The structure of the stratified part of the earth's
crust conveniently studied by the examination of a
single formation--A coal-field selected for this
purpose--Illustration of the principles necessary to such
an investigation--The antiquities of a country of value in
compiling its pre-historic annals--Geological antiquities
equally valuable and more satisfactorily arranged--Order
of superposition of stratified formations--Each formation
contains its own suite of organic remains--The age of
the boulder defined by this test from fossils--Each
formation as a rule shades into the adjacent ones--Mineral
substances chiefly composing the stratified rocks few
in number--Not of much value in themselves as a test of
age--The Mid-Lothian coal-basin--Its subdivisions--The
limestone of Burdiehouse--Its fossil remains--Its probable
origin--Carboniferous limestone series of Mid-Lothian--Its
relation to that of England--Its organic remains totally
different from those of Burdiehouse--Structure and
scenery of Roman Camp Hill--Its quarries of the mountain
limestone--Fossils of these quarries indicative of an ancient
ocean-bed--Origin of the limestones--Similar formations still
in progress--Coral-reefs and their calcareous silt--Sunset
among the old quarries of Roman Camp Hill.
Among the standard jokes of ancient Athens was that of the simpleton
who, with the intent of selling his house, carried about a brick as a
specimen. In this and the following chapter I propose to follow his
example, and, for the purpose of giving my reader a correct notion of
the structure displayed in the stratified portion of the earth's crust,
to select therefrom a single formation whose details will connect
together the subjects discussed in the previous pages. And in so doing
it will, I trust, be found that what was ludicrous in the hands of
the Greek becomes sober sense in those of the geologist. The "brick,"
then, which I would humbly present to the thoughtful consideration of
the reader as really a specimen of the house of which it forms a part,
has been termed the "Carboniferous System," and consists of a series
of stratified rocks sometimes nearly 15,000 feet thick. The plants
and animals found in these strata have been already described somewhat
in detail, and we have turned aside to look at the processes whereby
such masses of sedimentary rock came to be accumulated. But we shall
probably better understand the habits of the animals and the general
aspect of the vegetation, as well as the agencies at work in depositing
vast beds of mineral matter, if we take a coal-field and analyse it
stratum by stratum, marking as we go their varied and ever-changing
character, and the corresponding diversity of the included organic
remains. Such an examination will bring before us some of the more
striking and important laws of geological research, and while of use
to the young observer, may be not without some share of interest
to the general reader. Before beginning, however, let me endeavour
to illustrate the principles that will guide us by a simple though
hypothetical story.
Suppose the bed of the Firth of Forth were raised above the level of
the sea and covered over with verdure, and that, in ignorance of the
previous topography of the locality, a mason were to excavate on the
lately-born land the foundation for a dwelling-house. Immediately
below the grass he would com? upon layers of hardened mud containing
oyster-beds, with detached valves of cockles, mussels, fish-bones, and
perhaps the tooth of an anchor or the timber of some old herring-boat.
Now, if he were gifted with but ordinary intelligence, what would he at
once conclude from these remains? Plainly, that the spot on which he
stood had once been the bed of the sea. And if in place of appearing
as dry mud and sand these deposits had got hardened into shale and
sandstone, and the shells, too, had become hard and stony, this would
not alter his convictions. He would still assert positively that he
stood upon an old sea-bottom. And suppose further, that all this were
far away from any sea, still such a circumstance could make no change
in his opinion; he would rightly assert that the place of sea and
land might vary, and that the ocean's being now many miles distant
could be no argument against the waves having once rolled over the
site of the intended dwelling-house. Let us further imagine that he
continues his trench, and in sinking deeper comes to a bed of dark
peat with snail-shells and bones of sheep, deer, and oxen. What will
he infer from these? Clearly that they represent an old land-surface,
once covered with vegetation and browsed over by ruminant animals, and
that this old land-surface has at some distant period been submerged
beneath the sea. Suppose, moreover, that below the peat there were a
thin bed of reeds and rushes intermingled with the mouldering remains
of fresh-water shells. He would in that case infer that before the
formation of the peat the locality was occupied by a lake.
Putting now all these deductions together, our mason would have evolved
a very interesting history. He would have ascertained that in a bygone
age the spot on which he stood was the site of a lake, tenanted by
delicate shells and fringed with reeds and rushes, where the coot and
the mallard may have reared their young; that in process of time the
vegetation gained upon the water, choking up the lake, so as gradually
to form a soil firm enough to support sheep, deer, and oxen, and
yielding shady coverts whither the antlered stag could retire and lay
him down to die; that in after years the sea had encroached upon the
peat-moss, and oyster-beds begun to form where cattle had been wont to
browse; that again the ocean receded, and the land emerged to assume
new verdure and receive new inhabitants.
Now, in all this reasoning there is no hypothesis or speculation. The
mason proves himself an intelligent, honest fellow, and uses his eyes
and his head where many other men would perchance see very little need
for the use of either. There can be no setting aside of his story;
he can appeal to facts. "There," says he, "is a layer of peat with
the rush-stalks and moss-fibres matted together in the soft brown
mouldering substance, exactly as I have seen them a hundred times in
the peat-cuttings on the moors, and I cannot but believe that they
must both have had the same origin, that is, that they grew in swampy
hollows of the land. There, too, lies a stratum of fresh-water shells
identical with those that occur in our ponds and marshes. Although
mouldering now, they are evidently not fragmentary, but entire and
unbroken; some of them are young, others full-grown, and they lie
grouped together as in our present lakes. Such shells could only live
in fresh water, therefore the spot where I stand must have been at one
time a fresh-water lake. There, again," he continues, "is a bed of
oysters which cannot have been transported hither, for their valves are
together, lying just as they do in our present oyster-beds. This green
field, therefore, must have been at one period a muddy sea-bottom."
