-like fronds or branches of the Forfarshire plant rose by dozens from a common root, like the fronds of Zostera, and somewhat resembled a scourge of cords fastened to a handle.
Contemporary with this organism of the gray flagstone formation, and thickly occupying the planes on which it rests, there occur fragments of twisted stems, some of them from three to four inches in diameter (though represented by but mere films of carbonaceous matter), and irregularly streaked, or rather _wrinkled_, longitudinally, like the bark of some of our forest trees, though on a smaller scale. With these we find in considerable abundance irregularly-shaped patches, also of carbonaceous matter, reticulated into the semblance of polygonal, or, in some instances, egg-shaped meshes, and which remind one of pieces of ill woven lace. When first laid open, these meshes are filled each with a carbonaceous speck; and, from their supposed resemblance, in the aggregated form, to the eggs of the frog in their alb.u.minous envelop, the quarriers term them "puddock [frog] sp.a.w.n." The slabs in which they occur, thickly covered over with their vegetable impressions, did certainly remind me, when I first examined them some fifteen years ago, of the bottom of some stagnant ditch beside some decaying hedge, as it appears in middle spring, when paved with fragments of dead branches and withered gra.s.s, and mottled with its life-impregnated patches of the gelid substance regarding which a provincial poet tells his readers, in cla.s.sical Scotch, that
"Puddock-spue is fu" o" e"en, An" every e"e "s a pu-head."[49]
Higher authorities than the quarriers,--among the rest, the late Dr.
Mantell,--have been disposed to regard these polygonal markings as the fossilized sp.a.w.n of ancient Batrachians; but there now seems to be evidence enough from which to conclude that they are the remains, not of the eggs of an animal, but of the seed of a plant. Such was the view taken many years ago by Dr. Fleming,--the original discoverer, let me add, of fossils both in those Upper and Middle Old Red Sandstone deposits that lie in Scotland to the south of the Grampians. "These organisms," we find him saying, in a paper published in "Cheek"s Edinburgh Journal" (1831), "occur in the form of circular flat patches, not equalling an inch in diameter, and composed of numerous smaller contiguous pieces. They are not unlike what might be expected to result from a compressed berry, such as the bramble or the rasp. As, however, they are found adjacent to the narrow leaves of gramineous [looking]
vegetables, and chiefly in clay slate, originally lacustrine silt, it is probable that they const.i.tuted the conglobate panicles of extinct species of the genus Junicus or Sparzanium." From specimens subsequently found by Dr. Fleming, and on which he has erected his species _Parka decipiens_, it seems evident that the nearly circular bodies (which in all the better preserved instances circ.u.mscribe the small polygonal ones) were set in receptacles somewhat resembling the receptacle or calyx of the strawberry or rasp. Judging from one of the specimens, this calyx appears to have consisted of five pieces, which united in a central stem, and were traversed by broad irregularly diverging striae.
And the sp.a.w.n-like patches of Carmylie appear to be simply ill preserved specimens of this fruit, whatever its true character, in which the minute circular portions, divested of the receptacle and stem, had been thrown into irregular forms by the joint agency of pressure and decay.
The great abundance of these organisms,--for so abundant are they, that visitors to the Carmylie quarries find they can carry away with them as many specimens as they please,--may be regarded as of itself indicative of a vegetable origin.[50] It is not in the least strange, however, that they should have been taken for patches of sp.a.w.n. The large-grained sp.a.w.n of fishes, such as the lump-fish, salmon, or sturgeon, might be readily enough mistaken, in even the recent state, for the detached spherical-seed vessels of fruit, such as the bramble-berry, the stone-bramble, or the rasp. "Hang it!" I once heard a countryman exclaim, on helping himself at table to a spoonful of Caviare, which he had mistaken for a sweet-meat, and instantly, according to Milton, "with sputtering noise rejected,"--"Hang it for nasty stuff!--I took it for bramble berry jam."
[Ill.u.s.tration: Fig. 121.
PARKA DECIPIENS.]
[Ill.u.s.tration: Fig. 122.]
[Ill.u.s.tration: Fig. 123.]
