Town Geology 8

Town Geology 8

That was a sight not easily to be forgotten. But we need not have

gone so far from home, at least, a few hundred years ago, to see an

exactly similar one. The fens of Norfolk and Cambridgeshire, before

the rivers were embanked, the water pumped off, the forests felled,

and the reed-beds ploughed up, were exactly in the same state. The

vast deposits of peat between Cambridge and the sea, often filled

with timber-trees, either fallen or upright as they grew, and often

mixed with beds of sand or mud, brought down in floods, were formed

in exactly the same way; and if they had remained undrained, then

that slow sinking, which geologists say is going on over the whole

area of the Fens, would have brought them gradually, but surely,

below the sea-level, to be covered up by new forests, and converted

in due time into coal. And future geologists would have found--they

may find yet, if, which God forbid, England should become barbarous

and the trees be thrown out of cultivation--instead of fossil

Lepidodendra and Sigillariae, Calamites and ferns, fossil ashes and

oaks, alders and poplars, bulrushes and reeds. Almost the only

fossil fern would have been that tall and beautiful Lastraea

Thelypteris, once so abundant, now all but destroyed by drainage and

the plough.

 

We need not, therefore, fancy any extraordinary state of things on

this planet while our English coal was being formed. The climate of

the northern hemisphere--Britain, at least, and Nova Scotia--was

warmer than now, to judge from the abundance of ferns; and especially

of tree-ferns; but not so warm, to judge from the presence of

conifers (trees of the pine tribe), as the Tropics. Moreover, there

must have been, it seems to me, a great scarcity of animal-life.

Insects are found, beautifully preserved; a few reptiles, too, and

land-shells; but very few. And where are the traces of such a

swarming life as would be entombed were a tropic forest now sunk;

which is found entombed in many parts of our English fens? The only

explanation which I can offer is this--that the club-mosses, tree-

ferns, pines, and other low-ranked vegetation of the coal afforded

little or no food for animals, as the same families of plants do to

this day; and if creatures can get nothing to eat, they certainly

cannot multiply and replenish the earth. But, be that as it may, the

fact that coal is buried forest is not affected.

 

Meanwhile, the shape and arrangements of sea and land must have been

utterly different from what they are now. Where was that great land,

off which great rivers ran to deposit our coal-measures in their

deltas? It has been supposed, for good reasons, that north-western

France, Belgium, Holland, and Germany were then under the sea; that

Denmark and Norway were joined to Scotland by a continent, a tongue

of which ran across the centre of England, and into Ireland, dividing

the northern and southern coal-fields. But how far to the west and

north did that old continent stretch? Did it, as it almost certainly

did long ages afterwards, join Greenland and North America with

Scotland and Norway? Were the northern fields of Nova Scotia, which

are of the same geological age as our own, and contain the same

plants, laid down by rivers which ran off the same continent as ours?

Who can tell now? That old land, and all record of it, save what

these fragmentary coal-measures can give, are buried in the dark

abyss of countless ages; and we can only look back with awe, and

comfort ourselves with the thought--Let Time be ever so vast, yet

Time is not Eternity.

 

One word more. If my readers have granted that all for which I have

argued is probable, they will still have a right to ask for further

proof.

 

They will be justified in saying: "You say that coal is transformed

vegetable matter; but can you show us how the transformation takes

place? Is it possible according to known natural laws?"

 

The chemist must answer that. And he tells us that wood can become

lignite, or wood-coal, by parting with its oxygen, in the shape of

carbonic acid gas, or choke-damp; and then common or bituminous coal,

by parting with its hydrogen, chiefly in the form of carburetted

hydrogen--the gas with which we light our streets. That is about as

much as the unscientific reader need know. But it is a fresh

corroboration of the theory that coal has been once vegetable fibre,

for it shows how vegetable fibre can, by the laws of nature, become

coal. And it certainly helps us to believe that a thing has been

done, if we are shown that it can be done.