After this manner and upon this kind of evidence must all inquiries
into the past changes of the earth's surface be conducted. And provided
only we proceed cautiously, reasoning from positive facts, and
striving as far as possible to exhaust what Bacon calls the "negative
instances," our deductions possess all the certainty of truth. For in
much the same fashion do we derive no small part of our acquaintance
with the early history of our own land, as well as with the arts
and customs of other nations. The scattered relics turned up by the
operations of the farmer--wooden canoes, flint hatchets, gold torques,
bronze pots, fragments of pottery, and rusty coins--all have their
bearing upon the annals of the country, and so clear is the evidence
which they read out that an eminent antiquary has divided the early
ages of Scotland into three periods, distinguished, from the character
of their relics, as the "Stone Period," the "Bronze Period," and the
"Iron Period."[57] But in such a classification the historian has
little to guide him save the nature of the relics themselves. He places
the rudest first, and groups the rest in succession, according to the
degree of advancement in civilisation which they respectively indicate.
And the grouping seems just, though in some cases objects belonging
to two of these periods may have been to some extent contemporaneous,
just as thatched roofs gave way to tiles, tiles to slates, and slates
partly to lead, though at the present day a walk of half an hour in
some localities will bring before us specimens of all these styles
still in use. If, however, the relics of geological history lay
scattered about like those of early Scottish history, all hope of
ever attaining to anything like a correct chronology and arrangement
would have to be abandoned in despair. In truth, it would then be
impossible to conjecture whether any succession of ages preceded man,
during which other tribes of plants and animals lived and died, or
whether the whole mass of fossiliferous rocks had been accumulated
since the human era, or perhaps created just as we find them. But all
this uncertainty and confusion has been obviated simply by the fossils
being ranged in beds vertically above each other, the oldest at the
bottom and the latest at the top. So that if we find in a low cliff
along the shore blown sand and broken whelks immediately beneath the
vegetable mould, and oyster-valves in a clayey bed three feet below, we
pronounce the oysters to have lived before the whelks, and that between
their respective lifetimes a sufficient interval must have elapsed to
allow three feet of sand, clay, and gravel, to accumulate. What is thus
true on the small scale holds equally so on the large. The stratified
formations in which organic remains occur are found to be grouped
regularly over each other in a settled invariable order. If A be below
B in England it will be below B all over the world, and if C be above
D at the North Pole it will be so at the South Pole too, and at every
locality where the two rocks lie together. This order of superposition
forms one of the grand tests for the age of different rock masses. By
means of this simple rule the geologist has been enabled to arrange
the different stratified formations, supplying the missing portions of
one locality from the more complete series of another, so as to form a
chronological table of no small part of our planet's primeval history.
[Footnote 57: See Dr. Daniel Wilson's deeply interesting work _The
Pre-historic Annals of Scotland_.]
But this is not all. We must attend to the character of the organisms
as well as to their order of occurrence. We must distinguish the animal
from the vegetable, the terrestrial from the marine, and scrupulously
examine the peculiarities of each so as to recognise them again in
other strata. By such careful scrutiny we may trace out the successive
changes in the physical aspect of a district during past times,
viewing in terrestrial plants (when clearly occupying their original
site) evidence of an old land-surface; in _cyprides_, _unios_, and
_paludinæ_, traces of a former lake; and in corals and marine shells,
unmistakable proofs of an ancient sea-bottom. Still further, by marking
the specific character of such fossils we obtain a key to the age of
many rocks that otherwise would be unintelligible, for it is found that
each of the stratified formations, from the oldest upwards, has its own
peculiar and characteristic organisms recognisable all over the world.
This test of the geological position and age of any fossiliferous rock
has a peculiar value, for it can be applied with infallible success
where every other fails. The order of superposition is often obscured
by dislocations and other causes, and the mineralogical texture of a
formation may change entirely in a short space; but if the imbedded
fossils remain, we can be at no loss as to the relationship of the rock
which contains them. And hence, if in some lone island of the Hebrides,
haunted only by the screaming sea-fowl, we find a patch of shale
containing ammonites, belemnites, and a host of other shells in large
measure identical with those occurring among the clays and limestones
of Gloucestershire, we infer that they must all belong to one series
and be of the same age; that, as we know the English beds to form part
of a formation called Lias, of which, the exact place in the geological
scale has been ascertained, so in like manner the Scottish beds must
occupy a position in the same series; and that consequently there was
a time when the site of Cheltenham and part of the Hebrides lay each
beneath a sea which teemed with ammonites, belemnites, and many other
mollusca, along, too, with the bulky saurians of the Lias. And yet
no study of the surrounding rocks in the northern locality, even if
carried on for a thousand years, could ever have thrown one ray of
light upon the subject. In an earlier page our grey rounded boulder
was introduced to the reader as a mass of sandstone belonging to the
Carboniferous group of rocks. How could one be sure of the precise
geological age of a loose water-worn block that might have journeyed
all round the world? Simply by its included fossils. The calamite,
lepidodendron, and stigmaria, revealed the date of the stone as clearly
and unmistakably as if we had seen it lifted from its original bed
by the lever and crane of the quarryman. These plants are peculiarly
characteristic of the Carboniferous strata, and they consequently
stamp as undoubtedly of carboniferous age the rock which contains
them, whether it be sandstone or conglomerate, limestone or shale, and
whether we meet with it among the newly-raised blocks of the quarry,
or among the pebbles of the sea-shore. Each geological formation, I
repeat, beginning at the oldest known to us, and ending with those that
are still forming in our lakes and seas, has its own set of organic
remains whereby we can detect it wherever it may chance to occur, from
the equator to the poles. Each has its _style_, so to speak, just as
we can at once tell whether a drawing represents a Hindoo, Egyptian,
Assyrian, Greek, or Gothic temple, simply from the general _style_ of
the architecture.
