Town Geology 7

Town Geology 7

w suppose we laid out the wheat on a table in a regular series,

such as you may see in many exhibitions of manufactures; beginning

with the wheat plant at one end, and ending with the loaf at the

other; and called in to look at them a savage who knew nothing of

agriculture and nothing of cookery--called in, as an extreme case,

the man in the moon, who certainly can know nothing of either; for as

there is neither air nor water round the moon, there can be nothing

to grow there, and therefore nothing to cook--and suppose we asked

him to study the series from end to end. Do you not think that the

man in the moon, if he were half as shrewd as Crofton Croker makes

him in his conversation with Daniel O'Rourke, would answer after due

meditation, "How the wheat plant got changed into the loaf I cannot

see from my experience in the moon: but that it has been changed,

and that the two are the same thing I do see, for I see all the

different stages of the change." And so I think you may say of the

wood and the coal.

 

The man in the moon would be quite reasonable in his conclusion; for

it is a law, a rule, and one which you will have to apply again and

again in the study of natural objects, that however different two

objects may look in some respects, yet if you can find a regular

series of gradations between them, with all shades of likeness, first

to one of them and then to the other, then you have a fair right to

suppose them to be only varieties of the same species, the same kind

of thing, and that, therefore, they have a common origin.

 

That sounds rather magniloquent. Let me give you a simple example.

 

Suppose you had come into Britain with Brute, the grandson of AEneas,

at that remote epoch when (as all archaeologists know who have duly

read Geoffrey of Monmouth and the Arthuric legends) Britain was

inhabited only by a few giants. Now if you had met giants with one

head, and also giants with seven heads, and no others, you would have

had a right to say, "There are two breeds of giants here, one-headed

and seven-headed." But if you had found, as Jack the Giant-Killer

(who belongs to the same old cycle of myths) appears to have found,

two-headed giants also, and three-headed, and giants, indeed, with

any reasonable number of heads, would you not have been justified in

saying, "They are all of the same breed, after all; only some are

more capitate, or heady, than others!"

 

I hope that you agree to that reasoning; for by it I think we arrive

most surely at a belief in the unity of the human race, and that the

Negro is actually a man and a brother.

 

If the only two types of men in the world were an extreme white type,

like the Norwegians, and an extreme black type, like the Negros, then

there would be fair ground for saying, "These two types have been

always distinct; they are different races, who have no common

origin." But if you found, as you will find, many types of man

showing endless gradations between the white man and the Negro, and

not only that, but endless gradations between them both and a third

type, whose extreme perhaps is the Chinese--endless gradations, I

say, showing every conceivable shade of resemblance or difference,

till you often cannot say to what type a given individual belongs;

and all of them, however different from each other, more like each

other than they are like any other creature upon earth; then you are

justified in saying, "All these are mere varieties of one kind.

However distinct they are now, they were probably like each other at

first, and therefore all probably had a common origin." That seems

to me sound reasoning, and advanced natural science is corroborating

it more and more daily.

 

Now apply the same reasoning to coal. You may find about the world--

you may see even in England alone--every gradation between coal and

growing forest. You may see the forest growing in its bed of

vegetable mould; you may see the forest dead and converted into peat,

with stems and roots in it; that, again, into sunken forests, like

those to be seen below high-water mark on many coasts of this island.

You find gradations between them and beds of lignite, or wood coal;

then gradations between lignite and common or bituminous coal; and

then gradations between common coal and culm, or anthracite, such as

is found in South Wales. Have you not a right to say, "These are all

but varieties of the same kind of thing--namely, vegetable matter?

They have a common origin--namely, woody fibre. And coal, or rather

culm, is the last link in a series of transformations from growing

vegetation?"

 

This is our first theory. Let us try to verify it, as scientific men

are in the habit of doing, by saying, If that be true, then something

else is likely to be true too.

 

If coal has all been vegetable soil, then it is likely that some of

it has not been quite converted into shapeless coal. It is likely

that there will be vegetable fibre still to be seen here and there;

perhaps leaves, perhaps even stems of trees, as in a peat bog. Let

us look for them.

