Intercalation of coal seams among mountain limestone beds of
Mid-Loihian--North Greens seam--Most of our coal seams
indicate former land-surfaces--Origin of coal a debated
question--Erect fossil trees in coal-measures--Deductions to
be drawn therefrom--Difference between the mountain limestone
of Scotland and that of England--Coal-bearing character of the
northern series--Divisions of the Mid-Lothian coal-field--The
Edge coals--Their origin illustrated by the growth of modern
deltas--Delta of the Nile--Of the Mississippi--Of the
Ganges--Progress of formation of the Edge coals--Scenery
of the period like that of modern deltas--Calculations of
the time required for the growth of a coal-field--Why of
doubtful value--Roslyn Sandstone group--Affords proofs of
a general and more rapid subsidence beneath the sea--Its
great continuity--Probable origin--Flat coals--Similar
in origin to the Edge coals below--Their series not now
complete--Recapitulation of the general changes indicated by
the Mid-Lothian coal-field.
Among the old quarries of Roman Camp Hill and down the course of
several streams in the same county, the limestone beds of the mountain
limestone series are seen to be associated with strata of shale, some
of which are highly calcareous, and charged with the same organic
remains that occur in the limestones. Such shaly intercalations mark
as before the transport and deposition of muddy sediment around and
above the corals and stone-lilies of the sea-bottom. All these beds
must undoubtedly be regarded as marine. But there occur, besides,
seams of sandstone and black partially-bituminous shale, with layers
of coal and fire-clay. To this singular intermixture it may be well
to advert more particularly, since it forms one of the distinguishing
features of these northern rocks, as contrasted with those of central
and south-western England, and more especially since it will lead us
to mark again the value of fossil remains as evidence of the ancient
changes of land and sea.
The southern part of Mid-Lothian consists of a broad heathy moorland,
that slopes northward into the more cultivated country, and swells
upward to the south into the series of undulating ridges that form the
Moorfoot Hills. It is traversed by several streams which rise high
among the pasture grounds of the south, and flow some into the valley
of the Esk, and thence into the sea at Musselburgh; others past the
ancient fortalices of Borthwick and Crichton, and so by the valley of
the Tyne into the sea at Tyningham. In their upper course they traverse
a broad belt of the mountain limestone that stretches across this
part of the country from east to west, and dips away north under the
coal-field. Where the streams have been able to cut through the thick
mantle of heath, sand, gravel, and clay, by which these higher grounds
are covered, we sometimes obtain a complete section of the strata
displayed in regular sequence along the bottom of the channels. Thus,
one of the rivulets that trickles slowly through the swampy ground of
Middleton Muir, on approaching the line of limestone begins to descend
more rapidly, and has excavated its course through several feet of the
rock below. The limestones are well exposed along each side of the
stream, forming in some places steep walls tapestried with moss and
overhung with scraggy furze, and offering to the student an instructive
series of sections. Near the farm of Esperston, where the stream flows
through a narrow secluded valley, the limestones form a floor which the
water in the course of centuries has worn smooth, so that the rock with
its included encrinal stems and shells, polished by the ceaseless flow
of the current, shows like a sheet of variegated marble. At one point
on the side of the water-course the observer may notice a low ledge
of rock jutting out for a short way along the edge of the stream. The
upper part is a hard compact limestone, full of small crinoidal joints.
The bed underneath it has been greatly eroded by the rivulet, but
enough remains to show that the stratum is one of coal. It rests upon
the series of limestones and sandstones seen in the upper part of the
water-course, and is surmounted by the thick limestones of Arniston and
Middleton. A similar seam nineteen inches thick has been worked among
the limestone about three miles to the west at Fountain. The same bed
occurs among the quarries on Roman Camp Hill already mentioned, and I
have seen an equivalent stratum intercalated among sheets of cup-corals
and stone-lilies on the shore at Aberlady, where the waves have laid
open perhaps the finest section of Carboniferous limestone strata in
Scotland. In West-Lothian, too, the same intercalation of coal-seams
among the mountain limestone beds can be seen in many places. Thus, in
the bed of the River Almond, near Blackburn, the following section is
laid bare:--
Calcareous shale.
Limestone (marine), eight feet.
Calcareous shale, with _spirifers_, &c.
Coal, six to eight inches.
Fire-clay.
Sandstone.
A short way further down the stream another bed of limestone occurs
with several seams of coal below it, one of them attaining a thickness
of six feet.
In addition to the thin seam at Esperston, the Mid-Lothian field
contains several others. Of these by much the most important is that
known as the North Greens Seam. It varies in thickness from only a
few inches to fully 5 feet, and has been extensively worked for the
_parrot_ or gas-coal which it contains. It rests upon a pavement of
shale, sometimes of fire-clay, and occurs about midway between two
thick marine limestones, being from 80 to 90 feet distant from each.
I have laid open many a block of the parrot-coal at the pit mouth,
and marked the well-defined outlines of the stigmaria covered with a
yellowish efflorescence of iron pyrites, like gilded figures upon
a black velvet ground. The plants lie with their divergent rootlets
spread out regularly along the stem like teeth on the back of a comb,
thus seeming to indicate no hurried agglomeration by some tidal wave or
turbid river, but rather a slow and tranquil deposition.
The fossils of the coal-seams consist for the most part of the
plants above described, which we saw to belong to _terrestrial
species_. But the reader will now understand that in dealing with
organic remains we cannot infer, because a certain stratum contains
nothing but land-plants, that it must necessarily by consequence be a
land-formation. For we have seen that the plants of the Burdiehouse
limestone, though all terrestrial, gave no support to the idea that the
rock had originated on land. In all such cases regard must be had not
only to the nature of the imbedded organisms, but their condition and
mode of occurrence, and to the character of those associated with them.
Especial care must be taken to distinguish what has been transported
from what is _in situ_, otherwise, by attending only to one part of
the evidence, we shall miss the import of the whole, and altogether
misinterpret the records which we seek to decipher.