Could we but voyage back in time as we can sail forward in space,
we should find each of the geological formations not less clearly
defined than are the different nations and countries of the present
day.[58] Were the reader suddenly set down in an out-of-the-way street
of Paris, he would probably not be long in discovering that he stood
on French ground. Or if spirited away in his sleep he should awake on
the banks of the Nile, he would soon ascertain himself to be in the
land of the Ptolemies. And so if you transported a geologist blindfold
into a quarry where ammonites and belemnites abounded, mingled here
and there with bones of ichthyosaurs and plesiosaurs, he would tell
you at once that the quarry lay among liassic strata. Or if he were
placed in a ravine where the rocks on either hand displayed fern-stems,
lepidodendra, stigmariæ, and sigillariæ, he would tell you that the
surrounding district was one of carboniferous rocks, and that probably
at no great distance there might be found smoking engines and dozens
of coal-pits. Or could you set him down in some dark night upon a
wild coast-line, and show him, perchance by the flare of torch-light,
bones and scales of osteolepis, pterichthys, and dipterus, lying on
the rocks around, he would tell you that the grim crags which shot up
into the gloom were as ancient as the era of the Old Red Sandstone. In
any case the character of the rock would signify nothing, nor would
he care about the general features of the landscape, though these too
become important characteristics in certain cases. Show him but a few
recognisable fossils, and you give him, as it were, an "Open Sesame" to
which the rocks unfold their gates and reveal a store of wonders yet
more varied than those in the cave of Ali Baba.
[Footnote 58: See _ante_, pp. 31, 32, and the Table of Rocks at the end
of the volume.]
But though the geological systems stand thus strongly marked off from
each other when viewed as a whole, their boundary lines can often be
only approximately drawn, thereby reminding us that the divisions are
of man's device, and can have had no place in the plans of Him who
needs not to chronicle His working by years and ages, but with whom
there is no past and no future. One formation insensibly passes into
another just as one nation merges into those around it. There are
sometimes gaps, however, between the formations, serving to mark out
strongly the limits of each,[59] precisely as intervening seas and
mountain-chains serve to mark put the boundaries of different peoples
and tribes.
[Footnote 59: Such cases, however, are probably merely local, and may
have originated from some features in the ancient physical geography
of the districts where they occur. For instance, it has always been
thought that palæozoic ages were marked off by a strong line of
demarcation from succeeding secondary times. But the gap which occurs
in England, France, and Germany, is being slowly filled up from the
evidence furnished by other countries, and we shall probably find in
the end that the Permian dovetailed with the Trias as closely as the
Silurian with the Old Red, or the Lias with the Oolite. In truth, the
longer we study the past history of our planet the less do we see of
hiatus and chasm and sharp clearly defined boundary line; while the
doctrine of a uniform system of laws and arrangements in the physical
world, first philosophically propounded in the immortal "Principles" of
Sir Charles Lyell, is ever receiving fresh confirmation.]
The mineral substances of which these formations consist are
comparatively few in number, being chiefly varieties of sandstone,
shale, conglomerate, and limestone. One sandstone can often be scarcely
distinguished from another, and so also with the other rocks; hence
such tests as mineralogical texture supplies can seldom be relied
on to determine the age of rocks. We can prove, for example, that a
series of limestones in England may be identical in age with a set of
sandstones in Sweden, and with a group of shales in America, because
they all contain the same or representative genera and species of
organic remains. They occupy the same position in the geological scale;
that is, the animals whose fossilized remains lie buried in these rocks
were all living at the same time, while lime was gathering at the
sea-bottom over the site of part of England, and sand was being thrown
down upon a portion of what is now Sweden, and mud was accumulating
over a submerged area of America. In such cases the differences of
mineralogical character go for nothing in determining the age of the
rocks; we have to rely solely on the embedded fossils, and on the order
of superposition.
Keeping in view, then, that the formations into which the geologist
has grouped the stratified portion of the earth's crust have a settled
and invariable order of occurrence, that each of them contains its own
peculiar and characteristic group of organic remains whereby it can
be recognised in any part of the world, and that such remains form
often the sole test at once of the geologic age and of the origin of
the rocks wherein they lie, we may return to the plan above proposed
and endeavour to understand the structure of a coal-field. For this
purpose it may be well to select one of the northern coal-fields of
Britain, since these perhaps display a greater variety in their organic
contents, and bear evidence of more diversified changes in their mode
of formation than can be seen in those of the south. The strata that
compose the coal-basin of Mid-Lothian will probably best suit our
purpose, as they are free from the disturbing effects of those igneous
intrusions which play so important a part among similar rocks to the
north and west.