Along with these curious remains Dr. Fleming found an organism which in form somewhat resembles the spike of one of the gra.s.ses, save that the better preserved bracts terminate in fan or kidney-shaped leaflets, with a simple venation radiating from the base. It is probably a fern, more minute in its pinnules than even our smallest specimens of true maidenhair. Its stipes, however, seems proportionally stouter than that of any of the smaller ferns with which I am acquainted. But the state of keeping of the specimen is not good, nor do I know that another has yet been found. Further, in the same beds Dr. Fleming found a curious nondescript vegetable, or rather part of a vegetable, with smooth narrow stems, resembling those of the smooth-stemmed organism of the Caithness flagstones, but unlike it in the circ.u.mstance that its detached nearly parallel stalks anastomose with each other by means of cross branches, that unite them in the middle, somewhat in the style of the Siamese twins. I have heard the doctor suggest, but know not whether he has placed the remark on record, that these parallel stems may have been but the internal fibres of some larger plant, whose more succulent portions have disappeared; and certainly, while such instances of anastomosis are rare among the _stems_ of plants, they are common enough among their _internal fibres_, as all who have examined the macerated _debris_ of a kitchen-garden or a turnip-field must have had occasion to remark. We sometimes, however, find cases of anastomosis among the stems of even the higher plants. I have seen oftener than once, in neglected hawthorn hedges, the branch of one plant entering into the stem of another, and becoming incorporated with its substance; and we are told by Professor Balfour, that this kind of chance adhesion is often seen in the branches of the ivy; and that not unfrequently, by a similar process, the roots of contiguous trees are united. Nor does it seem improbable, that what occasionally takes place among the higher plants of the present time may have been common among some of the comparatively low plants of so ancient a period as that of the Middle Old Red Sandstone. This formation of the gray tilestones has furnished one vegetable organism apparently higher in the scale than those just described, in a well marked Lepidodendron, which exhibits, like the Araucarian of the Lower Old Red, though less distinctly, the internal structure. It was found about sixteen years ago in a pavement quarry near Clockbriggs,--the last station on the Aberdeen and Forfar Railway as the traveller approaches the town of Forfar from the north. I owe my specimen of this ancient Lepidodendron to Mr. William Miller, banker, Dundee, an accomplished geologist, who has taken no little trouble in determining its true history. He has ascertained that it occurred deep in the rock, seventy-one feet from the surface; that the beds which rested over it were composed, in the descending order, first, of a conglomerate thirty feet thick; secondly, of a red rock four feet thick; thirdly, of twenty-eight feet of the soft shaly substance known to the quarriers as caulm; and fourthly, of more than nine feet of gray pavement, immediately under which, in a soft, argillaceous stratum, lay the organism. It was about four feet in length, bulged out at the lower end into a bulb-like protuberance, which may have been, however, merely an accidental result of its state of keeping; and threw off, at an acute angle, two branches about a foot from the top. It was covered with a bark of brittle coal, which is, however, wanting in all the fragments that have been preserved; and was resolved internally into a brown calcareous substance of about the hardness of ordinary marble, and very much resembling that into which the petrifactive agencies have consolidated the fossil trees of Granton and Craigleith. From the decorticated condition of the surviving fragments, and the imperfect preservation of the interior structure, in all save the central portions of its transverse sections, it yields no specific marks by which to distinguish it; but enough remains in its irregular network of cells, devoid of linear arrangement, and untraversed by medullary rays, to demonstrate its generic standing as a Lepidodendron.
[It has been questioned whether the lower place in the Old Red System should be a.s.signed to the flagstones of Caithness and Ross, with their characteristic Dipterus and Coccosteus beds, or to the gray tilestones of Forfar and Kincardineshires, with their equally characteristic Cephalaspis. The evidence on the point is certainly not so conclusive as I deemed it fifteen years ago, when our highest authority on the subject not only regarded the tilestone of the Silurian regions of England as a member of the Old Red Sandstone (an arrangement which I am still disposed to deem the true one), but also held further, that there had been detected in this formation near Downtown Castle, Herefordshire, broken remains of _Dipterus macrolepidotus_, one of the best marked ichthyolites of the flagstones of Caithness and Orkney. A great and unbroken series of fossiliferous rocks, with Dipterus at its base, Cephalaspis in its medial s.p.a.ces, and Holoptychius at its top, might well be regarded as the a.n.a.logue of the Old Red of Scotland, with the Caithness flagstones ranged at its bottom, the Cephalaspis beds of Forfarshire placed in its middle, and the Holoptychius beds of Scot-Crag and Clashbinnie on its upper horizon; but since that time the tilestones have been transferred to the Upper Silurian division of rocks, and the evidence furnished by their supposed Dipterus has not been confirmed.
And as the Old Red Sandstones of Scotland have no true fossiliferous base, but rest on primary rocks both to the south and north of the Grampians, it may be regarded as in some degree a moot point whether the lowest fossiliferous beds to the north be older or newer than those to the south, or, what is quite possible, of the same age. Provisionally, however, I have arranged my paper on the supposition that the Coccostean formation of the north is the lowest and oldest of the three; and this from the following considerations. In the first place, the Coccosteus and its contemporaries appear in the north at a very short distance above the base of the system. I have disinterred an Osteolepis from a fish bed near Cromarty only thirty-three feet over the great conglomerate, and only a hundred and twenty-nine feet over the granitic gneiss beneath; whereas the Cephalaspis beds occur high above the primary base of the system in the south,--at some distance over even the thick conglomerate of Stonehaven and Dunnottar; and under this conglomerate, as shown in the section furnished by the valley of the North Esk, there lies a pale red sandstone member of the system, estimated by Colonel Imrie at seven hundred and eighty feet in thickness. The conglomerate itself he estimates at twelve hundred feet.