 

This fact explains, also, why in mines of wood-coal carbonic acid,

i.e. choke-damp, alone is given off. For in the wood-coal a great

deal of the hydrogen still remains. In mines of true coal, not only

is choke-damp given off, but that more terrible pest of the miners,

fire-damp, or explosive carburetted hydrogen and olefiant gases. Now

the occurrence of that fire-damp in mines proves that changes are

still going on in the coal: that it is getting rid of its hydrogen,

and so progressing toward the state of anthracite or culm--stone-coal

as it is sometimes called. In the Pennsylvanian coal-fields some of

the coal has actually done this, under the disturbing force of

earthquakes; for the coal, which is bituminous, like our common coal,

to the westward where the strata are horizontal, becomes gradually

anthracite as it is tossed and torn by the earthquake faults of the

Alleghany and Appalachian mountains.

 

And is a further transformation possible? Yes; and more than one.

If we conceive the anthracite cleared of all but its last atoms of

oxygen, hydrogen, and nitrogen, till it has become all but pure

carbon, it would become--as it has become in certain rocks of immense

antiquity, graphite--what we miscall black-lead. And, after that, it

might go through one transformation more, and that the most startling

of all. It would need only perfect purification and crystallisation

to become--a diamond; nothing less. We may consider the coal upon

the fire as the middle term of a series, of which the first is live

wood, and the last diamond; and indulge safely in the fancy that

every diamond in the world has probably, at some remote epoch, formed

part of a growing plant.

 

A strange transformation; which will look to us more strange, more

truly poetical, the more steadily we consider it.

 

The coal on the fire; the table at which I write--what are they made

of? Gas and sunbeams; with a small percentage of ash, or earthy

salts, which need hardly be taken into account.

 

Gas and sunbeams. Strange, but true.

 

The life of the growing plant--and what that life is who can tell?--

laid hold of the gases in the air and in the soil; of the carbonic

acid, the atmospheric air, the water--for that too is gas. It drank

them in through its rootlets: it breathed them in through its leaf-

pores, that it might distil them into sap, and bud, and leaf, and

wood. But it has to take in another element, without which the

distillation and the shaping could never have taken place. It had to

drink in the sunbeams--that mysterious and complex force which is for

ever pouring from the sun, and making itself partly palpable to our

senses as heat and light. So the life of the plant seized the

sunbeams, and absorbed them, buried them in itself--no longer as

light and heat, but as invisible chemical force, locked up for ages

in that woody fibre.

 

So it is. Lord Lytton told us long ago, in a beautiful song, how

 

 

The Wind and the Beam loved the Rose.

 

 

But Nature's poetry was more beautiful than man's. The wind and the

beam loved the rose so well that they made the rose--or rather, the

rose took the wind and the beam, and built up out of them, by her own

inner life, her exquisite texture, hue, and fragrance.

 

What next? The rose dies; the timber tree dies; decays down into

vegetable fibre, is buried, and turned to coal: but the plant cannot

altogether undo its own work. Even in death and decay it cannot set

free the sunbeams imprisoned in its tissue. The sun-force must stay,

shut up age after age, invisible, but strong; working at its own

prison-cells; transmuting them, or making them capable of being

transmuted by man, into the manifold products of coal--coke,

petroleum, mineral pitch, gases, coal-tar, benzole, delicate aniline

dyes, and what not, till its day of deliverance comes.

 

Man digs it, throws it on the fire, a black, dead-seeming lump. A

corner, an atom of it, warms till it reaches the igniting point; the

temperature at which it is able to combine with oxygen.

 

And then, like a dormant live thing, awaking after ages to the sense

of its own powers, its own needs, the whole lump is seized, atom

after atom, with an infectious hunger for that oxygen which it lost

centuries since in the bottom of the earth. It drinks the oxygen in

at every pore; and burns.

 

And so the spell of ages is broken. The sun-force bursts its prison-

cells, and blazes into the free atmosphere, as light and heat once

more; returning in a moment into the same forms in which it entered

the growing leaf a thousand centuries since.

 

Strange it all is, yet true. But of nature, as of the heart of man,

the old saying stands--that truth is stranger than fiction.

 

 

 

V. THE LIME IN THE MORTAR

 

 

 

I shall presume in all my readers some slight knowledge about lime.