The Mid-Lothian coal-field comprises a mass of stratified beds of
sandstone, shale, coal, ironstone, and limestone, the united depth of
the whole being above 3000 feet. By reference to the annexed Table it
will be seen that the lowest beds of the section are chiefly sandstones
and shales, extending downwards to an unknown depth, without any coal
that can be profitably worked. These under-strata form the Lower
Carboniferous group. Above them comes a middle zone in which the
characteristic beds are of limestone, comprising the middle portion or
Mountain Limestone of the Scottish Carboniferous rocks. The third and
highest subdivision forms the Upper Carboniferous group or true Coal
Measures, and constitutes the whole of what is properly the Mid-Lothian
coal-field. For the sake of noting some of the remarkable changes
exhibited in the character of the rocks, it may be well to begin our
survey among the upper beds of the under group. Let us take as our base
the famous limestone of Burdiehouse, and work our way upward through
the four thousand feet of strata that lie piled above it.
VERTICAL SECTION OF THE MID-LOTHIAN COAL-FIELD.
UPPER CARBONIFEROUS
OR COAL-MEASURES.
/ / | | A series of sandstones, shales,
| | | | and fire-clays, with interbedded
| =Flat Coal Group.= / | | seams of coal occupying the
| (Above 1000 feet.) \ | | central area of Mid-Lothian
| | | | coal-field.
| \ | |
| |—————————|
| / | |
| | | |
| | | | A great series of sandstones
| | | | and shales with three seams of
/ =Roslyn Sandstone / | | marine limestone (marked here
\ Group.= \ |·········| by dotted lines), With the
| (About 1500 feet.) | | | exception of one or two thin seams
| | |·········| it contains no coal, and serves in
| | | | this way to mark off the coal-
| | | | bearing beds above from the still
| | | | richer coal-bearing beds below.
| \ |·········|
| |—————————|
| / | |
| | | | A group of sandstones and
| =Edge Coal Group.= / | | shales similar to those at the
\ (800-900 feet.) \ | | top, and like them abounding in
| | | coal seams, some of which are
\ | | thick and valuable.
————————————————————————————————————————————————————————————————————————
CARBONIFEROUS LIMESTONES. | |
/ =Roman Camp / | | A set of marine limestones
| Limestones.= | | | intercalated with sandstones,
\ (150-200 feet.) \ | | shales, and a few seams of coal.
————————————————————————————————————————————————————————————————————————
LOWER CARBONIFEROUS. | |
/ | |
| | |
| =Burdiehouse / | | Sandstones and shales extending
| Limestone.= | |·········| to an unknown depth, often
| (27 feet.) \ | | with seams of dull-grey compact
/ | | limestone, rarely of coal. The
\ Thickness of Lower | | beds become very red towards
| Carboniferous Rocks | | the base, and wholly devoid of
| unknown, but | | fossils.
| probably greater | |
| than that of the | |
\ upper. | |
The Burdiehouse limestone is twenty-seven feet thick, of a yellowish
or bluish-grey colour, very compact, splintery, and often fissile
in structure, with a finely striped and laminated appearance, which
probably indicates a slow and tranquil origin. It is Crowded with
fossils, every fragment when taken up showing its seed-cone, fern-stem,
fish-scale, or minute _cyprides_. All the plants seem to belong to
terrestrial species, and have a broken and often a macerated look.
Manifestly they never grew where we now find their remains; they must
have come drifting down from swamp, or jungle, or hill-side. And so
we come to know that during the later ages of the Lower Carboniferous
period, there lay somewhere in the neighbourhood of Burdiehouse a land
clothed with ferns and club-mosses, and through whose swampy hollows
there spread a network of stigmariæ, while sigillariæ waved their
fronds high overhead. From what has been said on a previous page we may
infer that the climate of the old land was moist and equable like that
of New Zealand, nourishing a prolific growth of ferns and other plants
comparatively low in the botanical scale. The scenery of the vegetation
displayed perhaps no great variety of outline, but exhibited rather an
endless succession of the same graceful forms.
But the limestone presents us with other remains than merely those of
terrestrial plants. It displays in abundance the minute dissevered
cases of _cypris_, the small crustaceous animal described above.
Recent species of this genus inhabit stagnant ponds or the bottoms of
gently-flowing rivers, and we hence infer that the ancient species must
in like manner have possessed a similar habitat, and consequently that
the rocks which preserve their remains must have been deposited in
fresh (or, perhaps, brackish) water. Tried by this test the Burdiehouse
limestone must be regarded as a lacustrine, or more probably a
fluviatile formation, which gathered slowly on an undisturbed bottom
swarming with crustaceans and plentifully covered with leaves,
branches, rootlets, and other fragments of terrestrial plants brought
down by streams from the adjoining land. Thus the inferences drawn from
the numerous plants, and from the countless multitude of cypris-cases,
come to be mutually corroborative. The former tell us of some
neighbouring forest-covered country; the latter lead us, as it were,
into its river-mouths, whence we can descry the waving woods on either
side.
Still we have not exhausted all the fossil remains of the Burdiehouse
rocks. Mingled among the stems of ferns and lepidodendra, and the
scattered valves of the cyprides, lie the scales, teeth, and bones,
of several large ganoidal fishes, along with entire specimens of the
smaller genera. The scales of holoptychius are especially abundant,
often crowded together by dozens, and probably not far out of the
arrangement they had when grouped on the body of the living animal.
Detached teeth of the same fish also frequently occur along with
disjointed internal bones. The remains of the contemporary megalichthys
likewise abound, more particularly the scales, which have a fine
nut-brown colour, and dot the surface of the rock with their bright
glittering enamel. Several other smaller ganoids may be met with,
especially a small and elegant species of Palæoniscus (_P. Robisoni_),
and one of Eurynotus, a fish remarkable for the great size of its
dorsal fin. Not uncommon, too, are the ichthyodoralites of a gigantic
placoid--the _Gyracanthus formosus_--with all their delicately-fretted
ornament and a peculiar crystalline glistening surface when broken
across, whereby the smallest fragment can be easily distinguished from
any other bone in the limestone. Such are the ichthyic remains of the
Burdiehouse beds; what deductions can be legitimately drawn from them?
