[Footnote: From a paper lately read before the Association of Foremen
Engineers.]
By JAMES PYKE.
The records from which geologists draw their information can scarcely be
compared to written or printed histories. There are, however, nations
of whom no written account exists, who perhaps never had any written
history, but about whom we are still able to gather from other sources
a vast amount of information. Their houses, their monuments, their
weapons, and their tools have survived, and these tell us the kind of
life, the state of civilization, and the skill of the men to whom they
belonged; from the contents of their tombs we learn what manner of men
they were physically; sometimes a sudden change in the appointments and
belongings of the folk indicates that tribes which had for a long time
inhabited a district were driven out and replaced by a new race. Thus,
then, from waifs and strays we can piece together a fairly connected
account of the events of a period long antecedent to any written
history.
The investigations of Dr. Schliemann on the supposed site of the city of
Troy furnish a good example of this method of research. He found lying,
one on the top of another, traces of the existence of five successive
communities of men, differing in customs and social development, and was
able to establish the fact that some of the cities had been destroyed by
fire, and that later on other towns had grown up over the buried remains
of the earlier settlements. The lowest layers were, of course, the
oldest, and the position of each layer in the pile gives its date, not
in years, but with regard to the layers above and below it.
Now, from time immemorial nature has been at work building up monuments
and providing tombs which tell us what were the events going on,
and what kind of inhabitants the earth had long before man made his
appearance on its surface. The monuments are the rocks which compose the
ground under our feet, and these, like many ancient monuments of human
construction, are the tombs of the creatures that lived while they were
being built.
Many facts testify that the earth's crust did not come into existence
exactly as we find it now, but that its rocks have been built up by the
slow action of natural agencies. These rocks constantly inclose the
remains of plants and animals, and as it is evident that neither plant
nor animal could have lived in the heart of a solid rock, this fact
shows that the rock must in some way have gathered round the remains
that are now found in it. Again, many of these remains, or fossils,
belonged to animals that lived in water, the larger part, indeed, to
marine creatures. This indicates that the rock was formed beneath the
sea, and when we examine the way in which the constituents of the rock
are arranged, we frequently find it to correspond exactly with the
manner in which the sand and mud that rivers sweep down into the sea or
lakes are spread out over the bottom of the water. In a pile of rocks
formed in this way it is clear that the lowest is the oldest of all, and
that any one stratum lying above is younger than the one beneath it.
Further, the occurrence of rocks inland containing marine fossils far
above the sea level shows that the sea and land have changed places.
When, again, we find that the fossils of one group of rocks differ
entirely from those of a group lying above them, we learn that one race
of creatures died out and was supplanted by a new assemblage of animal
forms.
These general remarks will, I trust, give some notion of the evidence
which is available for reconstructing the history of those remote
periods with which geology deals, and of the kind of reasoning which the
geologist employs for interpreting the records that are submitted to
him.
We will now briefly examine, by aid of these methods, the group of rocks
in which coal occurs in Great Britain, and see how far we can read the
story they have to tell.
The group with which we have to deal is called the carboniferous or
coal bearing system, and it includes four classes of rocks, viz.: 1,
sandstone; 2, shale or bind; 3, limestone; 4, coal and underclay.
We will take the sandstones and shales first. They are grains of sand
known to mineralogists as quartz, and consisting of a substance called
silica by chemists. The grains of sand are bound together by a cement
which in some few cases is identical in composition with themselves, and
consists of pure silica, but usually is a mixture of sandy, clayey, and
other substances. The shales are made up very largely of clay, mixed,
however, usually with sand and other substances, forming a conglomerate.
Both sandstones and shales are divided into layers or beds, and are said
to be stratified. It is this stratified or bedded structure that gives
us the first clew to the way in which these rocks were formed. Rivers
are constantly carrying down sand and mud into the sea or lakes, and
when their flow is slackened on entering the still water the materials
they bring down with them sink and are spread out in layers over the
bottom. The structure of the sandstones and shales shows that they were
formed in this way; they often inclose the remains of plants that have
been carried down from land, and occasionally of animals that lived in
the water where they were deposited.
