Interior of the boulder Wide intervals of
Geology--Illustration--Long interval between the
formation of the boulder as part of a sand-bed,
and its striation by glacial action--Sketch of the
intervening ages--The boulder a Lower Carboniferous
rock--Cycles of the astronomer and the geologist
contrasted--Illustration--Plants shown by the boulder once
grew green on land--Traces of that ancient land--Its seas,
shores, forests, and lakes, all productive of material
aids to our comfort and power--Plants of the Carboniferous
era--Ferns--Tree-ferns--Calamites--Asterophyllites--
Lepidodendron--Lepidostrobus--Stigmaria--Scene in a ruined
palace--Sigillaria--Coniferæ, Cycadeæ--Antholites, the oldest
known flower--Grade of the Carboniferous flora--Its resemblance
to that of New Zealand.
I have likened the boulder to an old volume of the middle ages encased
in a modern binding. We have looked a little into the mechanism and
history of the boards; in other words, we have gone over the history of
the scratched surface of the boulder, of the clays and sands around it,
and of that still earlier cycle of denudation whereof the rock itself
is probably a relic. Before proceeding to open the volume itself, it
will be well that we clearly mark the wide interval in time between
the ages represented by the surface-striation and those indicated by
the interior of the boulder. When we proceed from the groovings on the
outside to the plants within, we pass, to be sure, over scarcely an
inch of space, but we make a leap over untold millenniums in point of
time. It is as if we had laid our hands on a volume of history which
had by some misfortune found its way into the nursery. The first page
that catches our eye relates the battle of the Reform Bill, and, on
turning the previous leaf, we find ourselves with Boadicea and her
woad-coloured soldiery. Now, if one utterly ignorant of the chronology
of the country were to be told that the volume related solely to one
people, he would at once see from the manners and customs delineated,
that the two pages referred to very different states of civilisation,
and consequently to widely-separated periods. But he could give no
account of how long an interval might have elapsed between the time
when London had its inhabitants massacred by Boadicea, and the time
when another generation of them was excited by the tardiness of King
William iv. He could form no conjecture as to what events might have
happened in the meanwhile. The interval might be a century or twenty
centuries, wherein the city might have been burnt down fifty times.
Clearly, if he wished to make himself acquainted with the intervening
history, he would have to betake himself to an unmutilated volume.
And just so is it with our boulder. We can easily believe, merely from
looking at it as it lies on its clayey bed, that a long time must have
elapsed between the time of its formation as part of a sandstone bed,
and the period of its transportation and striation by an iceberg. The
sand of which it is formed must have been washed down by currents, and
other sediment would settle down over it. It would take some time to
acquire its present hardness and solidity, while, in long subsequent
times, after being broken up and well-rounded by breaker or current
action, it may have lain on some old coast-line for centuries before it
was finally frozen into an ice-floe, and so freighted to a distance.
But the stone, with all its stories of the olden time, can tell us
nothing of this intervening period. It leads us from a dreary frozen
sea at once into a land of tropical luxuriance, and so, if we desire to
know anything of the missing portion of the chronology, we must seek it
elsewhere.
The Boulder-clay is one of the latest of geologic periods.[16] Beyond
it we get into Tertiary times, and learn from the caves of Yorkshire
how elephants, hyenas, rhinoceroses, hippopotami, bears, and wolves,
prowled over the rich valleys; while, from the quarries of the Isle of
Wight, we see how at an earlier time herds of uncouth palæotheres and
slimly-built anoplotheres browsed the plains of Old England. Beyond
the Tertiary ages come those of the Chalk, with its ocean that swarmed
with sea-urchins, terebratulæ, pectens, sponges, and many other forms.
Then arises the era of the Wealden, with its bosky land haunted by the
unwieldy iguanodon; the Oolite, with its land rich in a coniferous
flora, and tenanted by a race of small marsupial animals, and its seas
abounding in corals, encrinites of many a form, cidares, cuttle-fishes,
and ammonites. Further back still, come the times of the Lias, that
strange era in the history of our country, when reptiles huger than
those of the Nile swam the seas, and sped on wings through the air.
Then come the times of the Trias, when a vegetation still further
removed from existing types clothed the land, and frogs large as oxen
waddled along the shores. Then the times of the Permian, with its deep
sea tenanted by a meagre list of corals and shells, and by a type of
fishes that was slowly passing away. We arrive at last at the Coal or
Carboniferous period, to the older ages of which our boulder belongs.
[Footnote 16: For the names and succession of the rocks of which the
known part of the earth's crust is composed, see the Table at the end
of the volume.]