For years the subject of the origin of coal formed one of the many
battle-fields on which geologists delighted to break lances. They
ranged themselves under two banners, the "drift"-theory men and the
"growth"-theory men, the former maintaining strenuously that coal
was simply vegetation transported from the land and deposited in
large troughs at river-mouths or sea-bottoms, the latter as eagerly
contending that the vegetation had not been drifted, but grew on
the very locality where its remains are now exhumed. Neither party
lacked plausible arguments in support of its doctrines. The "drift"
combatants stoutly affirmed it to be contrary to all experience that
a land-surface should be so oscillating as their opponents required,
that in short it was absurd to hold each coal-seam as marking a period
of elevation, for there were often dozens of seams in as many yards
of strata, some of them scarcely an inch thick, and yet, according to
the "growth" theory, each would have required for its accumulation
a special uplifting of the land above the sea-level. These and many
other difficulties were thought to be triumphantly overcome by the
hypothesis of transport and deposition. The vegetation borne down
by some ancient Mississippi would collect in vast rafts, and these
becoming water-logged would sink to the bottom, where, by getting
eventually covered over with silt and sand, they would in process of
time be chemically altered into coal. This explanation was, however,
vigorously resisted by the opposite side. They alleged that the "drift"
theory could account neither for the wide extent of coal-seams nor
for their remarkable persistency in thickness. If the vegetation had
really been hurried out to sea by river-action, it seemed natural to
expect that the coal-seams should occur in sporadic patches of very
unequal thicknesses, according as the drifted plants had been more
densely or more loosely packed. But this was found not to be the case
in point of fact. The coal-seams were ascertained to be generally
singularly continuous, and to retain for the most part a pretty uniform
thickness over considerable areas. And what was still more worthy of
note, they were, as a whole, markedly free from extraneous matter,
such as sand and mud. Where these impurities did occur, it was usually
in the form of intercalated seams or partings, often quite as regular
and extensive as the coal itself. Had the vegetation, therefore, been
transported into the sea, it could hardly fail to get mixed up with
the fine impalpable mud which, like that of the Ganges or Mississippi,
might have discoloured the ocean for leagues from the river-mouth,
and settled down as a thickening stratum at the sea-bottom. And many
other arguments, derived from the nature and arrangement of the strata
interbedded among the coal-seams, were urged to prove that the latter
had originated from vegetation which grew on the spot.
[Illustration: Fig. 33.--Section from Cape Breton coal-field, showing
four planes of vertical stems, and seven ancient soils with their
covering of vegetation.
_a_, sandstones; _b_, shales; _c_, coal; _d_, fire-clays; _e_,
arenaceous shales.]
The warfare seems now pretty nearly at an end, and as often happens in
such cases, it has been found that each party was to some extent in the
right and to some extent in the wrong. It has been ascertained that
some coal-seams must have originated from the deposition of drift-wood
in the mud and ooze of the sea-bottom, while others undoubtedly arose
from the decay and entombment of vegetation in swampy plains of the
land. That the latter mode of formation has been the usual one in most
of our coal-fields has been generally acknowledged since Sir William
Logan's announcement that each coal-seam, for the most part, rests
upon a bed of fire-clay, which, with its embedded roots, marks the
site of an ancient soil. This fact has been abundantly confirmed in
every part of this country, and indeed wherever an extended series of
coal-seams has been examined. Not only have the underlying fire-clays
been found, but in not a few instances erect stems of trees, passing
down through the coal-seam and spreading out their divergent roots in
the clay below, exactly as they must have done when they flourished
green and luxuriant in the times of the Carboniferous system. This was
especially the case in the Parkfield Colliery, Wolverhampton, where
seventy-three trunks were laid bare in the space of about a quarter of
an acre, each with its roots attached. The same appearance was observed
some years ago in the Dalkeith coal-field, where a group of erect
trees was encountered covering a space of several square yards. Some
instructive sections of such fossil-forests are given by Mr. Brown from
the Cape Breton coal-field.[63] In one of them (Fig. 33) no fewer than
four planes occur, each supporting its group of erect steins. Now, no
one can glance over this and the other sections illustrative of the
same paper, or the descriptions given by Sir Charles Lyell and others
of the Nova Scotian coal-field, without being compelled to admit that
the trees in question grew just where their upright stems can still be
seen, and consequently that the accompanying coal-seams originated not
from vegetation drifted by river-action, but from vegetation that grew
upon the spot. And though erect stems do not exist in every coal-field,
we seldom fail to detect the not less important occurrence of the
fire-clays and hardened shales that support the coal-seams and prove
by their embedded rootlets their identity with ancient soils. Thus we
arrive at the inference that while in certain localities coal-seams
have resulted from drifted vegetable matter, they have nevertheless for
the most part been formed from plants that flourished where the collier
now excavates, amid damp and dripping caverns, their carbonized remains.
[Footnote 63: _Quart. Jour. Geol. Soc._ vol. vi. pp. 120, 130. The cut
given above (Fig. 33) is taken from one of these sections as modified
by the late Sir Henry de la Beche (_Geological Observer_, p. 582). In
the original the beds are inclined at a considerable angle, but for the
sake of clearness they are here reduced to horizontality.]
Applying, then, this deduction to the strata occurring on the horizon
of the mountain limestone in Mid-Lothian, we are led to believe that
the North Greens coal-seam marks the site of a former land-surface.
It shows no vertical stems, but has all the other accompaniments of
an ordinary seam, such as the underlying fire-clay and shale, with
their included stigmariæ. And this conclusion has more than ordinary
interest, for if it be true, we have evidence of a terrestrial
formation among strata unequivocally marine; in other words, we see
proofs either of an elevation or a filling-up[64] of the sea-bottom
carried slowly on until land-plants grew up in matted swamps where
once there swarmed corals and encrinites, and then of a gradual
subsidence, so that marine organisms flourished again in abundance
over the site of the submerged vegetation. It is not insisted that
each of the thin coal-seams among the limestone strata marks a former
terrestrial area. Some of them may possibly have resulted from the
transport and deposition of plants borne from the land. Yet there are
others of wide extent resting upon beds of fire-clay which contains
stigmaria rootlets, &c. These I cannot but regard as the remains of
plants that grew upon the spot. And so, while we recognise in the beds
of limestone undoubted evidence of a former sea-bottom, I am persuaded
we must equally admit that at least several of the coal-seams bear fair
evidence of a former land-surface, scarcely raised above the sea-level
indeed, but nourishing nevertheless a thickly matted vegetation. In
this way we shall see the mountain limestone series of the Lothians
to be not a purely marine formation, but one partly marine and partly
deltoid, showing in the succession of its strata proofs of a gradual
submergence, interrupted by movements of elevation, so that the area
which at one period formed the ocean-bed became at a later time low
delta-land, and after continuing perhaps for ages to stretch out its
verdant surface beneath the open sky, sank again amid the corals of a
wide-spread sea.
[Footnote 64: If it be correct to set down the North Greens coal-seam
as really representing a terrestrial surface, that is, of course, a
flat delta or plain scarcely raised above the sea-level, we must, I
suspect, call in the aid of a slight elevatory process, or else hold
that the depth of the sea at the locality where the lower limestone was
forming did not exceed 80 or 90 feet, and may have been considerably
less, and that this space came to be eventually filled up by the
detritus of the river. But the wide extent and sometimes the great
thickness of the limestone beds seem to indicate a greater depth, and
thus favour the idea of an elevation of the sea-bottom to form the
North Greens coal-seam.]