The next we have to consider is limestone, which is mainly made up of a
substance known to chemists as calcium carbonate, or carbonate of lime.
In some districts, especially in volcanic countries, springs occur very
highly charged with carbonate of lime. The warm springs of Matlock are
a case in point; they are probably the last vestige of volcanic action
which was in operation in that neighborhood during carboniferous times.
Limestone is chiefly formed by the agency of small marine creatures of
low organization. By the aid of these animals the carbonate of lime is
brought back to a solid form; at their death their hard parts fall to
the bottom and accumulate in a mass of pure limestone, which afterward
becomes solidified into limestone rock.
The information that limestone gives us is this:
When we find, as is often the case, a mass of limestone hundreds of feet
thick, and composed of little else but carbonate of lime, we know that
the spot where it occurs was, at the time it was formed, far out at sea,
covered by the clear water of mid ocean; and when we find that this
limestone grows in certain directions earthy and impure, and that layers
of shale and sandstone, thin at first, but gradually thickening out in
a wedge-shape form, come in between its beds, we know that in those
directions we are traveling toward the shore lines of that sea whence
the water was receiving from time to time supplies of muddy and sandy
sediment.
The next class of rocks are the clays that are found beneath every
bed of coal, and which are known as _underclays_, or _warrant_, or
_spavins_. They vary very much in mineral composition. Sometimes they
are soft clay; sometimes clay mixed with a certain portion of sand; and
sometimes they contain such a large proportion of silicious matters that
they become hard, flinty rock, which many of you know under the name
of _gannister_. But all underclays agree in two points: they are all
unstratified. They differ totally from the shales and sandstones in this
respect, and instead of splitting up readily into thin flakes, they
break up into irregular lumpy masses. And they all contain a very
peculiar vegetable fossil called _Stigmaria_.
This strange fossil was for a long time a sore puzzle to fossil
botanists, and after much discussion the question was fairly solved by
Mr. Binney by the discovery of a tree embedded in the coal measures,
and standing erect just as it grew, with its roots spread out into the
stratum on which it stood. These roots were Stigmaria, and the stuff
into which they penetrated was an underclay. Sir Charles Lyell mentions
an individual sigillaria 72 feet in length found at Newcastle, and a
specimen taken from the Jarrow coal mine was more than 40 feet in length
and 13 feet in diameter near the base. It is not often these trees are
found erect, because the action of water, combined with natural decay,
has generally thrown them down. They are, however, found in very large
numbers in the roof of the coal, evidently having been tossed over, and
lying there flat and squeezed thin by the pressure of the measures that
lie above them.
Lastly, we come to coal itself--a rock which constitutes a small portion
of the whole bulk of the carboniferous deposits, but which may be fairly
looked upon as the most important member of that group, both on account
of its intrinsic value and also from the interest that attaches to its
history. That coal is little else but mineralized vegetable matter is a
point on which there has for a long time been but small doubt. The
more minute investigations of recent years have not only placed this
completely beyond question, but have also enabled us to say what the
plants were which contributed to the formation of coal, and in some
cases even to decide what portions of those plants enter into its
composition. It is a thing so universally admitted on all hands, that I
shall take it for granted you are all perfectly convinced that coal has
been nothing in the world but a great mass of vegetable matter. The only
question is: How were these great masses of vegetable matter brought
together? And you must realize that they were very large masses indeed.
Just to take one instance. The Yorkshire and Derbyshire coal field is
somewhere about 700 to 800 square miles in area, and Lancashire about
200. Well, in both these coal fields you have a great number of beds of
coal that spread over the whole of them with tolerable regularity and
thickness, and very often with scarcely any break whatever. And this is
only a very small portion of what must have been the original sheet of
coal, so that you see we have to account for a mass of vegetable matter
perfectly free from any admixture of sand, mud, or dirt, and laid down
with tolerably uniform thickness over many hundreds of square miles.