 

You will not need to look far. The coal, and the sands and shales

which accompany the coal, are so full of plant-remains, that three

hundred species were known to Adolphe Brongniart as early as 1849,

and that number has largely increased since.

 

Now one point is specially noticeable about these plants of the coal;

namely, that they may at least have grown in swamps.

 

First, you will be interested if you study the coal flora, with the

abundance, beauty, and variety of the ferns. Now ferns in these

islands grow principally in rocky woods, because there, beside the

moisture, they get from decaying vegetable or decaying rock,

especially limestone, the carbonic acid which is their special food,

and which they do not get on our dry pastures, and still less in our

cultivated fields. But in these islands there are two noble species,

at least, which are true swamp-ferns; the Lastraea Thelypteris, which

of old filled the fens, but is now all but extinct; and the Osmunda,

or King-fern, which, as all know, will grow wherever it is damp

enough about the roots. In Hampshire, in Devon, and Cornwall, and in

the southwest of Ireland, the King-fern too is a true swamp fern.

But in the Tropics I have seen more than once noble tree-ferns

growing in wet savannahs at the sea-level, as freely as in the

mountain-woods; ferns with such a stem as some of the coal ferns had,

some fifteen feet in height, under which, as one rode on horseback,

one saw the blazing blue sky, as through a parasol of delicate lace,

as men might have long ages since have seen it, through the plumed

fronds of the ferns now buried in the coal, had there only been a man

then created to enjoy its beauty.

 

Next we find plants called by geologists Calamites. There is no

doubt now that they are of the same family as our Equiseta, or horse-

tails, a race which has, over most parts of the globe, dwindled down

now from twenty or thirty feet in height, as they were in the old

coal measures, to paltry little weeds. The tallest Equisetum in

England--the beautiful E. Telmateia--is seldom five feet high. But

they, too, are mostly mud and swamp plants; and so may the Calamites

have been.

 

The Lepidodendrons, again, are without doubt the splendid old

representatives of a family now dwindled down to such creeping things

as our club-mosses, or Lycopodiums. Now it is a certain fact, which

can be proved by the microscope, that a very great part of the best

coal is actually made up of millions of the minute seeds of club-

mosses, such as grow--a few of them, and those very small--on our

moors; a proof, surely, not only of the vast amount of the vegetation

in the coal-making age, but also of the vast time during which it

lasted. The Lepidodendra may have been fifty or sixty feet high.

There is not a Lycopodium in the world now, I believe, five feet

high. But the club-mosses are now, in these islands and elsewhere,

lovers of wet and peaty soils, and so may their huger prototypes have

been, in the old forests of the coal.

 

Of the Sigillariae we cannot say as much with certainty, for

botanists are not agreed as to what low order of flowerless plants

they belong. But that they rooted in clay beds there is proof, as

you will hear presently.

 

And as to the Conifers, or pine-like trees--the Dadoxylon, of which

the pith goes by the name of Sternbergia, and the uncertain tree

which furnishes in some coal-measures bushels of a seed connected

with that of the yew--we may suppose that they would find no more

difficulty in growing in swamps than the cypress, which forms so

large a portion of the vegetation in the swamps of the Southern

United States.

 

I have given you these hints, because you will naturally wish to know

what sort of a world it was in which all these strange plants grew

and turned into coal.

 

My answer is, that it was most probably just like the world in which

we are living now, with the one exception that the plants and animals

are different.

 

It was the fashion a few years since to explain the coal--like other

phenomena of geology--by some mere hypothesis of a state of things

quite unlike what we see now. We were brought up to believe that in

the Carboniferous, or coal-bearing era, the atmosphere was intensely

moist and hot, and overcharged with carbonic acid, which had been

poured out from the interior of the planet by volcanic eruptions, or

by some other convulsion. I forget most of it now: and really there

is no need to remember; for it is all, I verily believe, a dream--an

attempt to explain the unknown not by the known, but by the still

more unknown. You may find such theories lingering still in

sensational school-books, if you like to be unscientific. If you

like, on the other hand, to be scientific you will listen to those

who tell you that instead of there having been one unique

carboniferous epoch, with a peculiar coal-making climate, all epochs

are carboniferous if they get the chance; that coal is of every age,

from that of the Scotch and English beds, up to the present day. The

great coal-beds along the Rocky Mountains, for instance, are

tertiary--that is, later than the chalk. Coal is forming now, I

doubt not, in many places on the earth, and would form in many more,

if man did not interfere with the processes of wild nature, by

draining the fens, and embanking the rivers.