These eras may have been some longer, some shorter, but each had a
duration which, when tried by human standards, must be regarded as
immensely protracted. The cycles of astronomy are very vast, yet
I have often thought that the cycles of geology, though probably
of much less duration, impress us more forcibly with the antiquity
of our planet. The astronomer tells us of light that has taken two
millions of years to reach our earth, and of nebulæ that are millions
upon millions of miles distant, but these numbers are so vast that
we cannot bring ourselves to realize them. We _know_ that there is a
great difference between two millions and ten millions, but we cannot
fully _appreciate_ it, and so the periods of the astronomer, beyond a
certain point, cease adequately to impress us. So long as they can be
easily contrasted with our own standards of comparison, they have their
full force; but after that, every additional million, or ten millions,
or ten hundred millions, produces only a confused and bewildered sense
of immensity, and the comparative amount of each addition fails to be
realized. Will my reader forgive a homely illustration:--Some years
ago, I stood at the pier-head of one of our smaller sea-port towns, and
watched the sun as it sullenly sank behind the outline of the opposite
hills. The breadth of the channel, in the direction of sunset, was
several miles, but in the flush of evening one fancied he could almost
have thrown a stone across. The water lay unruffled by a ripple, and
reflected all the thousand varying tints that lighted up the sky. The
harbour, that had been a busy scene all evening, began to grow less
noisy, as one by one the herring-boats pushed out to sea. I found it
not a little interesting to mark, as the boats gained the open firth,
how the opposite coast-line gradually seemed to recede. The farther the
dark sails withdrew, the more remote did the adjacent shores appear,
until, as the last tinge of glory faded from the clouds, and a cold
grey tint settled down over the landscape, the hills lay deep in shade
and stretched away in the twilight as a dark and distant land from
whose valleys there rose troops of stars. The coast-line, as seen in
early evening, reminded me of the periods of the astronomer; as seen in
early night, it reminded me of the periods of the geologist. We fail to
appreciate the real duration of astronomical cycles, because they are
presented to us each as one vast period. They are not subdivided into
intervals, and contain no succession of events, by means of which, as
by milestones, we might estimate their extent; and so their unvaried
continuity tends to diminish the impression of their vastness, just
as the firth, without any islet or vessel on its surface, seemed
greatly narrower than it really was. For it is with time as it is with
space--the eye cannot abstractly estimate distance, nor can the mind
estimate duration. In either case, the process must be conducted by a
comparison with known standards. The geological periods exemplify the
same rule. They may not be greater, perhaps not so great, as those
revealed by astronomy, yet their vastness impresses us more, because we
can trace out their history, and see how step by step they progressed.
Thus, that the interval between the boulder-clay and the coal-measurer
was immense, we learn from the records of many successive ages that
intervened, in the same way that one began to perceive the real breadth
of the firth, by resting his eye on the succession of intervening
herring-boats. In the former case, the mind has ever and anon a sure
footing on which to pause in gauging bygone eternity; in the latter,
the eye had likewise a succession of points on which to rest in
measuring distance. Or, to return to a former illustration: Boadicea
lived eighteen hundred years ago, but who does not feel that the last
nine hundred years look a great deal longer than the first? The one
set has few marked incidents to fix the thoughts; the other is replete
with those of the most momentous kind. In the one, we have M meagre
list of conquerors and kings, from Julius Cæsar down to Athelstan; in
the other, events crowd upon us from the waning of the Saxon power down
through the rising glory of our country to the present plenitude of
its power and greatness. The early centuries, like the cycles of the
astronomer, pass through our mind rather as one continuous period; the
later centuries, like the cycles of the geologist, arrest our thoughts
by a succession of minor periods, and hence the idea of duration is
more vividly suggested by the diversified events of the one series,
than by the comparatively unbroken continuity of the other.
Let us now open the volume and try to decipher the strange legends
which it contains. On removing some of the upper layers of the boulder,
I found, as I have said, well-preserved remains of several kinds of
plants. One of them was ribbed longitudinally, with transverse notches
every three or four inches, us though a number of slender threads
had been stretched along a rod, and tied tightly to it at regular
intervals. Another, sorely mutilated, was pitted all over somewhat
after the fashion in which the confectioner punctures his biscuits. A
third had a more regular pattern, being prettily fretted with small
lozenge-shaped prominences that wound spirally round the stalk. Other
plants seemed to be present, but in a very bad state of preservation.
They were all jumbled together and converted into a black coaly
substance, in which no structure could be discerned.
These plants assuredly once grew green upon the land; but where now is
that land on which they flourished? Had it hills and valleys, rivers
and lakes, such as diversify our country? Was it tenanted by sentient
beings, and, if so, what were their forms? Did insects hum their way
through the air, and cattle browse on the plains, and fish gambol in
the rivers? Was the land shaded with forests, dark and rugged like
those of Norway, or fragrant as the orange-groves of Spain? What, in
fine, were its peculiar features, and how far did its scenery resemble
that of any country of the present day?
That old land has not entirely disappeared. Traces of it are found
pretty extensively in South Wales, in Staffordshire, around Newcastle,
and through central Scotland. Strange as it may seem, its forests
are still standing in many places. The fishes that disported in its
lakes, the insects that fluttered amid its woods, and the lizards that
crawled among its herbage, are still in part preserved to us. Nay,
more; we may sometimes see the sea-beaches of that ancient land pitted
with rain-drops, and roughened with ripple-marks, as freshly as if the
shower had fallen and the tide had flowed only yesterday. The peasants
along the Bay of Naples gathered grapes from the flanks of Vesuvius
for well-nigh seventeen centuries, before it was ascertained that they
daily walked over the site of buried cities, with temples, theatres,
and private houses still erect. It was many more centuries ere the
people of Great Britain discovered that not a few of their villages and
towns stood on the site of buried forests, and lakes, and seas. We have
now, however, become aware of the fact, and are making good use of it.
We dig into the earth and exhume these old forests to supply us with
light and fuel; we quarry into the ripple-marked shores which fringed
that old land, and build our houses with the hardened sand; we calcine
the ferruginous mud that gathered in its swampy hollows, and extract
therefrom our most faithful ally both in peace and war--metallic iron;
we burn the delicate corals and shells and lily-like zoophytes which
lived in the sea of that far-distant era, to enable us to smelt our
iron, to build our houses, and manure our fields; in short, every year
we are discovering some new and valuable material in the productions
of that period, or finding out some new use which can be made of the
substances already known. A more than ordinary interest, therefore,
attaches to the history of the land and sea which have furnished us
with so many aids to comfort as well as power; and we shall find, as we
go on, that that history is a very curious one.
