Book: The Story of a Piece of Coal

E >> Edward A. Martin >> The Story of a Piece of Coal




There is a class of deposit found among the coal-beds, which is known as
the "underclay," and this is the most regular of all as to the position
in which it is found. The underclays are found beneath every bed of coal.
"Warrant," "spavin," and "gannister" are local names which are sometimes
applied to it, the last being a term used when the clay contains such a
large proportion of silicious matter as to become almost like a hard
flinty rock. Sometimes, however, it is a soft clay, at others it is mixed
with sand, but whatever the composition of the underclays may be, they
always agree in being unstratified. They also agree in this respect that
the peculiar fossils known as _stigmariae_ abound in them, and in some
cases to such an extent that the clay is one thickly-matted mass of the
filamentous rootlets of these fossils. We have seen how these gradually
came to be recognised as the roots of trees which grew in this age, and
whose remains have subsequently become metamorphosed into coal, and it is
but one step farther to come to the conclusion that these underclays are
the ancient soils in which the plants grew.

No sketch of the various beds which go to form the coal-measures would be
complete which did not take into account the enormous beds of mountain
limestone which form the basis of the whole system, and which in thinner
bands are intercalated amongst the upper portion of the system, or the
true coal-measures.

Now, limestones are not formed in the same way in which we have seen that
sandstones and shales are formed. The last two mentioned owe their origin
to their deposition as sediment in seas, estuaries or lakes, but the
masses of limestone which are found in the various geological formations
owe their origin to causes other than that of sedimentary deposition.

In carboniferous times there lived numberless creatures which we know
nowadays as _encrinites_. These, when growing, were fixed to the bed of
the ocean, and extended upward in the shape of pliant stems composed of
limestone joints or plates; the stem of each encrinite then expanded at
the top in the shape of a gorgeous and graceful starfish, possessed of
numberless and lengthy arms. These encrinites grew in such profusion that
after death, when the plates of which their stems consisted, became
loosened and scattered over the bed of the sea, they accumulated and
formed solid beds of limestone. Besides the encrinites, there were of
course other creatures which were able to create the hard parts of their
structures by withdrawing lime from the sea, such as _foraminifera_,
shell-fish, and especially corals, so that all these assisted after death
in the accumulation of beds of limestone where they had grown and lived.

[Illustration: FIG. 20.--Encrinite.]

[Illustration: FIG. 21.--Encrinital limestone.]

There is one peculiarity in connection with the habitats of the
encrinites and corals which goes some distance in supplying us with a
useful clue as to the conditions under which this portion of the
carboniferous formation was formed. These creatures find it a difficult
matter, as a rule, to live and secrete their calcareous skeleton in any
water but that which is clear, and free from muddy or sandy sediment.
They are therefore not found, generally speaking, where the other
deposits which we have considered, are forming, and, as these are always
found near the coasts, it follows that the habitats of the creatures
referred to must be far out at sea where no muddy sediments, borne by
rivers, can reach them. We can therefore safely come to the conclusion
that the large masses of encrinital limestone, which attain such an
enormous thickness in some places, especially in Ireland, have been
formed far away from the land of the period; we can at the same time draw
the conclusion that if we find the encrinites broken and snapped asunder,
and the limestone deposits becoming impure through being mingled with a
proportion of clayey or sandy deposits, that we are approaching a
coast-line where perhaps a river opened out, and where it destroyed the
growth of encrinites, mixing with their dead remains the sedimentary
debris of the land.

[Illustration: FIG. 22.--Encrinites: various. Mountain limestone.]

We have lightly glanced at the circumstances attending the deposition of
each of the principal rocks which form the beds amongst which coal is
found, and have now to deal with the formation of the coal itself. We
have already considered the various kinds of plants and trees which have
been discovered as contributing their remains to the formation of coal,
and have now to attempt an explanation of how it came to be formed in so
regular a manner over so wide an area.