As before, we must have recourse to the analogy of living nature. The
existing ganoidal fishes chiefly inhabit lakes and rivers, especially
near the confluence of the latter with the ocean. They feed on
the decaying matter brought down from the land, or on the minute
Crustacea that swarm upon the river-bottom. If, as seems probable,
the ancient ganoids had habits similar to those of their present
representatives, then the rocks wherein their remains occur abundantly
may have originated on river-bottoms, and such may have been the case
at Burdiehouse. So that here again we have corroborative evidence
of the fluviatile origin of the limestone in question. But besides
the remains of ganoidal fishes there occur the defensive spines of
placoids. Now, the placoids are emphatically marine fishes, and the
sole living representative of the most ancient genera of this order
is the Port-Jackson shark, that haunts the seas round Australia. The
ichthyodorulites of Burdiehouse, therefore, if we would apply analogy
consistently, must be regarded as the relics of marine species. And
this conclusion, too, will be found in entire harmony with those
already obtained, for if we are right in assuming the Burdiehouse
strata to have originated at a river-bottom, particularly near the sea,
we may expect to find the remains of marine predaceous fishes imbedded
in the sediment that gathered there, just as the teeth of the shark may
be preserved among the mud forming in the upper reaches of many British
estuaries, seeing that not a few instances are known where that fish
has been stranded on such shores as those of the higher parts of the
Firth of Forth. These Burdiehouse ichthyodorulites give positive proof
that the limestone could not have originated in a lake, and the only
explanation left is that of a river-bottom.
But it may perhaps be objected that, after all, these fish-remains
are for the most part fragmentary, and may consequently be drifted
specimens, so that no conclusion as to the source of the rock can be
based on their occurrence there. The imbedded land-plants confessedly
came from some distance, why may not the same have been the case with
the bones and scales of the river-haunting ganoid fishes? And, indeed,
did we regard these fish-bones and scales merely in themselves, the
argument might not perhaps be very easily answered, although the great
numbers and perfect outline of the bones, teeth, and scales, afford
pretty strong evidence that the owners lived and died in the locality
where their remains are found. But there is a curious kind of evidence
to be gleaned from the rocks around them whereby this objection can be
at once set aside. In the limestone itself, and especially in some of
the shales above, there occur vast numbers of small oblong coprolitic
concretions of a dirty yellow or brown colour, full of scales and
fragments of bone. There can be no doubt that these are the excremental
remains of predaceous animals, while their great number and perfect
preservation assure us that they could not have been drifted from a
distance, but must rather have been deposited on the spot where we now
find them. And thus we conclude that the site of Burdiehouse must have
been a favourite haunt of these bone-covered fishes; that the bulkier
forms, armed with pointed teeth or barbed-spines, preyed upon their
humbler congeners, while these in turn may have fed on the cyprides
that swarmed by millions at the bottom of the estuary. I have often
detected in these coprolites the peculiarly-sculptured scales of the
palæoniscus. These graceful little animals must, therefore, have died
that their lordlier brethren might dine.
On a survey, then, of the whole evidence from fossils, we are led to
conclude that the Burdiehouse limestone was slowly elaborated at the
bottom of an estuary, into which the remains of terrestrial plants were
drifted from the land, while bone-covered fishes haunted the waters,
and into these busy scenes huge sharks ascended from the sea to share
in the decaying putrescent matter ever brought down from the interior.
The upper part of the limestone is shaly and argillaceous, and rests
below a series of shales and thin sandstones. If the question were
asked, what caused the change from limestone to shale, from the
deposition of a calcareous to that of a muddy sediment, several
answers might be given. The most probable seems to be the following.
The limestone on weathered surfaces displays the mouldering casts of
cypris-cases sometimes in such abundance as to show that the rock must
be largely made up of them. The cyprides of the present day probably
cast their shells annually; the integuments thus thrown off forming
under favourable circumstances a thin mouldering calcareous marl at
the bottom of the pond or marsh, along with the decaying shells of
_paludina_, _planorbis_, _limnea_, or other fresh-water molluscs. We
may conceive the Burdiehouse limestone to have had a similar origin.
The cyprides, inhabiting water that contained little argillaceous
matter, must have propagated by myriads, and during a long period of
repose, in which the conditions of land and sea, and the directions of
tidal currents and river-courses, appear not to have greatly varied
in the neighbourhood of Burdiehouse, the calcareous exuviæ of these
minute animals, along perhaps with the remains of other estuarine or
fluviatile organisms,[60] would form each year a scarce appreciable
stratum, until by slow aggregation a bed twenty-seven feet deep was
elaborated. Each successive annual layer would hardly settle down more
perceptibly or more rapidly than "the flickering dust that mottles the
floor of some old haunted chamber."
[Footnote 60: Though I have never observed molluscan remains in the
limestone of Burdiehouse, they are abundant twelve miles to the west,
in the equivalent strata around Mid-Calder, one little gastropod
being especially plentiful near the base of the calcareous rock in a
seam known to the quarrymen as the "Buckie fake." I have not met with
specimens sufficiently perfect for identification, the hard splintery
nature of the rock seldom allowing anything but a cross-section to be
seen save on weathered specimens, where the general contour of the
shells has sometimes reminded me of _Paludina multiformis_ grouped
together in a recent fresh-water marl. In the shales above the
Burdiehouse limestone, Dr Hibbert states he found a _unio_ (?), called
by him _U. nuciformis_. _Trans. Roy. Soc. Edin._ vol. xiii. p. 245.]