Now this condition of things differs entirely from what is presented
by the Mountain Limestone group of England. That formation, when
typically developed, attains a thickness of from 1000 to 2000 feet, and
gives rise to that green hilly kind of scenery whence it has derived
its name. It is unequivocally a marine deposit, since it abounds in
corals, echinoderms, brachiopodous molluscs, and other productions of
the deep. Northward, however, it undergoes a gradual change, getting
greatly thinner, and split up by a series of intercalations of shale
and sandstone. This alteration goes on until, on the border-land
between the two countries, the massive limestone of Derbyshire has
dwindled down into a series of thin beds, often widely separated by
intervening strata, which contain many seams of coal. After crossing
the Silurian district, and descending the northern slopes of the
Lammermuir Hills, we get into the Carboniferous system again, and find
its limestone series still farther diminished. With this decrease
of marine formations, we can detect an augmentation of coal-bearing
strata. Thus the Berwickshire coal-field lies in this lower set of
beds, far under the coal-measures of Newcastle. In the Lothians, too,
as has been shown, coal is extensively worked in the same series, and
these seams also find their representatives in Fife and Lanarkshire.
The gradual change from the kind of strata found on the horizon of
the Burdiehouse limestone, to those occurring on the horizon of the
Mountain limestone, indicates, as we saw, a gradual change of the
conditions of deposition; and the nature of this alteration is shown by
the difference in the character of the imbedded fossils. The passage
of the massive Derbyshire limestone into the thin limestones and
coal-bearing sandstones of the north, as decidedly marks another change
in the relative position of sea and land. The former was a succession
in time, the latter was one in space, but the mode of reasoning remains
the same for both. In the former case, we saw estuarine strata passing
upward into others wholly marine, and the order of superposition told
us that the locality was first an estuary, and then slowly became open
sea. In the latter case, we see marine beds not succeeded by estuarine
strata, but becoming estuarine strata themselves. The thick limestones
gradually thin out horizontally into a great series of sandstones and
shales, with interbedded coal-seams, so that what bears evidence of a
deep sea at the one end, gives proof of a muddy and sandy delta at the
other. In other words, during the ages represented by what we call the
Mountain Limestone, the central and south-western portions of England
lay far below a wide breadth of ocean, and nourished a luxuriant crop
of stone-lilies, mingled with the other denizens of the deep, while the
Border district, and the whole of central Scotland, exhibited all the
conditions of a vast delta, sometimes spreading out as broad verdant
jungles, anon showing only scattered irregular groups of low, bare
mud-banks and sand-spits, which at other times disappeared altogether
beneath the dun discoloured waves. Now the reader will not fail to
mark that this curious and interesting fact in the past history of our
country, is ascertained solely from a comparison of fossil remains.
The stone-lilies and shells of Derbyshire, and the lepidodendra and
land-plants of the Lothians, form our sole basis of evidence, and we
may rest on them with as perfect certainty as if they were so many duly
attested documents deposited among the archives of our State-Paper
Office.
In our survey of the coal-field of Mid-Lothian, we have passed from
the Lower Carboniferous estuary beds of Burdiehouse to the Middle
Carboniferous marine beds of Roman Camp Hill, and their associated
terrestrial strata,--the coal-seams and fire-clays. We come now, in our
upward progress, to the Upper Carboniferous group, or Coal-measures
proper.[65] These strata rest immediately upon the limestones, and
attain a depth here of over three thousand feet. They consist of a
great series of sandstones, shales, coals, and fire-clays, that vary
in thickness from less than an inch to many feet, or even yards. The
coal-seams are especially variable, many of them existing as mere films
of carbonaceous matter; others varying up to a depth of fourteen feet.
There are from fifty to sixty that exceed a foot, but the average
thickness throughout the whole series is about three and a half
feet.[66] They are nearly all underlaid by fire-clay or shale, and very
generally have a roof of the latter material.
[Footnote 65: These terms--Lower, Middle, and Upper Carboniferous, are
used for want of others, and for the sake of clearness. They must not
be regarded, however, as equivalent to similar groupings of the English
carboniferous rocks, for the Scottish series is probably much older
than the greater part of the English, and coeval, to a considerable
extent, with the mountain limestone and millstone grit of the latter
country.]
[Footnote 66: See Milne on Mid-Lothian Coal-field. _Trans. Royal Soc.
Edin._ vol. xiv. p. 256, whence the above details are taken.]
By referring to the diagram of this coal-field, given above at p.
196, the reader will notice that the series is divisible into three
groups:--1_st_, and undermost, a considerable depth of coal bearing
strata known as the _edge series_, because they lie along the western
limits of the coal-basin at a high angle, and sometimes even on
edge; 2_d_, A great thickness of sandstones nearly barren of coal,
but containing at least three beds of limestone this may be termed
the Roslyn sandstone group; 3_d_, and highest, another series of
coal-bearing strata, commonly called the _flat coals_, because they
occupy the centre of the basin where the beds repose at a low angle,
and are in places quite flat. It will be convenient to keep in mind
this three-fold division, for it will point us to some important
changes in the ancient conditions of this coal-field.
The edge series, which forms the lowest, and of course oldest of the
above groups, averages from 800 to 900 feet in thickness. It contains
about thirty seams of coal above a foot thick, and many more of less
size. They occur irregularly, some lying only a few inches apart,
others from eighty to ninety feet, the intervening space being occupied
by sandstone or shale.
Now as each coal-seam, with its associated under-clay, appears to mark
a former land surface, it will follow that there must be as many old
land surfaces in this series of strata as there are such coal-seams,
and that for every intervening mass of sandstone or shale, the area
of vegetation must have been submerged. This conclusion would have
been violently resisted by the supporters of the "drift" theory. They
would have roundly asserted that such an unsteady surface was a mere
supposition to suit a hypothesis, unsupported by fact, and contrary
to the analogy of existing nature; and they would not perhaps have
hesitated to maintain, that such an oscillating land could be little
fitted to nourish so rich and luxuriant a vegetation as that of the
Carboniferous period. But it will not be difficult to show that our
conclusion, so far from being contrary to analogy, is amply borne
out by the processes of existing nature, and that its opponents, and
even its original asserters, failed to perceive that what it demands
is not a rapidly oscillating crust, but one as steady and uniform as
that of many of the least disturbed countries at the present day;
and that we do not require to call in the aid of a special elevation
and submergence for every coal-seam, but that for the most part the
hypothesis of a steady sinking of the area of a coal-field, interrupted
perhaps by occasional elevatory movements, along with an active and
constant deposition of sediment by the varying currents of a large
river, is sufficient, if not thoroughly to explain, at least to
throw great light upon the origin of those enormous masses of strata
composing our present coal-basins. The oft-recurring variations in
the nature of the strata that form our coal-measures, sandstones
alternating with shales, these again with coals and fire-clays,
together also with the terrestrial origin of the coal-seams, and the
occasional presence of true marine organisms, make it evident that,
to obtain any modern analogue to such a condition of things, we must
examine those localities where large bodies of fresh water, carrying
sediment and vegetation from the land, mingle with the sea. Let us then
look for a little at the operations now in progress at the mouths of
the larger rivers, and mark how far they elucidate the structure and
history of a coal-field.