I shall describe some of the more common plants and animals of the
period, that we may be able, in some measure, to look back through the
ages of the past, and see how these plants would appear when they cast
their broad shadow over river and lake, and how these animals would
have seemed to human eye in the twilight of the forest, in the sluggish
flow of the river, and in the stagnant waters of the lagoon.
The _Flora_, or vegetation of the Carboniferous era, differed
widely from any that now exists. With the exception of the highest
or exogenous class, it possessed representatives of all the existing
classes of the botanic scale, but in very strange proportions. The
number of species of carboniferous plants already found in Great
Britain amounts to about three hundred, amongst which the ferns are
especially abundant. Some of them seem to have been low-growing
plants, like the bracken of our hillsides, but others must have shot
up to the height of forest trees. We can recognise a few coniferous
and cycadaceous plants, a good many stems resembling the "horse-tail"
of our marshy grounds, and some of large size akin to the creeping
club-moss of our heaths; but there are still many to which there exist
no living analogues.
When we examine the roof of a coal-pit, or split open plates of shale
in a quarry of the coal-measures, we are struck with the similarity
which the ferns in the stone bear to those among our woods and hills.
One of the most common, and, at the same time, most elegant forms,
is the _Sphenopteris_ or wedge-leaved fern, of which a large list
of species is known. One of them (_S. crenata_) had a strong stem,
from which there sprung straight tapering branches richly dight with
leaflets. The leaflets--somewhat like minute oak-leaves--were ranged
like those of our modern ferns, along two sides of the stalk, in
alternate order, and tapered gently away to its outer extremity. The
effect of the whole is singularly rich, and one can well believe that a
garland of this ancient fern would have wreathed as gracefully around a
victor's brow as the parsley of Nemea or the laurel-leaves of Delphi.
Another plant of the same genus (_S. affinis_, Fig. 3) has leaflets
like the petals of the meadow-daisy, arranged in clusters along its
slim diverging stalks. From a collection and comparison of many
specimens, the late lamented Hugh Miller was enabled to make a drawing
of this fern as it must have appeared when it waved green along the
old carboniferous hill-sides. I enjoyed the privilege of going over
these specimens with him, and marked how, under a master-hand, piece
by piece fell into its proper place, and yielded up its evidence. His
restoration, which forms the frontispiece to his last work, is a very
beautiful one, and it is as true as it is beautiful.
[Illustration: Fig. 3. Sphenopteris affinis.]
[Illustration: Fig. 4. Pecopteris.]
[Illustration: Fig. 5. Cyclopteris.]
[Illustration: Fig. 6. Neuropteris.]
The _Pecopteris_ (Fig. 4, _P. heterophylla_) or comb-fern, is so called
from its stiff thick leaflets being in some species arranged along the
stalk like the teeth along the centre of a comb. Of all the plants
of the coal-measures this is the one that approaches most closely to
living nature. It appears to be almost identical with the _pteris_,
of which one species is well known as the bracken of our hill-sides.
Dr. Hooker figures together a frond of a New Zealand species (_P.
esculenta_) and a fossil frond from the Newcastle pits. They are
so similar as to be easily mistaken at first sight for drawings of
the same plant.[17] The _Neuropteris_ (as _N. gigantea_, Fig. 6) or
nerve-leaved fern, is remarkable for its strongly-defined venation. It
is scarcely, perhaps, so elegant in its outline as the _sphenopteris_,
or some of the other ferns. Its leaflets are large and thick, with an
oblong or rounded form, and arranged either singly along the frond
stem, or along secondary foot-stalks, which diverge from the main stem.
Of the latter kind, some of the species have a good deal of resemblance
to our _Osmunda regalis_ or royal fern. A species of the former class
(_N. cordata_) might readily enough be mistaken for the young leaves
of the _Scolopendrium_ or hart's-tongue, which hangs out its glossy
green amid the gloom of dank and dripping rocks. There are, besides,
several other genera of ferns in the Carboniferous strata, such as the
_Cyclopteris_ (_C. dilatata_, Fig. 5) or round-leaved fern, and the
_Odontopteris_ or tooth-fern. Most of these seem to have been lowly
plants, like the ferns of our own country. But there was another class
to which no analogue can be shown in Europe. They rose high over their
humbler congeners as lofty trees, and must be studied by a reference to
the existing tree-ferns of intertropical countries.
[Footnote 17: Hooker, _Mem. Geol. Surv._ vol. ii. part ii. p. 400.]
[Illustration: Fig. 7. Living Tree-fern.]
Tree-ferns flourish in warm climates, and are met with in Brazil, the
East and West Indies, New Zealand, &c. They rise sometimes to the
height of fifty or sixty feet, with a long tapering stem surmounted
by a dense crown of graceful fronds, and might easily be mistaken at
a little distance for palms. All the known species belong to the same
division (_Polypodiaceæ_) with the common polypodium of our road-sides.