Adopting as correct Colonel Imrie"s section, taken along the banks of the North Esk,--and the colonel was unquestionably a truthful observer,--the Cephalaspis beds of the south lie nearly two thousand (nineteen hundred and eighty) feet above the Azoic slates on which the Old Red Sandstone of Forfarshire rests, whereas the Coccosteus and Osteolepis beds of the north lie only one hundred and twenty-nine feet over the Azoic gneiss on which the Old Red Sandstone of Cromarty rests.
There is thus at least _room_ in the south for an underlying fossiliferous formation between that of the Cephalaspis and the base of the system, but none in the north beneath that of the Coccosteus and its base. In the north we find the _room_ lying above, between the Coccostean and Holoptychian formations, and represented by that great unfossiliferous deposit of pale sandstone to which the hills of Hoy and the rocks of Duncansbay Head and of Tarbet Ness belong. Further, in the second place, while the upper or Holoptychian formation is found _directly_ overlying that of the Coccosteus in only one locality,--Moray,--we find it directly overlying that of the Cephalaspis in _two_ widely separated localities;--in the vast band of Old Red which runs diagonally across the island from sea to sea, parallel to the Grampian chain, and in the immensely developed Red Sandstones of England and Wales. And it is of course more probable that the two corroborative instances should represent the natural succession of the formations, and the single instance the accidental gap in the scale consequent on the missing formation, than that, _vice versa_, the solitary instance should represent the natural succession, while the two mutually corroborative ones should represent, in localities widely apart, the mere accident of the gap. But, in the third place, I attach more weight to a conclusion founded on the positive character of the groups of organic remains by which the three great formations of the Old Red System are characterized, than I do to either of these considerations. The organisms of the Cephalaspian deposits differ _generically_, and in their whole aspect, from both those of the Coccostean and Holoptychian formations; whereas the organisms of the Coccostean formations, while they resemble generically and in the group those of the Holoptychian one, mainly differ from them _specifically_. The extreme _generic_ difference in the one case argues evidently a great difference in _condition_,--the lesser specific difference in the other, a great difference in point _of time_. The Cephalaspian formation might, as a fresh water formation, be nearly contemporary with either of the other two, or, as seems more probable, interposed between them; while they themselves, on the other hand, generically similar and decidedly marine in their character, must have been so widely separated in time, that all the species of the lower group became extinct ere those of the upper one had been ushered into being. And such are some of the considerations that still lead me, notwithstanding the failure of previous evidence, to hold, at least, provisionally, that our Scottish flagstones to the north of the Grampians occupy a lower horizon than our Scottish tilestones to the south. It must, however, be stated, on the other hand, that the crustaceans of the gray tilestones of Forfar and Kincardine not a little resemble those of the Upper Silurian and red tilestone beds of England; and that, judging from the ichthyodorulites found in both, their fishes must have been at least generically allied. The crustaceans of the upper Silurian of Lesmahagow, too, seem certainly much akin to those of the Forfarshire tilestones.]
[Ill.u.s.tration: Fig. 124.
CYCLOPTERUS HIBERNICUS.]
Above this gray tilestone formation lies the Upper Old Red Sandstone, with its peculiar group of ichthyic organisms, none of which seem specially identical with those of either the Caithness or the Forfarshire beds. For it is an interesting circ.u.mstance, suggestive surely of the vast periods which must have elapsed during its deposition, that the great Old Red System has, as I have just said, its three distinct platforms of organic existence, each wholly different from the others. Generically and in the group, however, the Upper fishes much more closely resemble, I repeat, the fishes of the Lower or Caithness and Cromarty platform, than they do those of the Forfarshire and Kincardine one. The vegetable remains of the Upper formation in Scotland are both rare and ill preserved. I have seen what I deemed fucoidal markings dimly impressed on the planes of some of the strata, not in the carbonaceous form so common in the other two formations, but as mere colored films of a deeper red than the surrounding matrix.