At one time it was supposed that coal was formed out of dead trees and
plants which were swept down by rivers into the sea, just in the same
way as shales and sandstones were formed out of mud and sand so swept
down. The fatal objection to this theory, however, is that rivers would
not bring down dead wood alone, but they would bring down sand and mud,
and other matters, and that in the bottom of the sea the dead wood would
be mixed with these matters, and instead of getting a perfectly unmixed
mass of vegetable matter, we should get a mixture of dead plants, sand,
mud, and other things, which would give rise to something like coal, but
something very different, as any one who tries to burn such coal will
soon find out, from really good, pure house coal. So that this theory,
which is generally known as the "drift" theory, was totally inadequate
to account for the facts as we know them.
The other theory was that coal was formed out of plants and trees that
grew on the spot where we now find coal itself. On this supposition it
is easy to account for the absence of foreign admixtures of sand, mud,
and clay in the coal; and we can also understand very much better than
by the aid of the drift theory how the coal had accumulated with such
wonderful uniformity of thickness over such very large areas. This
theory was for some time but poorly received; but after the discovery
of Sir William Logan, that every bed of coal had a bed of underclay
beneath, and the discovery of Mr. Binney, that these underclays were
true soils on which plants had undoubtedly grown, there was no doubt
whatever that this was the real and true explanation of the matter.
I dare say many of you have had occasion to walk across peat bogs.
The peat bog is a great mass of vegetable matter, which is every year
growing thicker and thicker; and underneath it there is almost always a
bed of thin clay, in look very much like the underclays, and this thin
clay is penetrated by the rootlets of the moss forming the peat, exactly
the same way as the underclays of the coal measures are penetrated by
the stigmaria and its rootlets. But you must not suppose that the plants
out of which coal was formed were exactly the same low type of moss
which forms our present peat bogs. However, it is pretty certain that
they were for the most part of a loose, succulent texture, and that they
grew very rapidly indeed.
You will have noticed that there is one step more wanted to make good
this theory of the growth of coal on the spot where we now find it.
The coal is found, as already described, interbedded with shales and
sandstones. These shales and sandstones, as shown, were formed beneath
the water of the sea, and as long as they remained there of course no
plants could grow upon them. The question is, How was the land surface
formed for the growth of plants? It must have been formed in some way or
other by the sea bottom having been raised above the level of the water.
Now, we have distinct proof in many cases that elevation of the sea
bottom and depression of the land is now going on in many parts of the
earth's surface. And, therefore, we shall be assuming nothing beyond the
range of experience if we say that such elevations and depressions went
on during coal measure times. The coal measure times must have been
times during which the same spot was now below the sea, and now dry
land, over and over again. There was a land surface on which plants grew
fast and multiplied rapidly, and as they died fell and accumulated in
a great heap of dead vegetable matter. After a time this layer of
vegetable matter was slowly and gently let down beneath the waters of
the sea--so slowly that the water flowing over it did not, as a rule,
disturb the loose, pasty mass; and then, by the method I have described
to you, shales and sandstones were deposited on the top of this mass
of dead vegetable matter. By their weight they compressed it, and
by certain chemical changes (which we have not time to go into this
evening) this dense mass of vegetable matter became converted into coal.
After a time the shales and sandstones which had been piled above this
stuff, which was to form coal for the future, were again elevated to
form a land surface; upon this another forest sprang up, and by its
decay produced another mass of vegetable matter fit to form coal. This
again was let down below the water, more shales and sandstones were
deposited on the top, and this process went on over and over again till
the whole mass of our present coal measures was formed. You will now see
how it is that trees are so seldom found in an upright position in the
coal beds. As the land went down, they would in very many cases be
toppled over by the water as it flowed against them, or their base would
be rotted, and they would then either fall or be blown over; that is the
reason why in most cases they are found lying flat on the roof of the
coal bed. But in a few cases, when the depression was very gentle and
gradual, the trees were not overthrown, and the shales and sandstones
accumulated round them and preserved them in the position in which they
grew.
I do not know that I can point out to you anything nowadays that exactly
resembles the state of things that must have gone on during the times
these coal measures were being formed; but there are a great many cases
strikingly analogous to them. I shall not attempt to describe them to
you, but may just mention the mangrove swamps that very often fringe the
coasts in the tropics, and the cypress swamps of the Mississippi, which
are so well described by Sir Charles Lyell in his recent works; also
the great Dismal Swamp of Virginia, which appears to me to furnish the
nearest analogue to the state of things that existed during coal measure
times.