Each of the British coal-fields is fairly extensive. The Yorkshire and
Derbyshire coal-fields, together with the Lancashire coal-field, with
which they were at one time in geological connection, give us an area of
nearly 1000 square miles, and other British coal-fields show at least
some hundreds of square miles. And yet, spread over them, we find a
series of beds of coal which in many cases extend throughout the whole
area with apparent regularity. If we take it, as there seems every reason
to believe was the case, that almost all these coal-fields were not only
being formed at the same time, but were in most instances in continuation
with one another, this regularity of deposition of comparatively narrow
beds of coal, appears all the more remarkable.

The question at once suggests itself, Which of two things is probable?
Are we to believe that all this vegetable matter was brought down by some
mighty river and deposited in its delta, or that the coal-plants grew
just where we now find the coal?

Formerly it was supposed that coal was formed out of dead leaves and
trees, the refuse of the vegetation of the land, which had been carried
down by rivers into the sea and deposited at their mouths, in the same
way that sand and mud, as we have seen, are swept down and deposited. If
this were so, the extent of the deposits would require a river with an
enormous embouchure, and we should be scarcely warranted in believing
that such peaceful conditions would there prevail as to allow of the
layers of coal to be laid down with so little disturbance and with such
regularity over these wide areas. But the great objection to this theory
is, that not only do the remains still retain their perfection of
structure, but they are comparatively _pure,--i.e.,_ unmixed with
sedimentary depositions of clay or sand. Now, rivers would not bring down
the dead vegetation alone; their usual burden of sediment would also be
deposited at their mouths, and thus dead plants, sand, and clay would be
mixed up together in one black shaly or sandy mass, a mixture which would
be useless for purposes of combustion. The only theory which explained
all the recognised phenomena of the coal-measures was that the plants
forming the coal actually grew where the coal was formed, and where,
indeed, we now find it. When the plants and trees died, their remains
fell to the ground of the forest, and these soon turned to a black,
pasty, vegetable mass, the layer thus formed being regularly increased
year by year by the continual accumulation of fresh carbonaceous matter.
By this means a bed would be formed with regularity over a wide area; the
coal would be almost free from an admixture of sandy or clayey sediment,
and probably the rate of formation would be no more rapid in one part of
the forest than another. Thus there would be everywhere uniformity of
thickness. The warm and humid atmosphere, which it is probable then
existed, would not only have tended towards the production of an abnormal
vegetation, but would have assisted in the decaying and disintegrating
processes which went on amongst the shed leaves and trees.

When at last it was announced as a patent fact that every bed of coal
possessed its underclay, and that trees had been discovered actually
standing upon their own roots in the clay, there was no room at all for
doubt that the correct theory had been hit upon--viz., that coal is now
found just where the trees composing it had grown in the past.

But we have more than one coal-seam to account for. We have to explain
the existence of several layers of coal which have been formed over one
another on the same spot at successive periods, divided by other periods
when shale and sandstones only have been formed.

A careful estimate of the Lancashire coal-field has been made by
Professor Hull for the Geological Survey. Of the 7000 feet of
carboniferous strata here found, spread out over an area of 217 square
miles, there are on the average eighteen seams of coal.

This is only an instance of what is to be found elsewhere. Eighteen
coal-seams! what does this mean? It means that, during carboniferous
times, on no less than eighteen occasions, separate and distinct forests
have grown on this self-same spot, and that between each of these
occasions changes have taken place which have brought it beneath the
waters of the ocean, where the sandstones and shales have been formed
which divide the coal-seams from each other. We are met here by a
wonderful demonstration of the instability of the surface of the earth,
and we have to do our best to show how the changes of level have been
brought about, which have allowed of this game of geological see-saw to
take place between sea and land. Changes of level! Many a hard geological
nut has only been overcome by the application of the principle of changes
of level in the surface of the earth, and in this we shall find a sure
explanation of the phenomena of the coal-measures.