In some genera, as the _alsophila_ of the East Indies, the trunk is
ribbed by long creeping branches, or rather rootlets, which descend to
the soil, giving the tree somewhat of the appearance so often seen in
old woods, where venerable fir-trees have been firmly encased by the
bearded stems of the ivy. Another genus, the _Cyathea_, has its stem
covered with oblong scars where leaves were attached, and a circle of
rich outspread fronds surmounts its summit. One of the coal-measure
tree-ferns seems to have resembled this recent type. It is named
the _Caulopteris_ or stalk-fern, and had a thick stem picturesquely
roughened by irregular oblong leaf-scars, that wound spirally from its
base to its point. No specimen has hitherto been found showing the
fronds in connexion with the stem, so that we are still ignorant of
the kind of foliage exhibited by this ancient tree. There can be no
doubt, however, that it was crowned with a large tuft of boughs that
cast their shadow over the sward below, and we may, perhaps, believe
that some of the numerous detached ferns found in the shales of the
coal-series, once formed part of this lofty coronal.
An important section of the carboniferous plants is embraced under
the generic name of _Calamites_. They had smooth jointed stems, like
reeds, and terminated beneath in an obtuse curved point (Fig. 8),
from which there sprang broad leaflets or rather rootlets. After many
years of research our knowledge of these plants is still very scanty.
Some of them have exhibited a highly-organized internal structure,
from which it appears that they consisted--first, of a soft central
cellular pith; second, of a thick layer of woody tissue; and third, an
external cylinder of strong bark, ribbed longitudinally, and furrowed
transversely. They have been ranked with the common horse-tail of our
ponds, but they would rather appear to belong to a higher family. The
breadth of the stem is very various, some specimens being a foot or
more in diameter, others scarcely half an inch. From the discoveries of
Professor Williamson and Mr. Binny of Manchester, it seems not unlikely
that what we call calamites may be really the inner core of a plant not
yet named, just as a set of fossils were long called _sternbergiæ_,
before they were discovered to be really the pith of coniferous trees.
With regard to the branches of the calamites, Brongniart's conjecture
may be true, that they exist among the group of plants called
_asterophyllites_. It is not unlikely that many dissimilar plants have
been grouped together as calamites, and, on the other hand, that plants
allied to the typical species have been thrown into separate genera.
For it requires but a slight acquaintance with the vegetable kingdom to
know how many forms analogous parts of the same plant may assume, and
how impossible it would often be to guess the real relationship of such
varieties if they were not found growing together on one plant.
[Illustration: Fig. 8. Terminal portion of a calamite stem.]
[Illustration: Fig. 9.[18]]
[Footnote 18: The fossil given in Fig. 9 is named by Lindley (_Foss.
Flo._ t. 15, 16), _Calamites nodosus_. He admits, however, that it was
not found in actual contact with a calamite stem. It has exactly the
contour of an asterophyllite, and might, perhaps, be referred to that
genus. It is inserted here that the reader may see the general form of
the asterophyllites, and the close relationship that subsists between
these plants and the calamites.]
A remarkably graceful class of the coal-plants are known as
_asterophyllites_. They had slim fluted and jointed stalks, apparently
of humble growth. From each of the joints there sprang two thin
opposite branches with stellate clusters of leaflets arranged round
them at equal distances. If the reader will take a young rush-stalk,
and string along it a number of the flowers of the little star-wort,
keeping them a little distance apart, he may form some idea of the
appearance of a single branch of the star-bearing _asterophyllite_.
Some of the plants embraced under this genus are conjectured to have
been aquatic, spreading out their clusters of leaflets in the green
sluggish water of stagnant pools; but many of them are evidently
related to the calamites, and may possibly have formed part of these
plants.
Whoever has rambled much in a coal-country, scrambling through briars
and brambles in old quarries, or threading his way among the rocks
of river-courses, must often have noticed, on the exposed surface of
sandstone blocks, dark ribbon-like bands fretted over with little
diamond-shaped knobs. They are so common in some districts, that you
can scarcely light upon a piece of sandstone which does not show one
or more. They belong to a carboniferous plant known as _lepidodendron_
(Fig. 10) or scaly tree, from the peculiar style of ornamentation which
adorned its bark. Its structure and affinities have puzzled botanists
not a little. A well-preserved specimen reminds one of the appearance
presented by a twig of the Scotch fir, when stripped of its green
spiky leaflets. The scars thus left at the base of the leaflets are
of a wedge-like form, and run spirally up the branch in a manner very
like those on the branches of lepidodendron; and it was accordingly
supposed at one time that the latter plant belonged, or at least was
allied, to the conifers. But the branches of lepidodendron possessed a
peculiarity that is shared in by none of our present coniferous trees.
They were what botanists call _dichotomous_,--that is, they subdivided
into two equal branches, these again into other two, and so on. Their
internal texture,[19] too, differed from that of any known conifer.
The only tribe of existing plants with which the lepidodendron seems
to bear comparison, are the _Lycopodiaceæ_, or club-mosses, of which
we have several species in the moor-lands of our own country. They
are low trailing plants, with moss-like scaly branches, bearing at
their ends shaggy little tufts, whence the popular name of the genus.