Further, I have detected in the same beds, and existing in the same state, fragments of a striated organism, which may have formed part of either a true calamite, like those of the Coal Measures, or of some such striated but jointless vegetable as that of the Lower Old Red of Thurso and Lerwick.[51] With these markings ferns are occasionally found; and to one of these, from the light which it throws on the true place in the scale of a series of deposits in a sister country, there attaches no little interest. I owe my specimen to Mr. John Stewart of Edinburgh, who laid it open in a micaceous red sandstone in the quarry of Prestonhaugh, near Dunse, where it is a.s.sociated with some of the better known ichthyic organisms of the Upper Old Red Sandstone, such as _Pterichthys major_ and _Holoptychius n.o.bilissimus_. Existing as but a deep red film in the rock, with a tolerably well defined outline, but without trace of the characteristic venation on which the fossil botanist, in dealing with the ferns, founds his generic distinctions, I could only determine that it was either a Cyclopterus or Neuropterus. My collection was visited, however, by the late lamented Edward Forbes, only a few weeks before his death; and he at once recognized in my Berwickshire fern, so unequivocally an organism of the Upper Old Red, the _Cyclopterus Hibernicus_ of those largely developed beds of yellow sandstone which form so marked a feature in the geology of the south of Ireland, and whose true place, whether as Upper Old Red or Lower Carboniferous, has been the subject of so much controversy. I had been previously introduced by Professor Forbes, in the Museum of Economic Geology in Jermyn Street, London, to an interesting collection of plants from these yellow beds, and had an opportunity afforded me of examining the only ichthyic organism hitherto found a.s.sociated with them; and was struck, though I could not identify its species, with its peculiarly Old Red aspect; but the evidence of the Cyclopterus is of course more conclusive than that of the fish; and we may, I think, legitimately conclude, that in Ireland, as in our own country, it was a contemporary of the great Pterichthys (_P. major_),--the hugest, and at least one of the last, of his race,--and gave its rich green to the hill sides of what is still the Emerald Island during the latter ages of the Old Red Sandstone, and ere the Carboniferous period had yet begun. The _Cyclopterus Hibernicus_, as shown both by the Prestonhaugh specimen and those of Ireland, was a bipinnate fern of very considerable size,--probably a tree fern. Its pinnae, opposite in the lower part of the frond, are alternate in the upper; while its leaflets, which are of a sub-rhomboidal form, and so closely ranged as to impinge on each other, are at least generally alternate in their arrangement throughout. Among living plants it seems most nearly represented by a South American species,--_Didymocloena pulcherrima_,--one of the smaller tree ferns.
The leaves of this graceful species are bipinnate, like those of the fossil; and the pinnae (thickly set with simple, alternately arranged leaflets) are opposite in the lower part of the frond, and alternate in the upper. Widely as they are separated in time, the recent South American Didymocloena and the Old Red Sandstone Cyclopterus, that pa.s.sed into extinction ere the times of the Coal, might be ranged together, so far at least as appears from their forms, as kindred species. It were very desirable that we had a good monograph of the Irish Old Red plants, the contemporaries of the latter, as the completest and best preserved representatives of the Middle Palaeozoic flora yet found. Sir Roderick Murchison has figured a single pinnae of this Cyclopterus in his recently published "Siluria;" and Sir Charles Lyell, both that and one of its contemporary Lepidodendra, in the last edition of his "Elements." These interesting fragments, however, serve but to excite our curiosity for more. When urging Professor Edward Forbes on the subject, ere parting from him for, alas! what proved to be the last time, he intimated an intention of soon taking it up; but I fear his purposed monograph represents only one of many works, important to science, which his untimely death has arrested for mayhap long years to come.
In the uppermost beds of the Upper Old Red formation in Scotland, which are usually of a pale or light yellow color, the vegetable remains again become strongly carbonaceous, but their state of preservation continues bad,--too bad to admit of the determination of either species or genera; and not until we rise a very little beyond the system do we find the remains of a flora either rich or well preserved. But very remarkable is the change which at this stage at once occurs. We pa.s.s at a single stride from great poverty to great wealth. The suddenness of the change seems suited to remind one of that experienced by the voyager, when,--after traversing for many days some wide expanse of ocean, unvaried save by its banks of floating sea weed, or, where occasionally and at wide intervals, he picks up some leaf-bearing bough, or marks some fragment of drift weed go floating past,--he enters at length the sheltered lagoon of some coral island, and sees all around the deep green of a tropical vegetation descending in tangled luxuriance to the water"s edge,--tall, erect ferns, and creeping lycopodiaceae, and the panda.n.u.s, with its aerial roots and its screw-like cl.u.s.ters of narrow leaves, and, high over all, tall palms, with their huge pinnate fronds, and their curiously aggregated groups of ma.s.sive fruit. And yet the more meagre vegetation of the earlier time is not without its special interest. The land plants of the Old Red Sandstone seem to compose, all over the world, the most ancient of the terrestrial floras. It was held only a few years ago, that the Silurians of the United States had their plants allied to the Lepidodendron. But the group in which these occur has since been transferred from the Upper Silurian to the Old Red System; and we find it expressly stated by Professor H. D. Rogers, in his valuable contribution to the "Physical Atlas" (second edition, 1856), that "the Cadent [or Lower Old Red] strata are the oldest American formations in which remains of a true terrestrial vegetation have yet been discovered." It has been shown, too, by Sir Roderick Murchison, that the supposed Silurian plants of Oporto are in reality Carboniferous, and owe their apparent position to a reverse folding of the strata. I have already referred to the solitary spore-cases of the Ludlow Rocks; and beneath these rocks, says Sir Roderick (1854), "no remains of plants have been discovered which are recognizably of terrestrial origin." Scanty, too, as the terrestrial flora of the Old Red Sandstone everywhere is, we find it exhibiting very definitely the leading Palaeozoic features. Its prevailing plants are the ferns and their apparent allies. It has in our own country, as has been just shown, its ferns, its lepidodendra, its striated plants allied to the calamites, and its decided araucanite; in America, in the Cadent series, it had its "plants allied to ferns and lepidodendra;" and in the Devonian basin of Sabero in Spain, its characteristic organisms are, a lepidodendron (_L. Chemungensis_), and a very peculiar fern (_Sphenopteris laxus_).[52] But while in its main features it resembled the succeeding flora of the Carboniferous period, it seems in all its forms to have been specifically distinct. It was the independent flora of an earlier creation than that to which we owe the coal. For the meagreness of the paper in which I have attempted to describe it as it occurs in Scotland, I have but one apology to offer. My lecture contains but little; but then, such is the scantiness of the materials on which I had to work, that it could not have contained much: if, according to the dramatist, the "amount be beggarly," it is because the "boxes are empty." Partly, apparently, from the circ.u.mstance that the organisms of this flora were ill suited for preservation in the rocks, and partly because, judging from what appears, the most ancient lands of the globe were widely scattered and of narrow extent, this oldest of the floras is everywhere the most meagre.