At last, however, this condition of things came to be modified. The
direction of the river channel along some part of its course had
varied, or some analogous change had taken place, so that muddy
sediment transported from the land sank down amid the cyprides at
the bottom. In circumstances so uncongenial these tiny denizens
of the estuary diminished in numbers until the silt and sand came
down so rapidly and in such abundance that they eventually died out.
Alluvial matter still darkened the water and covered the river-bottom,
enveloping now the fronds of a delicate fern that had waved along
the margin of some sequestered lake far inland, anon a seed-cone
that had been shaken by the breeze from the spiky branches of some
tall club-moss. Among these muddy beds occur numerous coprolites and
fish-scales, along with cypriscases and a few shells of unio (?),
showing that though the cyprides were decreasing, the water still
presented the old estuary conditions and still swarmed with life.
Eventually there came other changes in the direction or rapidity of
river currents, and the accumulations of mud and silt were succeeded by
a long protracted deposition of yellow sand, now forming the sandstone
of Straiton. It enclosed many stems of stigmaria, lepidodendron, &c.,
and in certain limited areas these plants matted together in such
quantities that their remains now form thin irregular seams of coal.
It would appear, therefore, that notwithstanding these changes in the
matter transported and deposited at the locality in question, the
estuary character of the locality remained essentially the same. The
sand was at length replaced by fresh accumulations of mud and sandy
silt, which went to form the beds of shale and shaly sandstone now
found above the Straiton rock.
When in the course of many long centuries a depth of strata amounting
to fully 300 feet had been amassed, the area of Mid-Lothian underwent
a total change. Owing to a depression of the earth's crust, that seems
to have been general over the whole of central Scotland, the estuary
in which the Burdiehouse limestone and superincumbent strata were
deposited became open sea. As the evidence of this change rests solely
on the character of the imbedded organic remains, we shall pursue our
induction by examining the beds somewhat in detail.
Rather more than 300 feet above the limestone of Burdiehouse there
occurs in the Mid-Lothian coal-field a series of shales and seams
of limestone. The former are sometimes black and hard, sometimes
bluish-grey, soft, and frequently imbedding the remains of several
genera of mollusca and other organic remains. The limestones vary
considerably in the thickness and general aspect of their several
seams, some being highly crystallized and about two or three feet in
depth, others dull, compact, and ranging up to twenty and thirty feet
thick. The shales and limestones are intercalated with and sometimes
pass into each other, through the gradations of shaly limestone and
calcareous shales. The whole series may measure 150 to 200 feet,
resting on the Straiton sandstone below, and passing upwards into
the under part of the coal-bearing strata of Mid-Lothian known as
the _Edge series_. These limestones form the northern _marine_
equivalents of the mountain limestone of England, while the sandstones
and shales on which they rest, including the Burdiehouse beds and
all the Lower Carboniferous group, must probably be regarded as
_estuarine_ equivalents of the same formation. That is to say, while
marine limestones were accumulating over the site of central England,
sandstone, shale, and drifted plants, were slowly gathering in a wide
estuary over what is now central Scotland, and only at the close of the
period did marine limestones form simultaneously at both localities.
In examining these Mid-Lothian beds we are struck at once with the
great dissimilarity that obtains between their organic remains and
those of the underlying strata. All the land-plants disappear--ferns,
lepidodendra, sigillariæ, and stigmariæ. The cyprides, too, no longer
occur, though the shales seem, at a first glance, to differ in no
respect from those underneath, in some of which the cypris-cases were
seen to abound. Neither can we detect the glittering scales and teeth
that stood out in such strong relief upon the rocks below. Yet the
fossils are scarcely less numerous than they were in the lower beds.
Nay, in some of the limestones they lie so crowded together that the
rock seems entirely made up of them. Plainly such a total renovation
of organic life points to some equally extensive change of a physical
kind. Let us examine for a little some of the fossil remains occurring
in the mountain limestone series of Mid-Lothian, and read off, if we
can, the revolutions which they chronicle.
[Illustration: Fig. 32.--Section from Gilmerton to Crichton; _a_, Lower
Carboniferous; _b_, Mountain Limestone; _c_, Edge Series; _d_, Roslyn
Sandstone Group; _e_, Flat Coals; _y_, Drift.]
The neighbourhood of Edinburgh affords many facilities for the study of
these rocks. They can be seen, for instance, at many points along the
ridge of the Roman Camp Hill, near Dalkeith, exposed in the operations
of quarrying. That ridge is formed by what is known technically as
an anticlinal axis (Fig. 32); in other words, the lower beds of the
coal-measures rise up here into a sort of broad wave-like undulation,
round the sides of which the higher parts of the series are folded. The
elevated area has either been pushed up from below, or the more level
country around has subsided into two trough-like hollows, so that now
the strata, which geologically speaking are lowest, come to occupy the
highest ground in the district. Seated on some of the opener spots
of this woody eminence the observer has a noble prospect on which to
expatiate. The ground around him is rich in historic associations,
and links itself to many a varied page in the annals of Scotland. The
hill on which he rests is crowned by the mouldering mounds of what
tradition reports to have been a Roman station, but which may perhaps
belong to a still earlier era. A few hundred yards north rise the
wooded slopes of Carberry Hill, where the hapless Mary surrendered to
her rebel lords, and whence she was led into her own capital amid the
insults of an infuriate rabble. Northward, too, lies the fatal field
of Pinkie, and eastward the less deadly but not less decisive field
of Prestonpans. To the west the eye can mark the grey smoke of the
Scottish metropolis, with its dusky towers and its lion-shaped hill,
and then the blue waving outline of the Pentlands that sweep away
south and lose themselves among the distant hills which girdle in the
coal-basin of Edinburgh and Haddington. The course of the Esk--that
_fabulosus amnis_--passes by many a time-honoured spot, from Habbie's
Howe and the scene of the Gentle Shepherd down by the haunted scenery
of Roslyn, the cliffs of Hawthornden, the grounds of Newbattle, and
the old Roman station of Inveresk. East, west, and south, the broad
expanse of green field and clustering wood swells upward to the distant
hills that encircle the landscape with a wavy line of softest blue.