In warmer climates, they are both more numerous and attain a larger
size, sometimes standing erect to about the height of an ordinary
gooseberry-bush. But though the lepidodendron appears to have been
allied to these plants in structure, it greatly differed from them in
dimensions. The club-mosses of the coal-measures shot up as goodly
trees, measuring fifty feet and upwards in height, and sometimes
nearly five in diameter. Their general effect must have been eminently
picturesque. A shaggy covering of green spiky leaflets bristled over
their multitudinous pendant boughs; and where on the older stems these
leaflets had decayed and dropped off, the outer bark was laid bare,
fretted over with rows of diamond-shaped or oval scars, separated
by waving lines of ridge or furrow, that wound spirally round the
stem. From not a few of the branches there sprang oblong hirsute
cones called _lepidostrobi_ (Fig. 11), which bore the sporangia, or
seed-cases. These cones are of frequent occurrence in the shales of
the coal-measures, and may be readily recognised. They had a central
axis round which the oblong sporangia were built, the whole being
protected externally by a thick covering of pointed scales, imbricated
like the cone of the Scotch fir. The leaflets of lepidodendron, called
_lepidophylla_, were broader than those of the Scotch fir, and had a
stout mid-rib, which must have given them a rigidity like that of the
araucarian pine a plant they may also have resembled in the dark glossy
green of its leaves.
[Footnote 19: See Hooker, _Mem. Geol. Surv._ vol. ii. part ii. p. 436.]
[Illustration: Fig 10. Lepidodendron Sternbergii.]
[Illustration: Fig. 11. Lepidostrobus.]
Of all the common coal-measure plants, there is perhaps none so
abundant as that known by the name of _stigmaria_, or punctured-stem.
It is found spreading out its rootlets for several yards in beds of
shale and under-clay, and sometimes even limestone,[20] while, in
many sandstones, fragments of its blackened stems lie as thickly
strewn as twigs among the woods in autumn. I have said that several
of the plants above described have greatly puzzled botanists. None
of them, perhaps, has given rise to so much conjecture and variety
of opinion as the stigmaria. The history of the discussion regarding
its nature and affinities, would be not a little interesting as an
illustration of the slow hindered progress often attendant on the
researches of science, and an instance of how a few simple facts are
sometimes enough to overturn the most plausible theories and probable
conjectures. Many thousands of specimens had been examined ere one was
found that revealed the true nature of the stigmaria. It was by some
imagined to be a soft succulent marshy plant, consisting of a number
of long branches radiating from a sort of soft disk, like spokes from
the centre of a wheel. Analogies were suggested with dicotyledonous
tribes, as the _cacti_ and _euphorbiæ_, though it was at the same time
admitted that the ancient plant presented appearances which seemed very
anomalous.
[Footnote 20: The fresh-water limestone of Mid-Calder abounds in
long trailing stems and rootlets of stigmaria, mingled with other
terrestrial plants, and shells of _cyprides_.]
[Illustration: Fig. 12 Stigmaria rootlets springing from Sigillaria
stem.]
In the course of an extensive survey of the coal-field of South
Wales, Mr. (now Sir William) Logan ascertained the important fact,
that each coal-seam is underlaid by a bed of clay, in which the stems
of stigmaria, branching freely in all directions, may be traced to
the distance of many feet or even yards. They were recognised as
undoubtedly occupying the site on which they grew, and consequently
each coal-seam was held to rest upon an ancient soil. Some years
afterwards, in making a cutting for the Lancaster and Bolton Railway,
several upright massive stems belonging to a plant called _sigillaria_,
were found to pass downwards into true stigmaria stems (Fig. 12).
There could be no doubt that they were different parts of one and the
same plant. This fact has since been abundantly demonstrated from the
Nova Scotia coal-field. Many sigillariæ have been found there passing
down into the fire-clay below, where they branch out horizontally as
true stigmatiæ. It is evident, therefore, that the stigmaria was the
under-ground portion of a plant, which, judging from the nature of the
soil, and the free mode in which the tender rootlets branched off,
appears to have lived in aquatic or marshy stations.
[Illustration: Fig. 13. Stigmaria.]
The stigmaria is too well marked to be readily confounded with any
other coal-measure plant. It had a rounded stem, seldom more than
four or five inches across, which was marked by a series of circular
tubercules with a puncture in the centre, arranged in spiral lines
round the stem. Each of these tubercules is surrounded, in ordinary
specimens, by a circular depression,[21] and the whole plant (if
one may use the comparison) looks as if it had been smitten with
small-pox. From the hollow in the centre of each protuberance, there
shot out a long round rootlet, formerly thought to be a leaf, and
since the tubercules are pretty thickly set, the stigmaria must have
had a somewhat hirsute appearance as it crept through the mud. It
would resemble a thick bearded stem of ivy, save that the fibres,
instead of running up two sides, were clustered all round it. Along
the centre of the root, there ran a woody pith of a harder and more
enduring texture than the surrounding part of the plant. The space
between the outer tuberculed rind and the inner pith, seems to have
been of a soft cellular nature, and to have decayed first, for the pith
is sometimes hollow, and may not unfrequently be seen at a distance
from the centre, and almost at the outer bark--a circumstance that
seems only explicable on the supposition, that while the surrounding
portions were decaying, the firmer pith altered its position in the
hollow stem, sinking to the lower side, if the plant lay prostrate, and
that it did not itself begin to decay until the interior of the stem
had been at least partially filled up with sand or mud, or fossilized
by the infiltration of lime. From the root of the sigillaria, which
has a curious cross-shaped mark on its base, the stems of stigmaria
strike out horizontally, first as four great roots which subdivide as
they proceed. Their subdivisions are dichotomous, each root splitting
equally into two, and thus they want that intricate interlacing of
rootlet which is so familiar to us. The whole disposition of these
under-ground stems is singularly straight and regular, leading us to
believe that they shot out freely through a soft muddy soil.