I shall take for granted, for instance, that all are better informed

than a certain party of Australian black fellows were a few years

since.

 

In prowling on the track of a party of English settlers, to see what

they could pick up, they came--oh joy!--on a sack of flour, dropped

and left behind in the bush at a certain creek. The poor savages had

not had such a prospect of a good meal for many a day. With endless

jabbering and dancing, the whole tribe gathered round the precious

flour-bag with all the pannikins, gourds, and other hollow articles

it could muster, each of course with a due quantity of water from the

creek therein, and the chief began dealing out the flour by handfuls,

beginning of course with the boldest warriors. But, horror of

horrors, each man's porridge swelled before his eyes, grew hot,

smoked, boiled over. They turned and fled, man, woman, and child,

from before that supernatural prodigy; and the settlers coming back

to look for the dropped sack, saw a sight which told the whole tale.

For the poor creatures, in their terror, had thrown away their pans

and calabashes, each filled with that which it was likely to contain,

seeing that the sack itself had contained, not flour, but quick-lime.

In memory of which comi-tragedy, that creek is called to this day,

"Flour-bag Creek."

 

Now I take for granted that you are all more learned than these black

fellows, and know quick-lime from flour. But still you are not bound

to know what quick-lime is. Let me explain it to you.

 

Lime, properly speaking, is a metal, which goes among chemists by the

name of calcium. But it is formed, as you all know, in the earth,

not as a metal, but as a stone, as chalk or limestone, which is a

carbonate of lime; that is, calcium combined with oxygen and

carbonic-acid gases.

 

In that state it will make, if it is crystalline and hard, excellent

building stone. The finest white marble, like that of Carrara in

Italy, of which the most delicate statues are carved, is carbonate of

lime altered and hardened by volcanic heat. But to make mortar of

it, it must be softened and then brought into a state in which it can

be hardened again; and ages since, some man or other, who deserves to

rank as one of the great inventors, one of the great benefactors of

his race, discovered the art of making lime soft and hard again; in

fact of making mortar. The discovery was probably very ancient; and

made, probably like most of the old discoveries, in the East,

spreading Westward gradually. The earlier Greek buildings are

cyclopean, that is, of stone fitted together without mortar. The

earlier Egyptian buildings, though the stones are exquisitely squared

and polished, are put together likewise without mortar. So, long

ages after, were the earlier Roman buildings, and even some of the

later. The famous aqueduct of the Pont du Gard, near Nismes, in the

south of France, has, if I recollect right, no mortar whatever in it.

The stones of its noble double tier of circular arches have been

dropped into their places upon the wooden centres, and stand unmoved

to this day, simply by the jamming of their own weight; a miracle of

art. But the fact is puzzling; for these Romans were the best mortar

makers of the world. We cannot, I believe, surpass them in the art

even now; and in some of their old castles, the mortar is actually to

this day harder and tougher than the stones which it holds together.

And they had plenty of lime at hand if they had chosen to make

mortar. The Pont du Gard crosses a limestone ravine, and is itself

built of limestone. But I presume the cunning Romans would not trust

mortar made from that coarse Nummulite limestone, filled with gritty

sand, and preferred, with their usual carefulness, no mortar at all

to bad.

 

But I must return, and tell my readers, in a few words, the chemical

history of mortar. If limestone be burnt, or rather roasted, in a

kiln, the carbonic acid is given off--as you may discover by your own

nose; as many a poor tramp has discovered too late, when, on a cold

winter night, he has lain down by the side of the burning kiln to

keep himself warm, and woke in the other world, stifled to death by

the poisonous fumes.

 

The lime then gives off its carbonic acid, and also its water of

crystallisation, that is, water which it holds (as do many rocks)

locked up in it unseen, and only to be discovered by chemical

analysis. It is then anhydrous--that is, waterless--oxide of lime,

what we call quick-lime; that which figured in the comi-tragedy of

"Flour-bag Creek;" and then, as you may find if you get it under your

nails or into your eyes, will burn and blister like an acid.