Northward the eye rests on the Firth of Forth with its solitary sails,
bounded by the bosky heights of Fife, and opening outwards by the May
Island and the Bass Rock into the far-off hazy ocean. On every side
objects of historic interest lie crowded together, about which many
pleasant volumes have been and might still be written. If the observer
be a lover of geological science he will find an examination of the
structure of the hill to impart an additional interest to the scene.
From the wide panorama of hill and dale, river and sea, with all its
battle-fields, castles, and abbeys, and all its memories of the olden
time, let him turn into one of the quarries that indent the flanks
of the hill, and try to decipher there the records of a still older
history. An hour or two thus spent will pass swiftly and pleasantly
away, and on quitting the quarry he will have gained a new light in
which to look on the landscape that lies spread out below.
The mountain limestone of Mid-Lothian consists, as has been mentioned,
of several seams interbedded with black and calcareous shales. The
quarries on Roman Camp Hill have been opened in several of the thickest
of these seams. Let us enter one of the excavations. A vertical face
of rock forms the background, overhung above by long dangling tufts of
withered grass, and washed below by a pool of water having that milky
green tint peculiar to old lime-quarries. The lowest rock visible is
a dull grey limestone with a yellowish weathered surface. Above it
rests a mass of hard yellow calcareous shale, known to the workmen
as "bands." This rock is worthless as a source of lime, nor from its
irregular laminations and shivery structure has it much value in any
other way. A few inches of surface-soil form the upper part of the
section. It requires but a glance over the weathered surface of the
limestone to mark that the rock abounds in fossils. Of these by far
the most numerous are the joints of the stone-lily, for the most part
of small size, and when broken across, with their minute central
apertures, looking like so many fractured stems of tobacco-pipes. Other
organisms also occur, such as a small delicately-plaited productus,
a larger and more boldly-ribbed spirifer, a small cyathophyllum or
cup-coral, and the fragile interlacing meshes of one of the net-like
bryozoa--the fenestella. Of rarer occurrence are the whorled shells
called bellerophon, the long chambered shells of orthoceratites, and
the grooved tapering shells of pinnæ. Many of the same fossils can be
detected in the beds above, which thus evidently all form part of one
series with the rock below. What, then, were the circumstances under
which these strata originated?
The answer to such a question is not far to seek. The corals and
crinoids are exclusively marine families, and so any stratum in which
their remains occur must have had a submarine origin. It matters not
in this case though the specimens be fragmentary, showing a broken and
drifted appearance. For even supposing that they did not live at the
spot where their petrified relics are now exhumed by the operations of
the quarryman, granting that they were drifted from a distance, still
they could only have been drifted from one part of the sea-bottom to
another. The state of keeping of the specimen often tells vastly on the
value of its evidence when it belongs to a land or fresh-water tribe.
Thus, in one of the limestones of West-Lothian I have found a black
carbonized stem of sigillaria. Now, the sigillaria was a land-plant
as much as any of our hazels or willows, and where the evidence from
the associated organisms coincides, furnishes its own testimony as
to the origin of the rock which imbeds its remains. But the stem in
question was a mere fragment, and showed moreover a worn macerated
surface. Such a fossil had evidently no value as a test of the origin
of the limestone, which might have been elaborated either in an inland
lake or in open sea. That it had really a marine origin, and that the
sigillaria actually was, as it seemed to have been, a drifted plant,
I ascertained beyond a doubt by detecting on the same slab hundreds
of encrinal stems along with the shells, and thin, delicate, silvery
spines of productus. Thus, then, the organisms of the land may be
carried into the sea, and in dealing with their fossilized remains
in the deposits of former ages we must be very careful in the use of
evidence derived from fragmentary and drifted specimens. But no such
caution is needed in regard to the productions of the sea. If they be
fragmentary and drifted, we may believe they were rolled about by tides
and currents previous to their final entombment; but still they remain
as good a test as ever of the marine character of the rock in which
they occur.[61]
[Footnote 61: The exceptional instance, of the accumulation on the land
of blown sand imbedding the broken remains of marine shells, needs only
to be noticed here.]
The fossils of Roman Camp Hill are not drifted specimens. They must
have lived and died where the quarryman now finds them. We recognise
them as all unequivocally marine; corals, crinoids, and brachiopodous
molluscs, are all clearly the denizens of the sea, and hence we
conclude that they mark the site of an ancient ocean. The snail-shells
that swarm about the fruit-trees of our orchards not more unmistakably
indicate a land-surface than do these petrified relics evidence an old
sea-bottom. We can argue, too, from the crowded way in which they lie
grouped together, that life must have been prolific in these primeval
waters. Every fragment of the rock shows its dozens, nay, hundreds,
of stone-lily joints, disjointed indeed, yet easily recognisable.