[Footnote 21: Such is the usual aspect of the plant. But as the stems
have been, for the most part, greatly flattened by the pressure of
the superincumbent rocks, the sharpness of the pattern has been much
effaced. In some specimens described by Dr. Hooker, as having been
found in an upright position, the external ornamentation presents an
appearance somewhat different. What in the common specimens stand out
as tubercules, are there seen to be deep circular cavities, in which
the shrunk flagon-shaped bases of the rootlets are still observable.
(See above, Fig. 13 _b_, which is taken from one of Dr. Hooker's
plates. For a detailed description of the structure of stigmaria, see
the paper above referred to in the _Geological Survey Memoirs_.) A
very ornate species is mentioned by the late Hugh Miller, in which
each tubercule formed the centre of a sculptured star, and the whole
stem seemed covered over with flowers of the composite order. And what
is, perhaps, still more curious, the stem was seen to end off 7 in an
obtuse point, tuberculed like the rest of the plant.--_Testimony of the
Rocks_, p. 461.]
Some time ago I chanced to visit the remains of what had once been
a royal residence, and still looked majestic even in decay. It gave
a saddened pleasure to thread its winding stairs, and pass dreamily
from chamber to hall, and chapel to closet; to stand in its gloomy
kitchens, with their huge fire-places, whose blackened sides told of
many a roaring fagot that had ruddied merry faces in days long gone
by; to creep stealthily into the sombre dungeons, so dank, earthy, and
cold, and then winding cautiously back, to emerge into the light of the
summer sun. The silent quadrangle had its encircling walls pierced with
many a window, some of which had once been richly carved; but their
mullions were now sorely wasted, while others, with broken lintels and
shattered walls above, seemed only waiting for another storm to hurl
them among the roofless chambers below. In the centre of the court-yard
stood a ruined fountain. It had been grotesquely ornamented with heads
of lions and griffins, and was said to have once run red with wine. But
it was silent enough now; the hand of time, and a still surer enemy,
the hand of man, had done their worst upon it; its groined arches
and foliaged buttresses were broken and gone, and now its shattered
beauty stood in meet harmony with the desolation that reigned around.
I employed myself for a while in looking over the fragments, marking
now the head of some fierce hippogryph, anon the limbs of some mimic
knight clad in armour of proof, and ere long I stumbled on a delicately
sculptured _fleur-de-lis_, that might have surmounted the toilet-window
of some fair one of old. Turning it over, I found its unhewn side
exhibited a still more delicately sculptured stigmaria. The incident
was certainly simple enough, perhaps even trifling. And yet, occurring
in a spot that seemed consecrated to reverie, it awoke a train of
pleasant reflection. How wide the interval of time which was bridged
across in that sculptured stone! Its one side carried the mind back
but a few generations, the other hurried the fancy away over ages and
cycles far into the dim shadows of a past eternity. The one told of a
land of flowers, musical with the hum of the bee and the chantings of
birds, and gladdened by the presence of man; the other told of a land
luxuriant, indeed, in strange forms of vegetation--huge club-mosses,
tall calamites, and waving ferns--yet buried in a silence that was only
broken fitfully by the breeze as it shook the spiky catkins or the
giant fronds of the forest. The _fleur-de-lis_ recalled memories of
France--the sunny land of France--which stood out so brightly in the
dreams of our school-days; the stigmaria conjured up visions of a land
that was never gazed on by human eye, but rolled its rich champaign
during the long ages of the Carboniferous era, and sometimes rises up
dimly in the dreams of our maturer years. Between these two epochs
how many centuries, how many cycles must have slowly rolled away! The
_fleur-de-lis_ was carved but yesterday; the stigmaria flourished when
the earth was young, and had seen scarcely a third part of its known
history.
I have said that the stem of the stigmaria is called sigillaria.
The name may be translated _signet-stem_,[22] and has reference to
one of the distinguishing peculiarities of the plant. About twenty
British species are enumerated, some of them very dissimilar, yet
they all agree in having long fluted stems with parallel rows of
prominent seal-like tubercules. The sigillaria differed so widely in
its whole contour and ornamentation from every living plant, that it
is impossible to convey an idea of its form by reference to existing
vegetation. Some of the species, as _S. organum_ (Fig. 14), had their
trunks traversed longitudinally by broad ridges separated by narrow
furrows. Along the summit of each ridge there ran a line of tubercules,
set regularly at distances varying from a third or a quarter of an inch
to close contact. One may sometimes see no unfair representation of
the bark of this ancient tree, when looking at a newly ploughed field
in spring-time, having each of its broad ridges dotted with a row of
potato sacks. Other species, while exhibiting the same plan, differed
not a little in the details. In some the tubercules are round, in
others angular, and in a third set double or kidney-shaped. In some
they are far apart, in others they are strung together like a chain of
beads. Sometimes they exist as mere specks, while occasionally they
broaden out so as to equal in width the ridge that supports them.
One species (_S. reniformis_), instead of the broad ridge and narrow
furrow, exhibits an arrangement exactly the reverse. It looks not
unlike a cast of the species first described, save that its broad flat
furrows support rows of much larger tubercules. The breast of a lady's
chemisette, with a thick-set row of buttons down each plait, would
be somewhat like this species of sigillaria, with this difference,
however, that the buttons on the plant were of a form that does not
appear as yet to have come into fashion among the fair sex. Yet they
had no little elegance, and like many other objects in the geological
storehouse, might be a useful model for our students of design. They
were neither round nor quite oval, but rather of a kidney-shape, or
like a double cherry.