LECTURE TWELFTH.
ON THE LESS KNOWN FOSSIL FLORAS OF SCOTLAND.
PART II.
In the n.o.ble flora of the Coal Measures much still remains to be done in Scotland. Our Lower Carboniferous rocks are of immense development; the Limestones of Burdiehouse, with their numerous terrestrial plants, occur many hundred feet beneath our Mountain Limestones; and our list of vegetable species peculiar to these lower deposits is still very incomplete. Even in those higher Carboniferous rocks with which the many coal workings of the country have rendered us comparatively familiar, there appears to be still a good deal of the new and the unknown to repay the labor of future exploration. It was only last year that Mr.
Gourlay[53] of this city (Glasgow) added to our fossil flora a new Volkmannia from the coal field of Carluke; and I detected very recently in a neighboring locality (the Airdrie coal field), though in but an indifferent state of keeping, what seems to be a new and very peculiar fern. It presents at first sight more the appearance of a Cycadaceous frond than any other vegetable organism of the Carboniferous age which I have yet seen. From a mid stem there proceed at right angles, and in alternate order, a series of sessile, lanceolate, acute leaflets, nearly two inches in length by about an eighth part of an inch in breadth, and about three lines apart. Each is furnished with a slender midrib; and, what seems a singular, though not entirely unique, feature in a fern, their edges are densely hirsute, and bristle with thick, short hair, nearly as stiff as p.r.i.c.kles. The venation is not distinctly preserved; but enough remains to show that it must have been peculiar,--apparently radiating outwards from a series of centres ranged along the midrib.
Nay, the apparent hairs seem to be but prolongations of the nerves carried beyond the edges of the leaflets. There is a Stigmaria, too, on the table, very ornate in its sculpture, of which I have now found three specimens in a quarry of the Lower Coal Measures near Portobello, that has still to be figured and described. In this richly ornamented Stigmaria the characteristic areolae present the ordinary aspect. Each, however, forms the centre of a sculptured star, consisting of from eighteen to twenty rays, or rather the centre of a sculptured flower of the composite order, resembling a meadow daisy or sea-aster. The minute petals,--if we are to accept the latter comparison,--are of an irregularly lenticular form, generally entire, but in some instances ranged in two, or even three, concentric lines round the depressed centre of the areolae; while the inters.p.a.ces outside are occupied by numerous fretted markings, resembling broken fragments of petals, which, though less regularly ranged than the others, are effective in imparting a richly ornate aspect to the whole.
[Ill.u.s.tration: Fig. 125.]
[Ill.u.s.tration: Fig. 126.
STIGMARIA.]
[Ill.u.s.tration: Fig. 127.
THE SAME, MAGNIFIED.]
Ever since the appearance, in 1846, of Mr. Binney"s paper on the relations of stigmaria to sigillaria as roots and stems, I have been looking for distinguishing specific marks among the former; and, failing for a time to find any, I concluded that, though the stems of the sigillarian genus were variously sculptured, their roots might in all the species have been the same. The present rich specimen does seem, however, to bear the specific stamp; and, from the peculiar character of the termination of another specimen on the table, I am inclined to hold that the stigmaria may have borne the appearance rather of underground stems than of proper roots. This specimen suddenly terminates, at a thickness of two and a half inches, in a rounded point, abrupt as that of one of the ma.s.sier cacti; and every part of the blunt sudden termination is thickly fretted over with the characteristic areolae. The slim tubular rootlets must have stuck out on every side from the obtuse rounded termination of this underground stem, as we see, on a small scale, the leaflets of our larger club mosses sticking out from what are comparatively the scarce less abrupt terminations of their creeping stems and branches. In at least certain stages of growth the sub-aerial stems of Lepidodendron also terminated abruptly (see Fig. 24); and the only terminal point of Ulodendron I ever saw was nearly as obtuse as that of Stigmaria.