Having explained the way in which coal measures have been formed, we
will now take a brief sketch of its uses and products. The year 1259 is
memorable in the annals of coal mining. Hitherto the mineral had not
been raised by authority, but in that year Henry III. granted a charter
to the freemen of Newcastle-on-Tyne for liberty to dig coal, and a
considerable export trade was established with London, and it speedily
became an article among the various manufacturers of the metropolis. But
its popularity was but short lived. An impression became general
that the smoke arising therefrom contaminated the atmosphere and was
injurious to public health. Years of experience have proved the fallacy
of the imputation; but in 1306 the outcry became so general that a
proclamation was issued by Edward I forbidding the use of the offending
fuel, and authorizing the destruction of all furnaces, etc., of those
persons who should persist in using it. Prejudice gradually gave way as
the value of the fossil fuel became better known, and from that time
downward its use has become more and more extended down to the enormous
extent of our present trade. The annual increase in the production of
coal in the British Isles since the year 1854 is over 2½ million tons.
In that year the coal produce was about 65 million tons, and it has
grown up to the year 1880 to the grand total of 135 million tons.
We will now deal with some of the uses that this valuable black diamond
is now being put to. It is, in the first place, the center of all our
enterprise and prosperity, and upon it depends our chief success as a
manufacturing nation for the future. When it is exhausted we shall have
to look forward to the condition of things which now obtains in those
regions where there is no coal--that is to say, instead of our being a
nation full of manufacturing and mercantile enterprise, a great nation
to which all the people of the earth resort, we shall be merely a people
who live for ourselves by the cultivation of the ground. The duration of
our coal fields has been ascertained within certain limits. Mr. Hall, an
accomplished geologist, tells us that in England at the present time we
have a stock of coal sufficient for our consumption for no less than
1,000 years. On the other hand, Professor Jevons, whose opinion is
worthy of the very greatest weight on such questions, calculates that
100 years is about the tenure of our coal fields, according to the
present rate of increase in the consumption. Whichever view we take,
sooner or later the end must ultimately come when the coal will be
exhausted; when the great mainspring of our commercial enterprise will
be gone, and we shall revert to that condition in which we were before
the coal fields were worked. In this point of view, therefore, coal has
an especial interest to us as engineers. If coal is important in this
direction, it is no less important in a purely scientific point of view,
apart from any mercantile end.
The chemist or physicist will tell you the wondrous story that the black
substance which you burn is simply so much light and heat and motion
borrowed from the sun and invested in the tissues of plants. He will
tell you that when you sit round your firesides the flame which enlivens
you, and the gas which enables you to read, and which civilizes you, is
nothing in the world but so much sunlight and so much sunheat bottled up
in the tissues of vegetables, and simply reproduced in your grates and
gas burners. Very few persons, I am afraid, realize this, which is one
of the many stories which science in its higher teachings shows us--one
of those fairy tales which are the result of the most careful scientific
investigation. Of the hundred and odd million tons of coal which we in
this country burn in the course of a year, about 20,000,000 tons are
thrown on our house fires; 30,000,000 tons find their way into our blast
furnaces, or are otherwise used in the smelting and manufacture of
metals; about 48,000,000 are burnt under steam boilers; 6,000,000 are
used in gas-making; while the remainder is consumed in potteries, glass
works, brick and lime kilns, chemical works, and other sundries which I
need not speak of.
To go into the chemistry of coal is quite sufficient to take up more
time than I have at my disposal this evening, therefore I will briefly
touch on a few of the main points. Coal gas is made, as you are all
aware, by heating coal or cannel, which is the special form of coal
most valued for the purpose, on account of the high quality of gas it
produces in cylindrical fireclay retorts.
The by-products obtained in the manufacture of coal gas, the tar and the
ammonia water, are nowadays scarcely less important than the coal gas
itself. The ammonia water furnishes large quantities of salts to be
used, among other applications, as food for plants. We thus restore
to-day to our vegetation the nitrogen which existed in plants of
primeval times. The tar, black and noisome though it be, is a marvelous
product, by the reason of scores of beautiful substances which are
concealed within it.