[Footnote 22: The word sigillaria is really plural, and was used by the
Romans to denote the little images which friends were wont to present
to each other at the end of the Saturnalia. They answered pretty nearly
to christmas-boxes and new year's gifts among ourselves. It is not
uninteresting thus to find among the hard dry names of science, one
that two thousand years ago was synonymous with all the kindliness of
friendship.]
[Illustration: Fig. 14. Sigillaria, with black carbonized bark
partially removed.]
There can be no doubt that these tubercules must once have supported
leaflets. They are true leaf-scars, like those on the Scotch fir, and
the lozenge-shaped knobs on the bark of lepidodendron. But of the form
of these leaves we are still in ignorance, for no part of the plant,
save the stem and roots, has yet been found. The sigillaria must have
been a tree that could not long withstand maceration, for not only are
its leaves gone, but, in many cases, the outer bark has partially or
wholly decayed, leaving a scarcely distinguishable mass of carbonized
matter.[23] When this outer rind is peeled off, the inner surface of
the stem is seen to be ridged, furrowed, and tuberculed in the same
way, but the markings are much less distinct than on the outside. The
bark sometimes attains the thickness of an inch, and is always found
as a layer of pure coal enveloping the stem where it stands erect, or
lying as a flat cake without any central cylinder where the stem is
prostrate. (See Fig. 14.)
[Footnote 23: Another proof of the looseness of the texture of this
ancient vegetable may be gathered from the almost invariable truncation
of even the largest erect stems; they are snapped across at the height
of a few feet from their base. The famous "Torbanehill Mineral"
contains many such fragmentary stems, often of considerable thickness.
Their interior consists of the same material as the surrounding bed,
and displays many dissevered plants that may have been washed into
the decaying trunks. For the internal structure of sigillaria see Dr
Hooker's _Memoir_, and the authorities therein cited.]
Another remarkable feature in this carboniferous plant is that it
appears to have had no branches along its stem. Trunks have been found
four and five feet in diameter, and have been traced to a distance of
fifty, sixty, and even seventy feet, without any marks of branches
being detected. Brongniart examined the portion of one stem, which,
at its thicker end, had been broken across, but still measured a
foot in breadth. It ran for forty feet along the gallery of a mine,
narrowing to a width of not more than six inches, when it divided into
two, each branch measuring about four inches across. The sigillaria
stems, accordingly, must have shot up, slim and straight, to a height
of sometimes seventy feet before they threw out a single branch. We
know nothing of the coronal of these strangely-formed trees. From
Brongniart's observations, it would seem that the upper part of the
stem, like that of the lepidodendron, was dichotomous, that is, it
branched out into two minor stems; but how these were disposed is
unknown. We are wholly ignorant, too, of the foliage of these branches,
though, from the general structure of the plant, as well as from the
number of fern-fronds often found around the base of the stems, it has
been conjectured that the sigillaria was cryptogamous, and, like the
tree-ferns, supported a group of sweeping fronds. If so, it differed in
many respects from every known member of the cryptogamic tribes.
Putting together, then, all that we know of the exterior of the
sigillaria, we find that it was a tall slender tree, with, palm-like,
a clump of foliaged branches above, its stem bristling thickly, in at
least its upper part, with spiky leaves, and its roots equally hirsute,
shooting out to a distance of sometimes forty feet through the soft
muddy soil. Future researches may bring us better acquainted with this
ancient organism. In the meanwhile, enough of it is known to mark it
out as one of the most ornate forms of vegetation that the world has
ever seen.
In addition to the above, the coal strata have yielded many other
fragmentary remains, to which names have been given, but of which very
little is known. It is pleasant, amid such a wide sea of doubt and
uncertainty, to alight upon some well-known form of whose affinities
there can be no question, since it still finds its representatives in
living nature. Of such a kind are the coniferous stems occasionally met
with in the sandstones of the coal-measures. .
It is now many years since the operations of the quarryman in the
carboniferous sandstones of Edinburgh and Newcastle disclosed the
remains of huge gnarled trunks deeply imbedded in the rock. The
neighbourhood of the latter town yielded, in 1829,[24] the stem of
a tree seventy-two feet long, without branches, but roughened with
numerous knobs, indicative of the places whence branches had sprung.
At Craigleith, near Edinburgh, a trunk thirty-six feet long, and three
feet in diameter at the base, was disinterred in the year 1826. Since
then, several others have been found in the same neighbourhood; some
of them sixty and even seventy feet in length, and from two to six in
breadth. They were, for the most part, stripped of roots and branches,
and lay at a greater or less angle among the white sandstone beds,
which they cut across obliquely. It was unknown for some time to what
division of the vegetable kingdom these trunks should be referred.
Their irregular branched surface and undoubted bark indicated a higher
kind of structure than that possessed by any of the other carboniferous
plants; but the conjecture remained unverified until an ingenious
and beautiful method was discovered of investigating their internal
organization. Two Edinburgh geologists, Mr. Nichol and Mr. Witham,
succeeded in obtaining slices of the plants sufficiently transparent to
be viewed under the microscope by transmitted light, and in this way
their true structure was readily perceived. The method of preparing
these objects was simply as follows:--A thin slice of the plant to be
studied was cut by the lapidary, or detached by the hammer. One side
having been ground down smooth, and polished, was cemented by Canada
balsam to a piece of plate-glass, and the upper surface was then ground
down and polished in like manner, so as to leave the slice no thicker
than cartridge-paper.[25] When the preparation was then placed under
a magnifying power, the minute cells and woody fibre of the plant
could be detected as clearly as those of a recent tree. The Craigleith
fossils were in this way recognised as belonging to the great
coniferous family, and to that ancient[26] division of it which is, at
the present day, represented by the pine of Norfolk Island--"a noble
araucarian, which rears its proud head from 160 to 200 feet over the
soil, and exhibits a green and luxuriant breadth of foliage rare among
the coniferæ."[27] Some of these plants have yielded faint traces of
the annual rings shown so markedly in the cross section of our common
forest-trees; whence it would appear, that even as far back as the
times of the coal-measures, there were seasons of alternate heat and
cold, though probably less defined than now.