"Egypt is the gift of the Nile." Such was the conclusion arrived at by
one of the most diligent observers of ancient Greece--the venerable
Herodotus.[67] He sailed up the river marking all the leading features
in its scenery, and noting the more apparent evidences of ancient
physical changes. His remarks on these subjects form one of the
earliest specimens of scientific reasoning that have come down to us,
and are remarkable for their correctness and the truly inductive mode
of thought which they evince. Modern travellers have amply confirmed
the opinions of the father of history, and we now know that but for its
central river, Egypt would be a vast dreary expanse of arid sand like
the neighbouring deserts of Lybia. The Nile, by annually inundating
the country, deposits over it a stratum of rich loam, and thus not
only waters the land, but continually renews the soil. The sediment
in this way brought down has gradually encroached upon the waters of
the Mediterranean, being heaped up at the river mouth into shifting
sand-banks, islets, and great tracts of low, swampy ground, slightly
raised above the sea-level. Through this series of silting deposits,
the river sends a number of branches, often winding in labyrinthine
convolutions, and ever changing their course, by wearing away the silt
at one place, and throwing it down at another. The area traversed by
the mouths of the Nile was called by the Greeks the Delta, from its
similarity in form to the Greek letter, and the name has since been
given to all such fluviatile deposits, whether they have this general
form or not.
[Footnote 67: _Euterpe_, 5.--His words are very emphatic. "To one of
ordinary intelligence, who has not heard of it before, but sees it,
Egypt is manifestly land acquired by the inhabitants, and a gift from
the river--δωρον τον ποταμον." The 10th and 12th chapters of the same
book deserve especial study for the admirable inductive style in which
the historian compares the phenomena observable in Egypt with what were
well known as the results of river action in other lands. The passages
might be quoted word for word in the most rigid scientific argument of
any modern geologist.]
The sediment annually deposited by the Nile varies in thickness in
different years. The mean thickness of the annual layers at Cairo has
been calculated not to exceed that of a sheet of thin pasteboard, so
that "a stratum of two or three feet must represent the accumulation
of a thousand years."[68] Such thin laminæ must resemble greatly some
of the more fissile shales in the Carboniferous system, which were,
perhaps, formed by as slow a process, and in their aggregate depth
probably took many thousand years to accumulate. But those fluviatile
depositions of the Nile vary little in kind, for when cut through they
are found regularly stratified down to their base, which rests upon the
great underlying sand. They show us how the argillaceous seams of the
coal-measures may have originated; but the diversity of character in
these Carboniferous rocks indicates a more varied kind of sediment, and
probably more rapid and active transporting currents. A closer analogy
to such a condition of things meets us on the shores of the New World.
[Footnote 68: Lyell's _Principles_, p. 262.]
The Mississippi, so magnificent in all its proportions, has raised
a delta which covers a tract of about 14,000 square miles, equal to
almost half the area of Ireland. The lower parts of this delta are
formed of low, shifting banks, traversed by innumerable streams that
diverge from the main river, and alternately throw down and remove vast
quantities of earthy sediment, intermingled with rafts of drift-wood.
These swamps are covered with a rank growth of long grass and reeds,
and for about six months of the year are more or less submerged below
the waters of the river, while liable at the same time to continual
inundation and encroachment from the sea. The higher parts of the
delta, though also subject to a similar periodical submergence,
nourish a more luxuriant vegetation. Vast tracts of level sandy soil
are densely overgrown with pine, which is used extensively for making
pitch. Large districts of the swampy ground are covered with willows,
poplars, and thickets of the deciduous cypress, an elegant tree that
rises more than 100 feet above the soil. When in hot seasons these
swamps get dried up, "pits are burnt into the ground many feet deep,
or as far down as the fire can descend without meeting with water,
and it is then found that scarcely any residuum or earthy matter is
left. At the bottom of all these 'cypress swamps' a bed of clay is
found, with roots of the tall cypress, just as the underclays of the
coal are filled with stigmaria."[69] In this way a thick accumulation
of vegetable matter goes on forming for years, until either the river
changes its course, and inundating the swamp gradually covers it over
with sand and mud, or until, owing to oscillations of the earth's
crust, the district is either permanently submerged, so as to be silted
over, or elevated to nourish a new and different kind of vegetation.
[Footnote 69: Lyell's _Elements_, p. 386.]
That such changes have taken place in the past history of the river we
have several interesting proofs. Thus, owing to the great earthquakes
of 1811, 1812, an area of more than 2000 square miles was permanently
submerged.[70] Since then it has gone under the name of the "Sunk
Country;" and Sir Charles Lyell, who visited the locality in 1846, that
is, thirty-four years afterwards, tells us that he saw innumerable
submerged trees, some erect, others prostrate. Now, it is easy to see
how such an area may, when the climate suits, become the receptacle of
vast accumulations of peat, which, by pressure and chemical action,
will ultimately pass into coal. If we suppose the submergence carried
on more rapidly at some periods, the plants might have been unable to
keep pace with the ever-increasing inroads of sand and mud. In such
cases the layer of vegetation would become eventually entombed beneath
succeeding deposits of earthy matter. Were the amount of sediment thus
thrown down sufficient in the end to counteract the downward motion of
the earth's crust, and so raise the bottom of the river or lake to the
level of the water, vegetation would spring up afresh and clothe the
new raised surface as densely as in former years. This alternation,
according as the amount of sinking or the amount of sediment
predominated, might go on for thousands of years, until a series of
strata many thousand feet thick were accumulated, and tranquilly
carried down bed after bed below the level of the waters.
[Footnote 70: See Sir Charles Lyell's _Second Visit to United Stales_,
chap, xxxiii.]