 

Let me by a few words prove this statement. They will give you,

beside, a fresh proof of Sir Charles Lyell's great geological rule--

that the best way to explain what we see in ancient rocks is to take

for granted, as long as we can do so fairly, that things were going

on then very much as they are going on now.

 

When it was first seen that coal had been once vegetable, the

question arose--How did all these huge masses of vegetable matter get

there? The Yorkshire and Derbyshire coal-fields, I hear, cover 700

or 800 square miles; the Lancashire about 200. How large the North

Wales and the Scotch fields are I cannot say. But doubtless a great

deal more coal than can be got at lies under the sea, especially in

the north of Wales. Coal probably exists over vast sheets of England

and France, buried so deeply under later rocks, that it cannot be

reached by mining. As an instance, a distinguished geologist has

long held that there are beds of coal under London itself, which

rise, owing to a peculiar disturbance of the strata, to within 1,000

or 1,200 feet of the surface, and that we or our children may yet see

coal-mines in the marshes of the Thames. And more, it is a provable

fact that only a portion of the coal measures is left. A great part

of Ireland must once have been covered with coal, which is now

destroyed. Indeed, it is likely that the coal now known of in Europe

and America is but a remnant of what has existed there in former

ages, and has been eaten away by the inroads of the sea.

 

Now whence did all that enormous mass of vegetable soil come? Off

some neighbouring land, was the first and most natural answer. It

was a rational one. It proceeded from the known to the unknown. It

was clear that these plants had grown on land; for they were land-

plants. It was clear that there must have been land close by, for

between the beds of coal, as you all know, the rock is principally

coarse sandstone, which could only have been laid down (as I have

explained to you already) in very shallow water.

 

It was natural, then, to suppose that these plants and trees had been

swept down by rivers into the sea, as the sands and muds which buried

them had been. And it was known that at the mouths of certain

rivers--the Mississippi, for instance--vast rafts of dead floating

trees accumulated; and that the bottoms of the rivers were often full

of snags, etc.; trees which had grounded, and stuck in the mud; and

why should not the coal have been formed in the same way?

 

Because--and this was a serious objection--then surely the coal would

be impure--mixed up with mud and sand, till it was not worth burning.

Instead of which, the coal is usually pure vegetable, parted sharply

from the sandstone which lies on it. The only other explanation was,

that the coal vegetation had grown in the very places where it was

found. But that seemed too strange to be true, till that great

geologist, Sir W. Logan--who has since done such good work in Canada-

-showed that every bed of coal had a bed of clay under it, and that

that clay always contained fossils called Stigmaria. Then it came

out that the Stigmaria in the under clay had long filaments attached

to them, while when found in the sandstones or shales, they had lost

their filaments, and seemed more or less rolled--in fact, that the

natural place of the Stigmaria was in the under clay. Then Mr.

Binney discovered a tree--a Sigillaria, standing upright in the coal-

measures with its roots attached. Those roots penetrated into the

under clay of the coal; and those roots were Stigmarias. That seems

to have settled the question. The Sigillarias, at least, had grown

where they were found, and the clay beneath the coal-beds was the

original soil on which they had grown. Just so, if you will look at

any peat bog you will find it bottomed by clay, which clay is pierced

everywhere by the roots of the moss forming the peat, or of the

trees, birches, alders, poplars, and willows, which grow in the bog.

So the proof seemed complete, that the coal had been formed out of

vegetation growing where it was buried. If any further proof for

that theory was needed, it would be found in this fact, most

ingeniously suggested by Mr. Boyd Dawkins. The resinous spores, or

seeds of the Lepidodendra make up--as said above--a great part of the

bituminous coal. Now those spores are so light, that if the coal had

been laid down by water, they would have floated on it, and have been

carried away; and therefore the bituminous coal must have been

formed, not under water, but on dry land.