Coal tar when distilled yields three main products: naphtha, dead oil,
and pitch or asphalt. The naphtha on redistillation yields benzine, from
which are prepared some of our most beautiful dyes; the dead oil, as
the less volatile portion is termed, furnishes carbolic acid, used as a
disinfectant and antiseptic, together with anthracene and naphthaline;
all three substances the starting points of new series of coloring
matters.
This discovery of these coloring matters marks an era in the history
of chemical science; it exercised an extraordinary influence on the
development of organic chemistry. Theoretical and applied chemistry were
knit together in closer union than ever, and dye followed dye in quick
succession; after mauve came magenta, and in close attendance followed a
brilliant train of reds, yellows, oranges, greens, blues, and violets;
in fact, all the simple and beautiful colors of the rainbow.
But there is still another story of coal tar to be told. Among the
many curious substances that wonderful fluid contains is the beautiful
wax-like body called paraffine, the development of which chiefly owes
its origin to the genius and energy of Mr. James Young. As early as
1848, Mr. Young had worked a small petroleum spring in a coal mine in
Derbyshire, and had produced oils suitable for burning and lubricating
purposes, but the spring gave out, and then Mr. Young sought to obtain
these oils by distilling coal. After many trials, in conjunction with
other gentlemen connected therewith, he proved successful, and the
present magnitude of this industry is without parallel in the history of
British manufactures.
In Scotland alone there are about sixty paraffine oil works, one alone
occupying a site of nearly forty acres. Here about 120,000 gallons of
crude oil are produced weekly, and among the various works in Scotland
about 800,000 tons of shale are distilled per annum, producing nearly
30,000,000 gallons of crude oil, from which about 12,000,000 gallons of
refined burning oil are obtained in addition to the large quantities
of naphtha, solid paraffine, ammonia, and other chemical products.
Twenty-five years ago scarcely a dozen persons had seen this paraffine,
and now it is turned out by the ton, fashioned into candles delicately
tinted with colors obtained from coal tar.
I might dwell on this subject until it becomes wearisome to you,
therefore I will not trespass too much on your time. But from every
point we look we reach this fact, that our coal trade is one which
develops itself according to laws that we are perfectly powerless to
control; if it seems to promise a less rapid increase here, it is only
that it may spread abroad with accelerated vigor elsewhere; if it is our
slave in some aspects, it seems as if it were our master in others.
Finally, we have to ask, What of our export coals? Rapid as has been the
growth of our total production during the last twenty-three years, the
growth of our export of coals has been greater still. Beginning at
4,300,000 tons in '54, we find it reaching 16,250,000 tons in '76, and
an increase at a corresponding ratio up to the present date as far as
statistics will carry us. At such a rate of increase it would seem as
if our whole annual production would be ultimately swallowed up in our
exports, and it is not, perhaps, impossible that after we have ceased to
be to any great extent a manufacturing people, a certain export trade
in coal may still continue. Just the same as the export trade in coal
preceded by centuries our own uses for it other than domestic, so may
it also survive these by a period as prolonged. If our descent from
our present favored position be a gradual one, much may be done in the
interval to adapt ourselves to the future outcome, but it is certain
that nothing will be done except under the stern persuasion of
necessity.
When our coal fields become exhausted, be it soon or late, he would be
a wise or, perhaps, a rash speculator who fixed himself to a year or a
generation. Being inevitable, the best philosophy is to make our decline
more gradual and less bitter. Sentimental regrets that these hills and
valleys will no longer resound with the din of labor, or be blackened by
the smoke of the factory, would surely be out of place. What we might
regret is that Britain, which we know and are proud of, the Britain
of great achievements in politics and literature, of free thought and
self-respecting obedience, of a thousand years of high endeavor and
constant progress, was indeed to perish when these factories and
furnaces whirled and blazed their last. But, it is not so. This
country's fortunes are gradually being merged into those of a Greater
Britain, which largely, through the aid of coal, whose prospective
loss we are lamenting, has grown beyond the limits of these islands to
overspread the vastest and richest regions of the earth; and we have no
reason to fear that the great inheritance that America and Australia
and New Zealand have accepted from us will in their hands be dealt
unworthily with in the future.
* * * * *