It is interesting to know that the case supposed here has actually
been realized in the delta of the Ganges. Some years ago an Artesian
well was attempted to be made near Calcutta, and the auger was sunk
to a depth of 481 feet.[71] The material passed through consisted of
sand, clay, and nodules of argillaceous limestone, and at various
depths, from 50 to 380 feet, several seams of decaying wood and peat
were found, along with bones of various animals, such as deer and
fresh-water tortoises, and fragments of lacustrine shells. Each of
these vegetable layers evidently formed at one time a forest-covered
swamp like those of the surrounding delta at the present day; and
hence it follows, that during the accumulation of the Gangetic delta,
the ground in that locality must have undergone a depression of more
than 300 feet, and that this sinking has been interrupted by slight
elevations, or by periods when the ground remained stationary, so as
to admit of a dense and prolonged growth of vegetation, at successive
intervals, upon the swampy flats and shifting islands. The general
appearance of these old forests is pretty well shown by the mangrove
swamps along the mouths of the river. These trees flourish in dense
jungles on the banks, and extend even below high water mark, being
covered in places by shell fish. So that were these maritime parts
of the delta inundated by the ocean, and buried beneath a mass of mud
and silt, the peaty layer that would be formed would display trunks of
trees still occupying their original erect position, and spreading out
their roots in the clay below, exactly as the sigillaria is found to do
in the coal-seams of the carboniferous rocks, while clustered round the
carbonized stems, or scattered among the decayed leaves and branches,
there might be detected limpets and barnacles (as lingulæ and pectens
occur in the coal-seams), showing, by their mode of occurrence, that
they lived and died upon the spot.
[Footnote 71: See Lyell's _Principles_, p. 280.]
If my reader will now suppose this sand of the Indian river to be
hardened into sandstone, the mud in like manner compressed into
shale, and the peat beds chemically altered into coal, can he fail to
perceive the striking analogy between the section thus displayed and
those already given from the Mid-Lothian and Cape Breton coal-fields?
The differences between the ancient and modern strata are not in kind
but in degree. The Scottish series reaches to more than six times the
thickness of the Indian one, and the coal-seams in the one exceed in
individual thickness the peat-beds in the other. We must remember,
however, that the climate of Hindustan is not remarkably favourable to
the accumulation of vegetable matter, the heat being so great that the
plants decay almost as rapidly as they grow. And it should likewise
be borne in mind, that were the conditions of subsidence and of the
gradual accumulation of sedimentary matter to continue even in the same
ratio as heretofore, the Ganges might, in the course of ages, heap up
a series of stratified sands, clays, and peat-beds, many thousand feet
in thickness, and many thousand square miles in extent, rivalling,
or perhaps surpassing in depth, the largest coal-field in the world.
The parallelism between this delta and an ordinary coal-field holds
singularly close, not merely as regards the nature of the stratified
deposits. The alluvial plain of Bengal has undergone a process of
subsidence to an unknown depth, whereby successive areas of terrestrial
vegetation have been carried down to be entombed beneath fluviatile
sand and mud. It is likewise subject to the more sudden operation
of earthquakes, whereby large tracts of country become permanently
altered, and changes are effected on the direction, rapidity, and
detritus of the streams. It is, moreover, liable to wide-spread
inroads of the sea, which sometimes covers cultivated districts to
a depth of several feet, laying waste the fields and destroying the
inhabitants. These and other features help us to understand the origin
of such vast masses of sedimentary strata as those of our coal-fields,
where terrestrial, fluviatile, and marine remains alternate in rapid
sequence, or sometimes occur together.
The origin of the constant succession of coal seams, sandstones, and
shales, of the Edge series may be thus accounted for. The area of
Mid-Lothian formed part of a great delta, which, like that of the
Ganges, was undergoing a gradual subsidence during the Carboniferous
era. The rate of this movement probably varied at different times, and
might even be occasionally interrupted by short periods of elevation.
When the ever-increasing accumulations of silt brought down by the
river reached or nearly reached the surface of the water, they would
become the site of wide tracts of swampy vegetation that flourished
for hundreds or thousands of years. Eventually, however, these
jungles, invaded by the changing currents of the river, were buried
beneath a thick deposit of fluviatile sediment, or more probably the
vegetation might become unable to keep pace with an accelerated rate
of submergence, and the forests would then be tranquilly carried
down beneath the water, and soon covered over with sand and mud. The
detrital matter might in like manner continue to be deposited over the
sunk forest for many years, perhaps centuries, until the muddy bottom
again reached the surface, and once more waved green with sigillariæ,
calamites, and lepidodendra. Another long interval might here elapse,
in which a thick bed of vegetable matter might accumulate, much after
the manner of the formation of peat among the bogs and mosses of our
own country. The periodical inundations of the river probably gave
rise to wide marshes and lagoons, often tenanted by lacustrine shells,
and thickly overgrown with aquatic vegetation. The decaying plants
decomposed the red ochreous matter with which the water was charged,
and re-deposited it among the mud and rotting leaves at the bottom as
a carbonate of iron. Such ferruginous accumulations, often entombing
fern-stems and other plants, with scales and teeth of ganoidal fishes,
sometimes _conulariæ_ and _lingulæ_, and, in certain localities, whole
acres and miles of fresh-water shells, are known now as our _clay-band_
and _black-band ironstones_. We can easily conceive that, in shallower
parts of the lagoons, a dense growth of marshy plants might spring
up, preventing any deposition of iron, and when the whole came to be
covered over with later accumulations of sand or mud, the deeper parts
of the old lake would be covered with a seam of ironstone, and the
shallower portions would display a bed of coal. In some such way we
may account for the frequent passage of ironstone into coal, and coal
into ironstone in many of our coal-fields. If undisturbed by the ever
changing currents of the river, these wide expanses of marsh and lake
might continue for many long years, the constant evaporation being
counterbalanced by continual supplies of water from the main stream.
Eventually, however, owing perhaps to another period of more rapid
submergence, the water gained the ascendency, and once more rolled over
prostrate stems and matted thickets of ferns, that sank slowly down
beneath a deepening sheet of sand and mud. Often, too, the sea must
have flooded, perhaps for years, the flat delta-lands, carrying with it
its own productions, such as the lingulæ and cardiniæ, which we find
among the coal seams. And thus the process went on during the long ages
of the Carboniferous system. Forest after forest spread its continuous
mantle of green athwart the low swampy lands of that old delta, and
each in succession foundered amid the muddy waters, now of the ocean
and now of the river, that strewed over its site a rich detritus which
went to form the soil of new jungles and forests.