[Footnote 24: Witham's _Foss. Veget._ p. 31.]
[Footnote 25: For a more detailed description of the process, see
Witham's _Foss. Veget._ p. 45.]
[Footnote 26: The solitary lignite of the Lower Old Red Sandstone,
seems to have been araucarian. Miller's _Footprints of the Creator_, p.
203.]
[Footnote 27: _Footprints of the Creator_, p. 192.]
These coniferous trees do not appear to occur among the erect stems of
the coal-beds, at least they are very rare in such a position. Their
more usual appearance is that of drifted, branchless trunks, imbedded
along with other fragmentary plants in deep strata of sandstone.
They probably grew on higher ground than the swamps which supported
the sigillariæ and their allies, and might have been carried down by
streams, freighted out to sea, and so deposited among the sediment that
was gathering at the bottom.
The remains of cycadaceous plants have been described among the
vegetation of the coal-measures; but only fragments have as yet been
found. The modern _Cycadeæ_ are low shrubs or trees, with thick stems
of nearly uniform breadth, crowned with a dense clump of spreading
fronds which resemble both those of the palms and the ferns. They are
natives of the warmer regions of both hemispheres.
So long ago as the year 1835, Dr. Lindley figured a flower-like
plant, to which he gave the name of _Antholites_, ranking it among
the _Bromeliaceæ_, or pine-apple group. It was afterwards suspected
by Dr. Hooker to belong rather to the coniferæ; and he supposed that
the so-called flowerets might be really tufts of young unexpanded
leaves. An examination of a more perfect specimen, however, has induced
that distinguished botanist to alter his convictions and return to
the original decision of Lindley, that the antholites are really
flowers.[28] In Fig. 15, therefore, which represents one of these
coal-measure fossils, the reader beholds the oldest flower that has
yet been found; and surely it is of no little interest to know, that
amid the rank, steaming forests of the Carboniferous era, with all
their darkness and gloom, there were at least some flowers--flowers,
too, that were allied to still living forms, and breathed out a rich
aromatic fragrance.
[Footnote 28: See Dr. Hooker's remarks in the Supplement to the fifth
edition of Lyell's Manual, p. 31.]
[Illustration: Fig. 15.--Antholites.]
In fine, from all the genera and species of plants that have been
detected in the strata of the coal-measures, it would appear that the
flora of that ancient period was in a high degree _acrogenous_--that
is to say, consisted in great measure of ferns, club-mosses, and other
members of the great group of plants known as _acrogens_. This word
literally means _top-growers_, and is applied to those plants which
increase in height, but not in width, since they attain at first
nearly their ultimate diameter. Such plants occupy a low position in
the botanical scale. Mingled with the numerous genera of carboniferous
ferns and club-mosses, we find the remains of a much higher grade
of vegetation--that of the _gymnogens_, or plants that bear naked
seeds--such as the firs and pines. There also seem to have been a
few _endogenous_ flowering plants. Viewing, then, this flora on the
whole, it presents us with many striking resemblances to certain
botanical regions of the present day. Many of the tropical islands
abound in ferns, and contain very few flowering plants. But New Zealand
affords perhaps the closest parallel. That island is in certain parts
highly mountainous, its loftiest summits being covered with glaciers.
The hills throughout large districts are bare, or covered with a
scanty herbage, while in other localities they are densely clothed
with forests of pine, beech, and other trees. These forests sweep
on to the lower grounds, where they are replaced by a thick growth
of fern and flax-plant intermingled with dragon-trees and graceful
tree-ferns, while the more swampy regions support a rich profusion of
reeds and rushes. Such a condition of things affords a close parallel
to the probable vegetation of the Carboniferous period--an immense
preponderance of ferns and arborescent acrogens, with an intermixture
of large coniferous trees. From the general scantiness of a flora
where ferns predominate, it has been argued that the swamps of the
coal-measures nourished a luxuriant repetition of comparatively few
species; and this hypothesis also receives confirmation from the
vegetation of New Zealand. Another deduction founded on the resemblance
of the ancient to the modern flora, refers to the conditions of heat
and moisture. It has been inferred that the climate of the coal period
was equable and humid, like that of New Zealand--a supposition much
more natural and simple than that, once so much in vogue, of a heated
atmosphere densely charged with carbonic acid gas. That the air of the
Carboniferous period differed in no material respect from the air of
the present day, seems at last proved by the remains of air-breathing
animals having been found among the coal-beds; and there seems no
reason why the higher mountain-tops of the same epoch may not have been
clothed with glaciers as those of New Zealand are. As yet we have no
evidence of the fact, but it is by no means beyond the possibility of
proof.[29]
[Footnote 29: See Professor Ramsay's suggestive Memoir on Permian
Breccias in _Quarterly Journal of the Geological Society_, vol. xi. p.
185.]