[Ill.u.s.tration: Fig. 128.
STIGMARIA.]
I have been long desirous of acquainting myself with the true character of this latter plant (Ulodendron), but hitherto my labors have not been very successful. A specimen of _Ulodendron minus_, however, now on the table, which I disinterred several years ago from out a bed of ferruginous shale in the Water of Leith, a little above the village of Colinton, exhibits several peculiarities which, so far as I know, have not yet been described. Though rather less than ten inches in length by about three inches in breadth, it exhibits no fewer than seven of those round, beautifully sculptured scars, ranged rectilinearly along the trunk, by which this ancient genus is so remarkably characterized. It is covered with small, sharply relieved, obovate scales, most of them furnished with an apparent midrib, and with their edges slightly turned up; from which peculiarities, and their great beauty, they seem suited to remind the architect of that style of sculpture adopted by Palladio from his master Vitruvius, when, in ornamenting the Corinthian and composite torus, lie fretted it into closely imbricated obovate leaves.
These scales are ranged in elegant curves, not unlike those ornamental curves,--a feat of the turning-lathe,--which one sees roughening the backs of ladies" watches of French manufacture. My fossil exhibited, as it lay in the rock, what I never saw in any other specimen,--a true branch sticking out at an acute angle from the stem, and fretted with scales of a peculiar form, which in one little corner appear also on the main stem, but which differ so considerably from those of the obovate, apparently imbricated type, that, if found on a separate specimen, they might be held to indicate difference of species. It has been shown by Messrs. Lindley and Hutton, on the evidence of one of the specimens figured in the "Fossil Flora," that the line of circular scars so remarkable in this genus, and which is held to be the impressions made by a rectilinear range of almost sessile cones, existed in duplicate on each stem,--a row occurring on two of the sides of the plant directly opposite each other. The branch in my specimen struck off from one of the intermediate sides at right angles with the cones. We already know that these were ranged in one plane; nor, if the branches were ranged in one plane also,--certainly the disposition of branch which would consort best with such a disposition of cone,--would the arrangement be without example in the vegetable kingdom as it even now exists. "Our host," says the late Captain Basil Hall, in his brief description of the island of Java, "carried us to see a singular tree, which had been brought from Madagascar, called familiarly the _Traveller"s Friend_, Urania being, I believe, its botanic name. We found it to differ from most other trees in having all its branches in one plane, like the sticks of a fan or the feathers of a peac.o.c.k"s tail." I may further mention, that the specimen which showed me the abrupt cactus-like terminations of Ulodendron repeated the evidence of Messrs. Lindley and Hutton"s specimen regarding the arrangement of the cone scars on opposite sides, and showed also that these scars ascended to within little more than an inch of the top of the plant.
As there are cases in which the _position_ of a fossil plant may add, from its bearing on geologic history, a threefold interest to the fossil itself, regarded simply as an organism, I may be permitted to refer to a circ.u.mstance already recorded, that there was a well marked Bechera detected about two years ago by Dr. Macbean of Edinburgh, an accomplished naturalist and careful observer, in a thin argillaceous stratum, interposed, in the Queen"s Park, between a bed of columnar basalt and a bed of trap-tuff, in the side of the eminence occupied atop by the ruins of St. Anthony"s Chapel. The stratified bed in which it occurs seems, from its texture and color, to be composed mainly of trappean materials, but deposited and arranged in water; and abounds in carbonaceous markings, usually in so imperfect a state of keeping that, though long known to some of the Edinburgh geologists, not a single species, or even genus, were they able to determine. All that could be said was, that they seemed fucoidal, and might of course belong to any age. The Bechera, however, shows that the deposit is one of the Lower Coal Measures. There was found a.s.sociated with it a tooth of a Carboniferous Holoptychius, whose evidence bore out the same conclusion; and both fossils derive an importance from the light which they throw on the age of the bed of tuff which underlies the stratum in which they occur. At least this trap-rock must be as old as the fossiliferous layer which rests upon it, or rather, as shown by its underlying position, a little older: it must be a trap of the earlier Carboniferous period.
Further, it must have been, not injected among the strata, but poured out over the surface,--in all probability covered at the time by water; and there must have formed over it, ere another overflow of trap took place, a thin sedimentary bed charged with fragments of the plants of the period, and visited, when in the course of deposition, by some of its fishes.
[Ill.u.s.tration: Fig. 129.
SPHENOPTERIS BIFIDA.