The Edge series measures from 800 to 900 feet in depth, so that the
depression must have been carried on till the forest that once grew
nearly on the sea-level had sunk 800 feet below it This process was
undoubtedly a very slow and tranquil one. Yet geologists used to regard
these frequent changes of sedimentary matter as so many proofs of
repeated catastrophic submergences, when the ocean came rolling over
the land, prostrating forests, uprooting the hugest trees, and leaving
the scattered bones and scales of fishes amid vast accumulations of mud
and sand, where but lately there had bloomed a luxuriant vegetation.
But the sober and diligent student of geologic fact will read in these
rocks no such record of cataclysms. He will see in them evidences of
the same gradual and sure operation which marks the processes of Nature
at the present day. He will note how during a tranquil and probably
imperceptible submergence of the river-bottom, forest after forest
sprang up, flourished perhaps for ages, and eventually settled down
beneath the waters of the river and sometimes of the ocean, amid ever
increasing accumulations of mud and sand. Musing on these ancient
changes he will be lost in wonder at the immense duration of the period
during which they were in progress; and he will try in some measure
to realize the features of their scenery. He will picture the delta
with its ever-varying islets and sand-banks, its lakes and submerged
forests, its leafless trunks peering above the water and sticking along
the shoaling mud, and its crowded jungles that cover every drier spot.
He will cast his eyes to where the delta opens out into the ocean, and
mark how the waves encroach upon the mud-banks, cutting away what the
river has piled up, and washing the roots of gigantic trees that wave
their green coronal of fronds above, and overshadow the rippling of
the green sea below. He will try to thread the windings of the stately
river through brakes of ferns and calamites, and banks richly hung
with tree-ferns and sigillariæ, and then upward through dark shaggy
pine-woods, silent and gloomy, with the water creeping lazily through
the shade or dashing in white cascades over dripping rocks, and onward
still, far away among the distant hills till the fountainhead of the
great stream is reached, gushing from the splintered sides of some
lone rock, or pouring perchance out of the glimmering caverns of some
massive glacier high amid the regions of perpetual snow.
Many attempts have been made to estimate the amount of time which
some of our coal-fields may have required for their accumulation.
But so large a number of conjectural elements must necessarily enter
into such calculations, that the results come to be of very doubtful
value. By estimating the amount of sediment annually transported by
such rivers as the Ganges or Mississippi, we may ascertain how long a
mass of similar sedimentary strata would take to form under similar
conditions. And if our calculation had to do merely with such detrital
accumulations, we might hope to arrive at some approach to accuracy.
But besides these sedimentary strata, the formation of which must have
been wholly analogous to that of similar deposits at the present day,
we have to deal with the problems suggested by the coal-seams. We know
nothing of the climate of the Carboniferous period save what may be
conjectured from the analogy of existing climates; and in a question
regarding the accumulation of decaying vegetable matter climate is a
subject of the first importance. We are ignorant, too, of the rate of
growth peculiar to the carboniferous flora; and even if we hold that
it was probably rapid, the process of decay may have been equally
speedy, and so a forest might go on shooting up fresh trees as the old
ones rotted away, yet at the end of a thousand years there might be a
scarcely greater thickness of vegetable matter on the ground than at
the commencement. A seam of coal two feet thick might thus represent,
say the accumulation of a hundred years, and another of exactly the
same thickness might stand as the accumulation of a thousand years.
Until we know more of the vegetation and climate of the coal period,
the thickness of a coal-seam can hardly be held as a certain guide to
the lapse of time required for its formation.
For the sake of illustration, let me take the following fragment of a
coal-measure section:--
Shale, 20 feet.
Coal, 4 "
Fire-clay, 6 "
Sandstone, 40 "
Beginning at the bottom, we may compute the period of the forty feet
of sandstone variously, according to the river selected as the type
of a transporting agent. Tried by the standard of the Nile, all other
conditions being similar, such a deposit would require perhaps not
less than 14,000 years; by that of the Mississippi, 5000; and by that
of the Ganges, nearly 2000.[72] We come, then, to the superincumbent
fire-clay and coal, representing an ancient soil and the forest that
grew on it. The occurrence of these seams shows us that the river-bed
had become a swampy tract clothed with vegetation; but who shall
say how long it may have continued so? Like the sunk country of the
Mississippi, it may have been submerged, and to some extent cut off
from the sediment-transporting channels of the river, and thus, as
a vast lake, have nourished a prolific growth of marshy and aquatic
plants. If the temperature resembled that of our own country, the
growth of peaty matter, other circumstances being favourable, might
be comparatively rapid. If, however, as seems probable, the climate
were more warm and humid, giving rise to a more luxuriant vegetation,
and at the same time to a more rapid decay, a long interval might have
elapsed without adding materially to the thickness of the vegetable
accumulations, and the eventual entombment of peaty matter sufficient
to consolidate into four feet of coal, might be owing in some measure
to the submergence of the swamp beneath the waters of the river,
whereby a quantity of detrital matter was deposited that arrested the
process of putrefaction, and entombed the thickly matted plants which
were growing on the spot at the time. Hence, until we know more of
the conditions under which vegetation may accumulate at river-mouths
in such a climate as the coal plants are conjectured to have enjoyed,
calculations of the amount of time required for the formation of a
great series of coal-bearing strata must be regarded as premature.
In the present instance, we can but affirm that the growth of the
four-foot coal-seam probably occupied many long years, even at the most
rapid rate of accumulation known to us. The forest-covered swamp on
which the plants grew was eventually invaded by muddy detritus brought
down by the river; and during another period of indefinite extent--five
hundred years or five thousand years--fine mud continued to settle down
over the foundered forest, hardening eventually into twenty feet of
shale.
[Footnote 72: Some observers have pointed to the occurrence of vertical
and inclined trunks of trees in the Carboniferous sandstones, and
deduced therefrom what has seemed to them a triumphant argument in
favour of the rapidity wherewith our coal-fields must have formed. A
foundered tree, they say, sank with its heavy-laden roots among the
sand at the bottom, its stem pointing up into the water like the snags
of the Mississippi, so that the sand must have come rapidly down to
entomb the whole before it had time to decay, and thus thirty or forty
feet of sediment must have been deposited in a few years, perhaps even
months. But this is somewhat like a begging of the question. We have
yet to learn how long a water-logged trunk will resist decomposition.]
The Edge coals of the Mid-Lothian coal-field are succeeded by a group
of sandstones and thin shales, with three or more seams of limestone.
This group of strata, which we may call the Roslyn Sandstone Series,
reaches a thickness of from 1200 to 1500 feet, and serves as a middle
zone to divide the Edge coals below from the Flat coals above. It
contains only a few thin laminations of coal, and these chiefly at its
upper and under portions. Such a great intercalation of beds, without
coal-seams, points, we might readily conjecture, to some change in the
physical conditions of the ancient delta. The nature of this change can
be easily made out from an examination of the rocks, and the reader
will see that here again we are indebted to fossil remains for the most
conclusive and satisfactory evidence of these old physical revolutions.