Great changes of the level of the land are undoubtedly taking place even
now on the earth's surface, and in assuming that similar changes took
place in carboniferous times, we shall not be assuming the former
existence of an agent with which we are now unfamiliar. And when we
consider the thicknesses of sandstone and shale which intervene beneath
the coal-seams, we can realise to a certain extent the vast lapses of
years which must have taken place between the existence of each forest;
so that although now an individual passing up a coal-mine shaft may
rapidly pass through the remains of one forest after another, the rise of
the strata above each forest-bed then was tremendously slow, and the
period between the growth of each forest must represent the passing away
of countless ages. Perhaps it would not be too much to say that the
strata between some of the coal-seams would represent a period not less
than that between the formation of the few tertiary coals with which we
are acquainted, and a time which is still to us in the far-away future.

The actual seams of coal themselves will not yield much information, from
which it will be possible to judge of the contour of the landmasses at
this ancient period. Of one thing we are sure, namely, that at the time
each seam was formed, the spot where it accumulated was dry land. If,
therefore, the seams which appear one above the other coincide fairly
well as to their superficial extent, we can conclude that each time the
land was raised above the sea and the forest again grew, the contour of
the land was very similar. This conclusion will be very useful to go
upon, since whatever decision may be come to as an explanation of one
successive land-period and sea-period on the same spot, will be
applicable to the eighteen or more periods necessary for the completion
of some of the coal-fields.

We will therefore look at one of the sandstone masses which occur between
the coal-seams, and learn what lessons these have to teach us. In
considering the formation of strata of sand in the seas around our
river-mouths, it was seen that, owing to the greater weight of the
particles of the sand over those of clay, the former the more readily
sank to the bottom, and formed banks not very far away from the land. It
was seen, too, that each successive deposition of sand formed a
wedge-shaped layer, with the point of the wedge pointing away from the
source of origin of the sediment, and therefore of the current which
conveyed the sediment. Therefore, if in the coal-measure sandstones the
layers were found with their wedges all pointing in one direction, we
should be able to judge that the currents were all from one direction,
and that, therefore, they were formed by a single river. But this is just
what we do not find, for instead of it the direction of the wedge-shaped
strata varies in almost every layer, and the current-bedding has been
brought about by currents travelling in every direction. Such diverse
current-bedding could only result from the fact that the spot where the
sand was laid down was subject to currents from every direction, and the
inference is that it was well within the sphere of influence of numerous
streams and rivers, which flowed from every direction. The only condition
of things which would explain this is that the sandstone was originally
formed in a closed sea or large lake, into which numerous rivers flowing
from every direction poured their contents.

Now, in the sandstones, the remains of numerous plants have been found,
but they do not present the perfect appearance that they do when found in
the shales; in fact they appear to have suffered a certain amount of
damage through having drifted some distance. This, together with the fact
that sandstones are not formed far out at sea, justify the safe
conclusion that the land could not have been far off. Wherever the
current-bedding shows itself in this manner we may be sure we are
examining a spot from which the land in every direction could not have
been at a very great distance, and also that, since the heavy materials
of which sandstone is composed could only be transported by being
impelled along by currents at the bed of the sea, and that in deep water
such currents could not exist, therefore we may safely decide that the
sea into which the rivers fell was a comparatively shallow one.

Although the present coal-fields of England are divided from one another
by patches of other beds, it is probable that some of them were formerly
connected with others, and a very wide sheet of coal on each occasion was
laid down. The question arises as to what was the extent of the inland
sea or lake, and did it include the area covered by the coal basins of
Scotland and Ireland, of France and Belgium? And if these, why not those
of America and other parts? The deposition of the coal, according to the
theory here advanced, may as well have been brought about in a series of
large inland seas and lakes, as by one large comprehensive sea, and
probably the former is the more satisfactory explanation of the two. But
the astonishing part of it is that the changes in the level of the land
must have been taking place simultaneously over these large areas,
although, of course, while one quarter may have been depressed beneath
the sea, another may have been raised above it.