 

I have dwelt at length on these further arguments, because they seem

to me as pretty a specimen as I can give my readers of that regular

and gradual induction, that common-sense regulated, by which

geological theories are worked out.

 

But how does this theory explain the perfect purity of the coal? I

think Sir C. Lyell answers that question fully in p. 383 of his

"Student's Elements of Geology." He tells us that the dense growths

of reeds and herbage which encompass the margins of forest-covered

swamps in the valley and delta of the Mississippi, in passing through

them, are filtered and made to clear themselves entirely before they

reach the areas in which vegetable matter may accumulate for

centuries, forming coal if the climate be favourable; and that in the

cypress-swamps of that region no sediment mingles with the vegetable

matter accumulated from the decay of trees and semi-aquatic plants;

so that when, in a very dry season, the swamp is set on fire, pits

are burnt into the ground many feet deep, or as far as the fire can

go down without reaching water, and scarcely any earthy residuum is

left; just as when the soil of the English fens catches fire, red-hot

holes are eaten down through pure peat till the water-bearing clay

below is reached. But the purity of the water in peaty lagoons is

observable elsewhere than in the delta of the Mississippi. What can

be more transparent than many a pool surrounded by quaking bogs,

fringed, as they are in Ireland, with a ring of white water-lilies,

which you dare not stoop to pick, lest the peat, bending inward,

slide you down into that clear dark gulf some twenty feet in depth,

bottomed and walled with yielding ooze, from which there is no

escape? Most transparent, likewise, is the water of the West Indian

swamps. Though it is of the colour of coffee, or rather of dark

beer, and so impregnated with gases that it produces fever or cholera

when drunk, yet it is--at least when it does not mingle with the salt

water--so clear, that one might see every marking on a boa-

constrictor or alligator, if he glided along the bottom under the

canoe.

 

But now comes the question--Even if all this be true, how were the

forests covered up in shale and sandstone, one after another?

 

By gradual sinking of the land, one would suppose.

 

If we find, as we may find in a hundred coal-pits, trees rooted as

they grew, with their trunks either standing up through the coal, and

through the sandstone above the coal; their bark often remaining as

coal while their inside is filled up with sandstone, has not our

common-sense a right to say--The land on which they grew sank below

the water-line; the trees were killed; and the mud and sand which

were brought down the streams enveloped their trunks? As for the

inside being full of sandstone, have we not all seen hollow trees?

Do we not all know that when a tree dies its wood decays first, its

bark last? It is so, especially in the Tropics. There one may see

huge dead trees with their bark seemingly sound, and their inside a

mere cavern with touchwood at the bottom; into which caverns one used

to peep with some caution. For though one might have found inside

only a pair of toucans, or parrots, or a whole party of jolly little

monkeys, one was quite as likely to find a poisonous snake four or

five feet long, whose bite would have very certainly prevented me

having the pleasure of writing this book.

 

Now is it not plain that if such trees as that sunk, their bark would

be turned into lignite, and at last into coal, while their insides

would be silted up with mud and sand? Thus a core or pillar of hard

sandstone would be formed, which might do to the collier of the

future what they are too apt to do now in the Newcastle and Bristol

collieries. For there, when the coal is worked out below, the

sandstone stems--"coal-pipes" as the colliers call them--in the roof

of the seam, having no branches, and nothing to hold them up but

their friable bark of coal, are but too apt to drop out suddenly,

killing or wounding the hapless men below.

 

Or again, if we find--as we very often find--as was found at

Parkfield Colliery, near Wolverhampton, in the year 1814--a quarter

of an acre of coal-seam filled. with stumps of trees as they grew,

their trunks broken off and lying in every direction, turned into

coal, and flattened, as coal-fossils so often are, by the weight of

the rock above--should we not have a right to say--These trees were

snapped off where they grew by some violent convulsion; by a storm,

or by a sudden inrush of water owing to a sudden sinking of the land,

or by the very earthquake shock itself which sank the land?