 

This has to be turned again into a hard and tough artificial

limestone, in plain words, into mortar; and the first step is to

slack it--that is, to give it back the water which it has lost, and

for which it is as it were thirsting. So it is slacked with water,

which it drinks in, heating itself and the water till it steams and

swells in bulk, because it takes the substance of the water into its

own substance. Slacked lime, as we all know, is not visibly wetter

than quick-lime; it crumbles to a dry white powder in spite of all

the water which it contains.

 

Then it must be made to set, that is, to return to limestone, to

carbonate of lime, by drinking in the carbonic acid from water and

air, which some sorts of lime will do instantly, setting at once, and

being therefore used as cements. But the lime usually employed must

be mixed with more or less sand to make it set hard: a mysterious

process, of which it will be enough to tell the reader that the sand

and lime are said to unite gradually, not only mechanically, that is,

by sticking together; but also in part chemically--that is, by

forming out of themselves a new substance, which is called silicate

of lime.

 

Be that as it may, the mortar paste has now to do two things; first

to dry, and next to take up carbonic acid from the air and water,

enough to harden it again into limestone: and that it will take some

time in doing. A thick wall, I am informed, requires several years

before it is set throughout, and has acquired its full hardness, or

rather toughness; and good mortar, as is well known, will acquire

extreme hardness with age, probably from the very same cause that it

did when it was limestone in the earth. For, as a general rule, the

more ancient the strata is in which the limestone is found, the

harder the limestone is; except in cases where volcanic action and

earthquake pressure have hardened limestone in more recent strata, as

in the case of the white marbles of Carrara in Italy, which are of

the age of our Oolites, that is, of the freestone of Bath, etc.,

hardened by the heat of intruded volcanic rocks.

 

But now: what is the limestone? and how did it get where it is--not

into the mortar, I mean, but into the limestone quarry? Let me tell

you, or rather, help you to tell yourselves, by leading you, as

before, from the known to the unknown. Let me lead you to places

unknown indeed to most; but there may be sailors or soldiers among my

readers who know them far better than I do. Let me lead you, in

fancy, to some island in the Tropic seas. After all, I am not

leading you as far away as you fancy by several thousand miles, as

you will see, I trust, ere I have done.

 

Let me take you to some island: what shall it be like? Shall it be

a high island, with cliff piled on cliff, and peak on peak, all rich

with mighty forests, like a furred mantle of green velvet, mounting

up and up till it is lost among white clouds above? Or shall it be a

mere low reef, which you do not see till you are close upon it; on

which nothing rises above the water, but here and there a knot of

cocoa-nut palms or a block of stone, or a few bushes, swarming with

innumerable sea-fowl and their eggs? Let it be which you will: both

are strange enough; both beautiful; both will tell us a story.

 

The ship will have to lie-to, and anchor if she can; it may be a

mile, it may be only a few yards, from the land. For between it and

the land will be a line of breakers, raging in before the warm trade-

wind. And this, you will be told, marks the edge of the coral reef.

 

You will have to go ashore in a boat, over a sea which looks

unfathomable, and which may be a mile or more in depth, and search

for an opening in the reef, through which the boat can pass without

being knocked to pieces.

 

You find one: and in a moment, what a change! The deep has suddenly

become shallow; the blue white, from the gleam of the white coral at

the bottom. But the coral is not all white, only indeed a little of

it; for as you look down through the clear water, you find that the

coral is starred with innumerable live flowers, blue, crimson, grey,

every conceivable hue; and that these are the coral polypes, each

with its ring of arms thrust out of its cell, who are building up

their common habitations of lime. If you want to understand, by a

rough but correct description, what a coral polype is: all who have

been to the sea-side know, or at least have heard of, sea-anemones.