(_Burdiehouse._)]
Even among the vegetable organisms of our Coal Measures, already partially described and figured, much remains to be accomplished in the way of restoration. Portions of _Sphenopteris bifida_, for instance, a fern of the Lower Carboniferous rocks have been repeatedly figured; but a beautiful specimen on the table, which exhibits what seems to be the complete frond of the plant, will give, I doubt not, fresh ideas respecting the general framework, if I may so speak, of this skeleton fern, to even those best acquainted with the figures; and an elaborate restoration of its contemporary, _Sphenopteris affinis_ (see frontispiece) which I completed from a fine series of specimens in my collection, will be new, as a whole, to those most familiar with this commonest of the Burdiehouse fossils. From comparisons inst.i.tuted between minute portions of this Sphenopteris and a recent fern, it has been held considerably to resemble a Davallia of the West Indies; whereas it will be seen from the entire frond that it was characterized by very striking peculiarities, exemplified, say some of our higher botanical authorities, to whom I have submitted my restoration, by no fern that now lives. The frond of _Davallia Canariensis_, though unlike in its venation, greatly resembles in general outline one of the larger pinnae of _Sphenopteris affinis_; but these pinnae form only a small part of the entire frond of this Sphenopteris. It was furnished with a stout leafless rachis, or leaf-stalk, exceedingly similar in form to that of our common brake (_Pteris aquilina_). So completely, indeed, did it exhibit the same club-like, slightly bent termination, the same gradual diminution in thickness, and the same smooth surface, that one accustomed to see this part of the bracken used as a thatch can scarce doubt that the stipes of Sphenopteris would have served the purpose equally well; nay, that were it still in existence to be so employed, a roof thatched with it, on which the pinnae and leaflets were concealed, and only the club-like stems exposed, row above row, in the style of the fern-thatcher, could not be distinguished, so far as form and size went, from a roof thatched with brake. High above the club-like termination of the rachis the stem divided into two parts, each of which, a little higher up, also divided into two; these in turn, in at least the larger fronds, also bifurcated; and this law of bifurcation,--a marked, mayhap unique, peculiarity in a fern,--regulated all the larger divisions of the frond, though its smaller pinnae and leaflets were alternate. It was a further peculiarity of the plant that, unlike the brake, it threw off, ere the main divisions of its rachis took place, two pinnae placed in the alternate order, and of comparatively small size. The frond of _Sphenopteris bifida_ was of a more simple form than that of its larger congener, and not a little resembled a living fern of New Zealand, _Coenopteris vivipara_. It was tripinnate; its secondary stems were placed directly opposite on the midrib, but its tertiary ones in the alternate arrangement; and its leaflets which were also alternate, were as rectilinear and slim as mere veins, or as the thread-like leaflets of asparagus. Like the fronds of Coenopteris when not in seed, it must have presented the appearance of the mere macerated framework of a fern.
I need scarce remark that, independently of the scientific interest which must attach to restorations of these early plants, they speak powerfully to the imagination, and supply it with materials from which to construct the vanished landscapes of the Carboniferous ages. From one such restored fern as the two now submitted to the a.s.sociation, it is not difficult to pa.s.s in fancy to the dank slopes of the ancient land of the Lower Coal Measures, when they waved as thickly with graceful Sphenopteres as our existing hill sides with the common brake; and when every breeze that rustled through the old forests bent in mimic waves their slim flexible stems and light and graceful foliage.
In 1844, when Professor Nicol, of Marischal College, Aberdeen, appended to his interesting "Guide to the Geology of Scotland," a list of the Scottish fossils known at the time, he enumerated only two vegetable species of the Scotch Oolitic system,--_Equisetum columnare_ and _Pinites_ or _Peuce Eiggensis_; the former one of the early discoveries of our distinguished President, Sir Roderick Murchison; the latter, of the late Mr. William Nicol of Edinburgh. Chiefly from researches in the Lias of Eathie, near Cromarty, and in the Oolites of Sutherland and the Hebrides, I have been enabled to increase the list from two to rather more than fifty species,--not a great number, certainly, regarded as the sole representative of a flora; and yet it may be deemed comparatively not a very small one by such as may remember, that in 1837, when Dr.
Buckland published the second edition of his "Bridgewater Treatise,"
Adolphe Brogniart had enumerated only seventy species of plants as occurring in all the Secondary formations of Europe, from the Chalk to the Trias inclusive. In a paper such as the present I can of course do little more than just indicate a few of the more striking features of the Scottish flora of the middle Secondary ages. Like that of the period of the true Coal, it had its numerous coniferous trees. As shown by the fossil woods of Helmsdale and Eigg, old Oolitic Scotland, like the Scotland of three centuries ago, must have had its mighty forests of pine;[54] and in one respect these trees seem to have more nearly resembled those of the recent pine forests of our country than the trees of the coniferous forests of the remote Carboniferous era. For while we scarce ever find a cone a.s.sociated with the coniferous woods of the Coal Measures,--Lindley and Hatton never saw but one from all the English coal fields, and Mr. Alexander Bryson of Edinburgh, only one from all the coal fields of Scotland,--tree-cones of at least four different species, more probably of five, are not rare in our Scottish deposits of the Lias and Oolite. It seems not improbable that in the Carboniferous genera Pinites, Pitus, and Anabathra, which approach but remotely to aught that now exists, the place of the ligneous scaly cone may have been taken, as in the junipers and the yews, by a perishable berry; while the Pines and Araucarians of the Oolite were, like their congeners in recent times, in reality coniferous, that is, cone-bearing trees. It is another characteristic of these Secondary conifers, that while the woods of the Palaeozoic periods exhibit often, like those of the tropics, none of the dense concentric lines of annual growth which mark the reign of winter, these annual lines are scarce less strongly impressed on the Oolitic woods than on those of Norway or of our own country in the present day. In some of the fossil trees these yearly rings are of great breadth; they seem to have sprung up in the rich soil of sheltered hollows and plains, and to have increased in diameter from half an inch to three quarters of an inch yearly; while in other trees of the same species the yearly zones of growth are singularly narrow,--in some instances little more than half a line in thickness.