They must have swarmed as thickly along the floor of the sea as the
strong-stemmed tangle that darkens the bottom of many a picturesque
bay along our western coasts, yet with a gracefulness of outline such
as none of our larger sea-weeds can boast. Less numerous but not less
markedly _in situ_ are the shells of productus and spirifer, the former
with its finely-striated surface fresh as if the creature had died but
yesterday, while the slender spines with which it was armed lie strewed
around. In short, the whole suite of organisms points to a period of
tranquil deposition in a sea of probably no great depth, where the
lower forms of the animal kingdom flourished in abundance, contributing
by their calcareous secretions to form continuous layers of limestone.
Such a condition of things finds a parallel in many parts of the globe
at the present day. Thus, the shores of the islands of the Pacific
are white with fine calcareous mud, that results from the action
of breakers on the surrounding coral-reefs. This mud, enveloping
fragments of coral, shells, sea-weed, drift-wood, and other extraneous
substances, hardens on exposure, and becomes eventually a limestone,
travertine, or calc-sinter. We may believe that the same process goes
on out at sea, around the edges of atolls or circular coral-reefs,
and that the sediment thus thrown down will enclose any zoophytes
or molluscan remains that may lie at the sea-bottom, along perhaps
with _fuci_, and occasional water-logged fragments of wood that have
been drifted from land. Along the shores of Guadaloupe a bed of this
calcareous silt has formed since America was colonized by man, for it
has been found to contain fragments of pottery, arrow-heads, and other
articles of human workmanship.[62] The same rock has yielded, besides,
the partially-petrified bones of several human skeletons, one of which,
though without the head, forms a prominent object among the fossil
treasures of the British Museum. The rock in which these remains are
embedded is described as harder than statuary marble, notwithstanding
its recent origin. By supposing the same process to be carried on over
a large area and for a long period, we may see how a continuous stratum
of limestone could be elaborated, full of fossil relics of corals,
molluscs, and other marine productions. And in some such way, we may
be permitted to believe, the seams of limestone on Roman Camp Hill
were accumulated. The billows of that old carboniferous ocean may not
have sent up their white surf against the margin of snowy coral-reefs,
but the currents below did their work of demolition as effectually,
and by sweeping through the submarine groves of stone-lilies and
cup-corals, as the night winds of autumn sweep athwart the heavy-laden
fields, would prostrate many a full-grown stem and scatter its
loosened joints among the thickening lime that covered the bottom.
Stone-lily, cup-coral, net-coral, productus, spirifer, pinna, nautilus,
orthoceratite, all would eventually be entombed amid the decaying
remains of their congeners, and thus produce a slowly-increasing seam
of limestone.
[Footnote 62: Lyell's _Manual of Elementary Geology_, p. 121. Fifth
edition.]
We still linger in the old quarry on Roman Camp Hill, but the day
draws rapidly to a close, and the long level beams of the setting sun
lighten up the higher grounds with a golden flush, while the valley
below lies deep in shade. The rays fall brightly on the abrupt face of
limestone at the further end of the quarry, every prominence standing
out in bold relief, and casting its shadow far behind. Our eye, in
passing over the sunlit rock, can detect the fractured joint of many an
encrinite glancing in the light; along, too, with the strongly defined
outlines of some of the lesser and more abundant molluscs--spirifers
or producti. Some of them, sorely effaced by the rains, have begun
to yield a scanty nestling place for creeping fibres of moss; others
yet bare, afford a rest to the _Vanessa_ whereon to spread its wings
in the mellow sunset ere flitting homewards among the dewy herbage.
The bushes overhead scarcely rustle in the light-breathing air that
comes fitfully across the land, and the long grass nods dreamily on
the margin of the pool below. There rests a calm stillness on all the
nearer landscape, and the distant ground blends away into the shades
of evening. The scene, in short, has about it that solemn impressive
repose which irresistibly arrests the fancy, and sets it to dress up
into fantastic shapes the massive clouds that float in the western
sky, to picture grim forms amid the misty shadows of the valley, or to
dwell half dreaming upon the memories of the past, that come crowding
through the mind in quick succession. Our labours among the fossils of
the old quarry, however, enable fancy to draw her stores from another
source. We muse on these petrified relics, gilded by the last rays
of the setting sun, when slowly, like a dissolving view, sunset and
herbage melt away, and the bottom of the old carboniferous ocean lies
before us with its corals and shells and stone-lilies, stretching out
their quivering arms, or expanding and contracting their flower-like
petals amid a scene of ceaseless animation and activity. Geology
delights in contrasts, and assuredly the contrast presented to us this
evening between the present and the past of Roman Camp Hill, will not
rank among the least striking of those which she has to reveal. There
is now spread over us the blue sky, richly hung with tinted clouds,
and melodious with the evening songs of the lark, the blackbird, and
the thrush. Not less surely did a wide expanse of sea during the
Carboniferous era roll over the hill on which we stand. And yonder
silvery moon that mounts up amid the violet twilight of the east, has
witnessed each scene and all the countless changes that have intervened
between them. The same pale light that now begins to steal through the
woods and athwart the fields, must have streamed down upon that old
sea and illumined its green depths. Oceans and continents, islands and
lakes, hills and valleys, have come and gone with all their successive
races of living things, and that same planet has marked them all. She
has seen, too, as but a thing of yesterday, the appearance of man upon
the scene, with all the successive centuries that have elapsed since
then. Truly the "goddess of the silver bow" would have a strange story
to tell us could we interrogate her about the past. But the days of
Endymion have gone by, and she now no longer visits in a personal form
the seat of beings who gaze at her crescent orb and daringly pronounce
it a scene of blasted ruin and desolation.