Now coral polypes are sea-anemones, which make each a shell of lime,

growing with its growth. As for their shapes, the variety of them,

the beauty of them, no tongue can describe them. If you want to see

them, go to the Coral Rooms of the British or Liverpool Museums, and

judge for yourselves. Only remember that you must re-clothe each of

those exquisite forms with a coating of live jelly of some delicate

hue, and put back into every one of the thousand cells its living

flower; and into the beds, or rather banks, of the salt-water flower

garden, the gaudiest of shell-less sea-anemones, such as we have on

our coasts, rooted in the cracks, and live shells and sea-slugs, as

gaudy as they, crawling about, with fifty other forms of fantastic

and exuberant life. You must not overlook, too, the fish, especially

the parrot-fish, some of them of the gaudiest colours, who spend

their lives in browsing on the live coral, with strong clipping and

grinding teeth, just as a cow browses the grass, keeping the animal

matter, and throwing away the lime in the form of an impalpable white

mud, which fills up the interstices in the coral beds.

 

The bottom, just outside the reef, is covered with that mud, mixed

with more lime-mud, which the surge wears off the reef; and if you

have, as you should have, a dredge on board, and try a haul of that

mud as you row home, you may find, but not always, animal forms

rooted in it, which will delight the soul of a scientific man. One,

I hope, would be some sort of Terebratula, or shell akin to it. You

would probably think it a cockle: but you would be wrong. The

animal which dwells in it has about the same relationship to a cockle

as a dog has to a bird. It is a Brachiopod; a family with which the

ancient seas once swarmed, but which is rare now, all over the world,

having been supplanted and driven out of the seas by newer and

stronger forms of shelled animals. The nearest spot at which you are

likely to dredge a live Brachiopod will be in the deep water of Loch

Fyne, in Argyleshire, where two species still linger, fastened,

strangely enough, to the smooth pebbles of a submerged glacier,

formed in the open air during the age of ice, but sunk now to a depth

of eighty fathoms. The first time I saw those shells come up in the

dredge out of the dark and motionless abyss, I could sympathise with

the feelings of mingled delight and awe which, so my companion told

me, the great Professor Owen had in the same spot first beheld the

same lingering remnants of a primaeval world.

 

The other might be (but I cannot promise you even a chance of

dredging that, unless you were off the coast of Portugal, or the

windward side of some of the West India Islands) a live Crinoid; an

exquisite starfish, with long and branching arms, but rooted in the

mud by a long stalk, and that stalk throwing out barren side

branches; the whole a living plant of stone. You may see in museums

specimens of this family, now so rare, all but extinct. And yet

fifty or a hundred different forms of the same type swarmed in the

ancient seas: whole masses of limestone are made up of little else

but the fragments of such animals.

 

But we have not landed yet on the dry part of the reef. Let us make

for it, taking care meanwhile that we do not get our feet cut by the

coral, or stung as by nettles by the coral insects. We shall see

that the dry land is made up entirely of coral, ground and broken by

the waves, and hurled inland by the storm, sometimes in huge

boulders, mostly as fine mud; and that, under the influence of the

sun and of the rain, which filters through it, charged with lime from

the rotting coral, the whole is setting, as cement sets, into rock.

And what is this? A long bank of stone standing up as a low cliff,

ten or twelve feet above high-water mark. It is full of fragments of

shell, of fragments of coral, of all sorts of animal remains; and the

lower part of it is quite hard rock. Moreover, it is bedded in

regular layers, just such as you see in a quarry. But how did it get

there? It must have been formed at the sea-level, some of it,

indeed, under the sea; for here are great masses of madrepore and

limestone corals imbedded just as they grew. What lifted it up?

Your companions, if you have any who know the island, have no

difficulty in telling you. It was hove up, they say, in the

earthquake in such and such a year; and they will tell you, perhaps,

that if you will go on shore to the main island which rises inside

the reef, you may see dead coral beds just like these lying on the

old rocks, and sloping up along the flanks of the mountains to

several hundred feet above the sea. I have seen such many a time.

 

Thus you find the coral being converted gradually into a limestone

rock, either fine and homogeneous, composed of coral grown into pulp,

or filled with corals and shells, or with angular fragments of older

coral rock. Did you never see that last? No? Yes, you have a

hundred times. You have but to look at the marbles commonly used

about these islands, with angular fragments imbedded in the mass, and

here and there a shell, the whole cemented together by water holding

in solution carbonate of lime, and there see the very same phenomenon  perpetuated to this day.