Rooted on some exposed hill side, in a shallow and meagre soil, they increased their diameter during the twelvemonth little more than a line in the severer seasons, and little more than an eighth part of an inch even when the seasons were most favorable. Further, whether the rings be large or small, we ordinarily find them occurring in the same specimens in groups of larger and smaller. In one of my Helmsdale specimens, indicative generally of rapid growth, there are four contiguous annual rings, which measure in all an inch and two twelfths across, while the four contiguous rings immediately beside them measure only half an inch.
"If, at the present day," says a distinguished fossil botanist, "a warm and moist summer produces a broader annual layer than a cold and dry one, and if fossil plants exhibit such appearances as we refer in recent plants to a diversity of summers, then it is reasonable to suppose that a similar diversity formerly prevailed." The same reasoning is of course as applicable to _groups_ of annual layers as to _single_ annual layers; and may we not venture to infer from the almost invariable occurrence of such groups in the woods of this ancient system, that that ill-understood law of the weather which gives us in irregular succession groups of colder and warmer seasons, and whose operation, as Bacon tells us, was first remarked in the provinces of the Netherlands, was as certainly in existence during the ages of the Oolite as at the present time?
[Ill.u.s.tration: Fig. 130.
CONIFERS?]
[Ill.u.s.tration: Fig. 131.
CONIFER TWIGS.]
Twigs which exhibit the foliage of these ancient conifers seem to be less rare in our Scotch deposits than in those of England of the same age. My collection contains fossil sprigs, with the slim needle-like leaves attached, of what seem to be from six to seven different species; and it is worthy of notice, that they resemble in the group rather the coniferae of the southern than those of the northern hemisphere. One sprig in my collection seems scarcely distinguishable from that of the recent _Altingia excelsa_; another, from that of the recent _Altingia cunninghami_. Lindley and Hutton figure in their fossil flora a minute branch of _Dacrydium cupressinum_, in order to show how nearly the twigs of a large tree, from fifty to a hundred feet high, may resemble some of the "fossils referable to Lycopodiaceae." More than one of the Oolitic twigs in my collection are of a resembling character, and may have belonged either to cone-bearing trees or to club mosses.
Respecting, however, the real character of at least one of the specimens,--a minute branch from the Lias of Eathie, with the leaflets attached,--there can be no mistake. The thicker part of the stem is in such a state of keeping, that it presents to the microscope, in a sliced preparation, the internal structure, and exhibits, as in recent coniferous twigs of a year"s growth, a central pith, a single ring of reticulated tissue arranged in lines that radiate outwards, and a thin layer of enveloping bark. Nothing, then, can be more certain than that this ancient twig, which must be accepted as representative of the foliage of whole forests of the Secondary ages in Scotland, formed part of a conifer of the Lias; and the foliage of several of the other twigs, its contemporaries, though I have failed to demonstrate their true character in the same way, bear a scarce less coniferous aspect. The cones of the period, from the circ.u.mstance that they are locked up in a hard limestone that clings closely around their scales, and from the further circ.u.mstance that the semi-calcareous lignite into which they are resolved is softer and less tenacious than the enclosing matrix, present, when laid open, not their outer surfaces, but mere sections of their interior; and give, in consequence, save in their general proportions and outline, but few specific marks by which to distinguish them. We see, however, in some cases in these sections what would be otherwise unseen,--the flat naked seeds lying embedded in their hollow receptacles between the scales, and in as perfect a state of keeping as the seeds of recent pines that had ripened only a twelvemonth ago. Had not the vitality of seeds its limits in time, like life of all other kinds, one might commit these perfect fossil germs to the soil, in the hope of seeing the old extinct forests called, through their agency, a second time into existence. Of three apparent species of cones which occur in the Eathie Lias, the smallest seems to have resembled in size and appearance that of the Scotch fir; the largest, which consisted from bottom to top, as seen in section, of from nine to ten scales, appears to have been more in the proportions of the oblong oval cones of the spruce family; while a cone of intermediate length, but of considerably greater breadth, a.s.sumed the rounded form of the cones of the cedar. I have found in the same deposit what seems to be the sprig of a conifer, with four apparently embryo cones attached to it in the alternate order.