The absence of coal-seams suffices to indicate that during the
formation of the middle group that part of the delta occupying the
site of Mid-Lothian was continually submerged, and never rose to the
surface so as to allow a covering of vegetation to form upon it.[73]
The large beds of sandstone prove a continued transport and deposition
of detritus during undisturbed periods of considerable length. The
intercalations of shale, pointing to local changes in the currents or
other modifying causes, are usually of small thickness and extent,
while the sandstone beds sometimes attain a depth of 150 or 200 feet,
and extend over wide areas of country. So far these mechanical rocks
indicate the deposition of sand and mud under water, but whether at
river-mouth or sea-bottom is left uncertain. From the fossil remains,
however, we learn that the deposition took place in the sea, but at
no great distance from land; in other words, the area of Mid-Lothian,
which, during the accumulation of the edge coals, had been alternately
clothed with vegetation and inundated by the river, sank down many
fathoms, so that the sea rolled over it and all its submerged forests.
The proof is two-fold, first, from the character of the organic remains
in the limestones; and second, from that of those in the sandstones and
shales.
[Footnote 73: Of course, this deduction is founded, as the reader
will notice, on the assumption that we have now the series, as it was
deposited, and that no peaty swamp or forest was denuded away, and its
site occupied by sand and silt. But the assumption is rendered probable
from the conditions of formation indicated by the Roslyn group.]
In some of the streamlets that flow into the beautifully wooded vale
of the Esk, south of Penicuik, these limestones can be well seen,
worn in the water-channel, or crusted over with moss along the banks.
Their organisms are singularly abundant, and consist of cyathophylla,
encrinites, spirifers, producti, &c., all exclusively marine. In a
picturesque brook that falls into the Esk near a saw-mill in the
grounds of Penicuik House, I have seen the little cup-corals clustered
by dozens on the weathered rock, showing their delicate striated
wrinkles in high relief among the scattered valves of productus and
innumerable joints of the stone-lily. They were all well preserved, and
in their grouping and general appearance differed in no respect from
similar organisms in the mountain limestone of Roman Camp Hill. The
inference to be drawn from them must accordingly correspond with what
has been deduced from the mountain limestone fossils, viz., that they
mark the site of a sea-bottom which remained free from mud and sand for
considerable periods, during each of which there abounded corals and
shells, whose exuviæ went to form several seams of limestone. But that
this sea-bottom was at no period very far distant from land, is proved
by the drifted plants that occur in the sandstones and shales both
below and above, and which often show so little trace of maceration,
that we can hardly believe they were carried far, or floated for a long
while previous to being enveloped in the sand or mud at the bottom. I
have never detected vegetable remains in the limestones themselves, but
there seems no reason why they should not be found there.
One of the most remarkable and difficult phenomena presented by these
limestones is their great persistency. I have traced them over a
large part of Mid-Lothian, from the highly inclined beds at Joppa to
the contorted and faulted strata near Carlops. I have found them,
too, in many parts of West-Lothian and Stirlingshire, from the sea at
Borrowstounness southwards into Lanarkshire. They likewise occur in
Fife, and seem to sweep away through Lanark and Ayrshire. The area
in which I have found them cannot be much under 700 square miles,
yet they are probably spread over a much greater extent of country.
Throughout this region they appear to continue on the whole at pretty
much the same vertical distance from each other, and average three or
four feet thick each. They vary in number, three being found in parts
of Mid-Lothian, in other parts only two. Throughout West-Lothian there
seem to be but two seams in the middle or moor-rock series, and the
same two seams are found passing over into Perth near Culross. There
are differences, too, in the structure and composition of the seams,
one running sometimes as a single bed of dull blue limestone, and
then gradually splitting up into three layers of a greyer and more
earthy texture, with soft shale between them. But making all these
abatements, the observer cannot fail to be struck with the general
regularity and continuity of these limestones. And the fact becomes
all the more remarkable when we consider the great irregularity, and
continual intercalations, and repetitions of the strata, both above and
below. Marine beds are usually persistent over large areas, especially
where extensively developed. As they decrease in thickness, their
continuity for the most part lessens, so that the rule is, on the
whole, a safe one, the thinner any particular stratum, the less likely
are we to trace it to a considerable distance. Yet, not only are these
Mid-Lothian limestones thin, but they occur in regular sequence among
a set of continually alternating and very irregular beds, and extend
over several hundred square miles of country. And this, too, not in a
single seam, but in two, three, or even more, so that the difficulty of
accounting for such intercalations is proportionately increased.
We have seen above that the area of a delta is often partially
submerged below the sea, and that such changes may become of the most
marked kind where the country is liable to be depressed by earthquakes.
There can accordingly be no difficulty in understanding how the ancient
carboniferous delta of Mid-Lothian may have likewise subsided. But the
limestones are unmistakable evidence that not only was the area of
the delta submerged, but that for a while no sediment was deposited
over it, and hence marine animals peculiar to clear water flourished
so long and so abundantly as to form by their remains several beds of
limestone. Had these beds been merely local we might have regarded them
as having been deposited in lagoon-like portions of the delta, shut out
from the detrital matter of the river on the one side and open to the
sea on the other. But their wide extent and nearly uniform thickness
preclude such a supposition. The following explanation appears to me
the most probable:--
After the series of the Edge coals had been brought to a close, the
coal-fields of Scotland underwent a complete submergence below the sea.
This depression was probably very gradual, yet more rapid than that
long-continued one which had been going on during the earlier part of
the Carboniferous series, and the consequence of this greater rapidity
was to prevent the growth of stigmaria swamps or reedy jungles, by
keeping the alluvial surface continually sunk to some depth below the
water. The amount of subsidence until the deposition of the lowest
limestone may not have been great, but even a slight depression would
tell vastly on an area of flat delta land. Mud banks would be brought
down into the region of waves and surface-currents, and speedily be
spread out over the floor of the sea. Forest-covered islands would in
like manner be levelled down, and their trees sent drifting seaward
or submerged amid the re-formed silt. Thus altered, the delta would
sink below the sea, and the sediment borne down by the river would
be scattered out over the older deposits as a slowly-forming sheet.