 

But what evidence have we of such sinkings? The plain fact that you

have coal-seam above coal-seam, each with its bed of under-clay; and

that therefore the land MUST have sunk ere the next bed of soil could

have been deposited, and the next forest have grown on it.

 

In one of the Rocky Mountain coal-fields there are more than thirty

seams of coal, each with its under-clay below it. What can that mean

but thirty or more subsidences of the land, and the peat of thirty or

more forests or peat-mosses, one above the other? And now if any

reader shall say, Subsidence? What is this quite new element which

you have brought into your argument? You told us that you would

reason from the known to the unknown. What do we know of subsidence?

You offered to explain the thing which had gone on once by that which

is going on now. Where is subsidence going on now upon the surface

of our planet? And where, too, upheaval, such as would bring us

these buried forests up again from under the sea-level, and make

them, like our British coal-field, dry land once more?

 

The answer is--Subsidence and elevation of the land are common now,

probably just as common as they were in any age of this planet's

history.

 

To give two instances, made now notorious by the writings of

geologists. As lately as 1819 a single earthquake shock in Cutch, at

the mouth of the Indus, sunk a tract of land larger than the Lake of

Geneva in some places to a depth of eighteen feet, and converted it

into an inland sea. The same shock raised, a few miles off, a

corresponding sheet of land some fifty miles in length, and in some

parts sixteen miles broad, ten feet above the level of the alluvial

plain, and left it to be named by the country-people the "Ullah

Bund," or bank of God, to distinguish it from the artificial banks in

the neighbourhood.

 

Again: in the valley of the Mississippi--a tract which is now, it

would seem, in much the same state as central England was while our

coal-fields were being laid down--the earthquakes of 1811-12 caused

large lakes to appear suddenly in many parts of the district, amid

the dense forests of cypress. One of these, the "Sunk Country," near

New Madrid, is between seventy and eighty miles in length, and thirty

miles in breadth, and throughout it, as late as 1846, "dead trees

were conspicuous, some erect in the water, others fallen, and strewed

in dense masses over the bottom, in the shallows, and near the

shore." I quote these words from Sir Charles Lyell's "Principles of

Geology" (11th edit.), vol. i. p. 453. And I cannot do better than

advise my readers, if they wish to know more of the way in which coal

was formed, to read what is said in that book concerning the Delta of

the Mississippi, and its strata of forests sunk where they grew, and

in some places upraised again, alternating with beds of clay and

sand, vegetable soil, recent sea-shells, and what not, forming, to a

depth of several hundred feet, just such a mass of beds as exists in

our own coal-fields at this day.

 

If, therefore, the reader wishes to picture to himself the scenery of

what is now central England, during the period when our coal was

being laid down, he has only, I believe, to transport himself in

fancy to any great alluvial delta, in a moist and warm climate,

favourable to the growth of vegetation. He has only to conceive

wooded marshes, at the mouth of great rivers, slowly sinking beneath

the sea; the forests in them killed by the water, and then covered up

by layers of sand, brought down from inland, till that new layer

became dry land, to carry a fresh crop of vegetation. He has thus

all that he needs to explain how coal-measures were formed. I myself

saw once a scene of that kind, which I should be sorry to forget; for

there was, as I conceived, coal, making, or getting ready to be made,

before my eyes: a sheet of swamp, sinking slowly into the sea; for

there stood trees, still rooted below high-water mark, and killed by

the waves; while inland huge trees stood dying, or dead, from the

water at their roots. But what a scene--a labyrinth of narrow

creeks, so narrow that a canoe could not pass up, haunted with

alligators and boa-constrictors, parrots and white herons, amid an

inextricable confusion of vegetable mud, roots of the alder-like

mangroves, and tangled creepers hanging from tree to tree; and

overhead huge fan-palms, delighting in the moisture, mingled with

still huger broad-leaved trees in every stage of decay. The drowned

vegetable soil of ages beneath me; above my head, for a hundred feet,

a mass of stems and boughs, and leaves and flowers, compared with