In connection with the question of the contour of the land during the
existence of the large lakes or inland seas, Professor Hull has prepared,
in his series of maps illustrative of the Palaeo-Geography of the British
Islands, a map showing on incontestible grounds the existence during the
coal-ages of a great central barrier or ridge of high land stretching
across from Anglesea, south of Flint, Staffordshire, and Shropshire
coal-fields, to the eastern coast of Norfolk. He regards the British
coal-measures as having been laid down in two, or at most three, areas of
deposition--one south of this ridge, the remainder to the north of it. In
regard to the extent of the former deposits of coal in Ireland, there is
every probability that the sister island was just as favourably treated
in this respect as Great Britain. Most unfortunately, Ireland has since
suffered extreme denudation, notably from the great convulsions of nature
at the close of the very period of their deposition, as well as in more
recent times, resulting in the removal of nearly all the valuable upper
carboniferous beds, and leaving only the few unimportant
coal-beds to which reference has been made.

[Illustration: FIG. 23.--_Cyathophyllum_. Coral in encrinital limestone.]

We are unable to believe in the continuity of our coal-beds with those of
America, for the great source of sediment in those times was a continent
situated on the site of the Atlantic Ocean, and it is owing to this
extensive continent that the forms of _flora_ found in the coal-beds in
each country bear so close a resemblance to one another, and also that
the encrinital limestone which was formed in the purer depths of the
ocean on the east, became mixed with silt, and formed masses of shaly
impure limestone in the south-western parts of Ireland.

It must be noted that, although we may attribute to upheaval from beneath
the fact that the bed of the sea became temporarily raised at each period
into dry land, the deposits of sand or shale would at the same time be
tending to shallow the bed, and this alone would assist the process of
upheaval by bringing the land at least very near to the surface of the
water.

Each upheaval, however, could have been but a temporary arrest of the
great movement of crust subsidence which was going on throughout the coal
period, so that, at its close, when the last coal forest grew upon the
surface of the land, there had disappeared, in the case of South Wales, a
thickness of 11,000 feet of material.

Of the many remarkable things in connection with coal-beds, not the least
is the state of purity in which coal is found. On the floor of each
forest there would be many a streamlet or even small river which would
wend its way to meet the not very distant sea, and it is surprising at
first that so little sediment found its way into the coal itself. But
this was cleverly explained by Sir Charles Lyell, who noticed, on one of
his visits to America, that the water of the Mississippi, around the rank
growths of cypress which form the "cypress swamps" at the mouths of that
river, was highly charged with sediment, but that, having passed through
the close undergrowth of the swamps, it issued in almost a pure state,
the sediment which it bore having been filtered out of it and
precipitated. This very satisfactorily explained how in some places
carbonaceous matter might be deposited in a perfectly pure state, whilst
in others, where sandstone or shale was actually forming, it might be
impregnated by coaly matter in such a way as to cause it to be stained
black. In times of flood sediment would be brought in, even where pure
coal had been forming, and then we should have a thin "parting" of
sandstone or shale, which was formed when the flood was at its height. Or
a slight sinking of the land might occur, in which case also the
formation of coal would temporarily cease, and a parting of foreign
matter would be formed, which, on further upheaval taking place, would
again give way to another forest growth. Some of the thicker beds have
been found presenting this aspect, such as the South Staffordshire
ten-yard coal, which in some parts splits up into a dozen or so smaller
beds, with partings of sediment between them.