By degrees this detrital matter must have been carried less and less
farther out to sea; in other words, the area of deposit or delta must
have crept gradually nearer to the land--a result owing partly to the
recession of the ancient coast-line, and partly perhaps to a greater
amount of depression inland than at the coast, which would of course
lessen the velocity of the streams and cause them to deposit their
burden of sediment at higher levels than before. The consequence
of this retreat of the delta from the sea would be to purify the
water over the site of the old swamps, and render it fitted for the
habitation of corals, molluscs, and other marine animals. A medium thus
prepared would not be allowed to remain long untenanted, and so we find
that it came to be densely peopled with the organisms peculiar to such
a station. Stone-lilies, cup-corals, net-like bryozoa, molluscs of many
kinds, and large predatory fish, swarmed in these old waters, and their
calcareous shells and skeletons are now broken up by the quarryman and
the collier as hard compact limestone.
After these animals had lived and died in successive generations,
perhaps for thousands of years, the downward movement of the earth's
crust seems to have ceased for a while or to have become greatly less.
The effect of this would be just to reverse what had been previously
done, especially if a slight elevatory movement took place. The streams
would in such circumstances descend from the uplifted ground with
renewed velocity and transport their detritus to gradually increasing
distances. The muddy and sandy sediment thus borne seawards would
slowly silt over the coral-banks at the bottom, and in conditions so
ungenial the organisms would dwindle down and finally die out. A great
thickness of sand and mud would be spread out over their remains so
long as the currents from the land continued to carry sediment out to
sea, and thus probably originated the sandstones and shales superposed
above the lowest limestone.
Eventually the old steady downward movement returned, and with it the
corals and stone-lilies. The detritus again sank to the bottom much
nearer the land, forming great banks and shoals that choked up the
river-mouth. Seaward the water regained its purity, and the bottom
once more swarmed with living things. Another lapse of many thousand
years may have here intervened during which the marine exuviæ gathered
into another seam of limestone, until again the process of subsidence
either ceased for a time, or what is perhaps more probable, became
considerably feebler. Detrital matter began to creep seaward as
before, and eventually entombed the corallines and crinoids to a great
depth. The calcareous bed thus formed is the second limestone, and the
superincumbent silt-beds represent the sandstones and shales that rest
above it.
In some such way as this does the Roslyn sandstone series appear to
have originated. I have indicated what seems to have been the main
features in the process, but it was probably a very complex one. There
may have been a great many oscillations of level of variable effects,
some of them raising the disturbed area to a much greater height at
one point than at another. This inequality would of course produce
marked effects along a low flat country such as that at the mouth of a
great river. New currents would be produced and the direction of old
ones changed; great shoals and banks of silt would be worn down only
to be thrown up again at some new point, where another oscillatory
movement would expose them afresh to destructive denudation. The
variations in the amount of elevation and depression would likewise
modify the transport of detritus to the sea, and give rise to a varied
and ever-changing sea-bottom. In short, the alternations and variations
must have been endless, for to the ordinary multiplied interchanges of
a delta we must add those induced by a constant and unequal oscillation
of the earth's crust.
The Roslyn sandstone series comes to a close, and passing onward in
ascending scale we meet with another great group of coal-bearing
strata. They occupy the central area of the Mid-Lothian coal-field,
and from their gentle inclination as compared with the lower strata
that rise up from under them on either side of the basin, are known as
the _Flat Coals_. Their total thickness--that is to say, all that has
escaped denudation--amounts to a thousand feet or more. They consist
chiefly of sandstones, shales, ironstones, and fire-clays, with from
twenty to twenty-five seams of coal, of which sixteen are thick enough
to be worked. Their similarity to the Edge coals below points to a
similarity in the conditions of formation. The frequent alternations
of sandstone and shale show how the delta gradually pushed outwards
again and re-occupied its ancient site above the successive forests of
the Edge series and the successive coral-beds of the Roslyn group. The
coal-seams indicate the further progress of the detrital accumulations,
and the eventual formation of vast swampy flats that nourished a rank
growth of stigmariæ, and tracts of drier ground waving with ferns, and
shadowed by the spiky foliage of the club-moss and the broader fronds
of the tree-fern.
The Flat coals are not succeeded by any other palæozoic strata. Above
them stretches the drift already described: sometimes in the form
of a stiff blue clay resting on a striated rock-surface; sometimes
as a coarse gravel containing fragments of all the rocks in the
neighbourhood; and sometimes as a fine white sand diagonally laminated,
and often showing dark partings of coal-fragments. From the section
given above (Fig. 32) at p. 196, the reader, will see that as the
upper limit of the Flat coals is formed by the drift, a large part of
that series may have been borne away by denuding agencies. Had there
been even a seam of limestone above them, it would have sufficed to
show their true thickness, for we should then have seen, that how
much soever had been removed in later times from above the limestone,
nothing had been removed from below it; and so it would mark the true
original limit of the series. We cannot now tell how much thicker the
upper part of the Mid-Lothian carboniferous system may have been.
Probably, during the long ages that intervened between palæozoic and
post-tertiary times, many hundred feet were borne away and carried to
other sites, there to grow up into new islands and continents, clothed
with other types of verdure, and peopled by other races of animals, and
fitted to become, in a long subsequent period, the dwelling-place of
man.
In fine, the evidence of these ancient changes in the history of the
Mid-Lothian coal-field is derived, as we have seen, from two sets of
facts; first, those of a mechanical, and, second, those of an organic
kind--the one class explaining and confirming the other. Beginning our
investigation at the horizon of the Burdiehouse limestone, we saw the
curtain rise slowly from off a wide estuary, in which there gambolled
large bone-covered fishes, while huge pine-trees--branchless and bare,
seed-cones, fern-fronds, and twigs of club-moss, floated slowly away
out to sea. The panorama moved on, and brought before us the ocean-bed
of the Roman Camp limestone, with its groves of stone-lilies and
bunches of coral; its tiny shells moored to the bottom, or creeping
slowly athwart the limy floor, or spreading out their many arms, and
rising or sinking at will. This picture passed slowly away, and then
came the delta of the Edge coals, with its sand-banks and ever-shifting
currents, its stigmaria swamps, and its forest-covered islets. We saw
the delta gradually sink beneath the sea, and corals and stone-lilies
cluster thick over its submerged area, to form the limestones of the
Roslyn group. Again, the mud-bars of the river crept out to sea, and
tangled forests waved green as of old, washed by the sea or inundated
by the river. How this last period came to a close, we shall probably
never know, and have no possible means of conjecturing. We pass at one
step from the ancient era of the coal to the comparatively modern one
of the drift--from a verdant palæozoic land, to an icy post-tertiary
sea. It is like a leap in history from the days of Pericles and Aspasia
to those of King Otho, or from the tents of Runnymede to the Crystal
Palace of Sydenham.