In the face of the stupendous movements which must have happened in order
to bring about the successive growth of forests one above another on the
same spot, the question at once arises as to how these movements of the
solid earth came about, and what was the cause which operated in such a
manner. We can only judge that, in some way or other, heat, or the
withdrawal of heat, has been the prime motive power. We can perceive,
from what is now going on in some parts of the earth, how great an
influence it has had in shaping the land, for volcanoes owe their
activity to the hidden heat in the earth's interior, and afford us an
idea of the power of which heat is capable in the matter of building up
and destroying continents. No less certain is it that heat is the prime
factor in those more gradual vertical movements of the land to which we
have referred elsewhere, but in regard to the exact manner in which it
acts we are very much in the dark. Everybody knows that, in the majority
of instances, material substances of all kinds expand under the influence
of heat, and contract when the source of heat is withdrawn. If we can
imagine movements in the quantity of heat contained in the solid crust,
the explanation is easy, for if a certain tract of land receive an
accession of heat beneath it, it is certain that the principal effect
will be an elevation of the land, consequent on the expansion of its
materials, with a subsequent depression when the heat beneath the tract
in question becomes gradually lessened. Should the heat be retained for a
long period, the strata would be so uplifted as to form an anticlinal, or
saddle-back, and then, should subsequent denudation take place, more
ancient strata would be brought to view. It was thus in the instance of
the tract bounded by the North and South Downs, which were formerly
entirely covered by chalk, and in the instance of the uprising of the
carboniferous limestone between the coal-fields of Lancashire,
Staffordshire, and Derbyshire.

How the heat-waves act, and the laws, if any, which they obey in their
subterranean movements, we are unable to judge. From the properties which
heat possesses we know that its presence or absence produces marked
differences in the positions of the strata of the earth, and from
observations made in connection with the closing of some volcanoes, and
the opening up of fresh earth-vents, we have gone a long way towards
establishing the probability that there are even now slow and ponderous
movements taking place in the heat stored in the earth's crust, whose
effects are appreciably communicated to the outside of the thin rind of
solid earth upon which we live.

Owing to the great igneous and volcanic activity at the close of the
deposition of the carboniferous system of strata, the coal-measures
exhibit what are known as _faults_ in abundance. The mountain limestone,
where it outcrops at the surface, is observed to be much jointed, so much
so that the work of quarrying the limestone is greatly assisted by the
jointed structure of the rock. Faults differ from joints in that, whilst
the strata in the latter are still in relative position on each side of
the joint, they have in the former slipped out of place. In such a case
the continuation of a stratum on the opposite side of a fault will be
found to be depressed, perhaps a thousand feet or more. It will be seen
at once how that, in sinking a new shaft into a coal-seam, the
possibility of an unknown fault has to be brought into consideration,
since the position of the seam may prove to have been depressed to such
an extent as to cause it to be beyond workable depth. Many seams, on the
other hand, which would have remained altogether out of reach of mining
operations, have been brought within workable depth by a series of
_step-faults_, this being a term applied to a series of parallel faults,
in none of which the amount of down-throw is great.

The amount of the down-throw, or the slipping-down of the beds, is
measured, vertically, from the point of disappearance of a layer to an
imaginary continuation of the same layer from where it again appears
beyond the fault. The plane of a fault is usually more or less inclined,
the amount of the inclination being known as the _hade_ of the fault, and
it is a remarkable characteristic of faults that, as a general rule, they
hade to the down-throw. This will be more clearly understood when it is
explained that, by its action, a seam of coal, which is subject to
numerous faults, can never be pierced more than once by one and the same
boring. In mountainous districts, however, there are occasions when the
hade is to the up-throw, and this kind of fault is known as an _inverted
fault_.

Lines of faults extend sometimes for hundreds of miles. The great Pennine
Fault of England is 130 miles long, and others extend for much greater
distances. The surfaces on both sides of a fault are often smooth and
highly polished by the movement which has taken place in the strata. They
then show the phenomenon known as _slicken-sides_. Many faults have
become filled with crystalline minerals in the form of veins of ore,
deposited by infiltrating waters percolating through the natural
fissures.

In considering the formation and structure of the better-known
coal-bearing beds of the carboniferous age, we must not lose sight of the
fact that important beds of coal also occur in strata of much more recent
date. There are important coal-beds in India of Permian age. There are
coal-beds of Liassic age in South Hungary and in Texas, and of Jurassic
age in Virginia, as well as at Brora in Sutherlandshire; there are coals
of Cretaceous age in Moravia, and valuable Miocene Tertiary coals in
Hungary and the Austrian Alps.

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