Town Geology 3

Town Geology 3

But suppose that when you took the bird up you found that it was

neither a jackdaw, nor a sparrow nor a swallow, as you expected, but

a humming-bird. Then you would be adrift again. The fact of it

being a humming-bird would be a new fact which you had not taken into

account, and for which your old explanation was not sufficient; and

you would have to try a new induction--to use your common sense

afresh--saying, "I have not to explain merely how a dead bird got

here, but how a dead humming-bird."

 

And now, if your imaginative friend chimed in triumphantly with: "Do

you not see that I was right after all? Do you not see that it fell

from the clouds? that it was swept away hither, all the way from

South America, by some south-westerly storm, and wearied out at last,

dropped here to find rest, as in a sacred-place?" what would you

answer? "My friend, that is a beautiful imagination; but I must

treat it only as such, as long as I can explain the mystery more

simply by facts which I do know. I do not know that humming-birds

can be blown across the Atlantic alive. I do know they are actually

brought across the Atlantic dead; are stuck in ladies' hats. I know

that ladies visit the cathedral; and odd as the accident is, I prefer

to believe, till I get a better explanation, that the humming-bird

has simply dropped out of a lady's hat." There, again, you would be

speaking common sense; and using, too, sound inductive method; trying

to explain what you do not know from what you do know already.

 

Now, I ask of you to employ the same common sense when you read and

think of Geology.

 

It is very necessary to do so. For in past times men have tried to

explain the making of the world around them, its oceans, rivers,

mountains, and continents, by I know not what of fancied cataclysms

and convulsions of nature; explaining the unknown by the still more

unknown, till some of their geological theories were no more

rational, because no more founded on known facts, than that of the

New Zealand Maories, who hold that some god, when fishing, fished up

their islands out of the bottom of the ocean. But a sounder and

wiser school of geologists now reigns; the father of whom, in England

at least, is the venerable Sir Charles Lyell. He was almost the

first of Englishmen who taught us to see--what common sense tells us-

-that the laws which we see at work around us now have been most

probably at work since the creation of the world; and that whatever

changes may seem to have taken place in past ages, and in ancient

rocks, should be explained, if possible, by the changes which are

taking place now in the most recent deposits--in the soil of the

field.

 

And in the last forty years--since that great and sound idea has

become rooted in the minds of students, and especially of English

students, geology has thriven and developed, perhaps more than any

other science; and has led men on to discoveries far more really

astonishing and awful than all fancied convulsions and cataclysms.

 

I have planned this series of papers, therefore, on Sir Charles

Lyell's method. I have begun by trying to teach a little about the

part of the earth's crust which lies nearest us, which we see most

often; namely, the soil; intending, if my readers do me the honour to

read the papers which follow, to lead them downward, as it were, into

the earth; deeper and deeper in each paper, to rocks and minerals

which are probably less known to them than the soil in the fields.

Thus you will find I shall lead you, or try to lead you on,

throughout the series, from the known to the unknown, and show you

how to explain the latter by the former. Sir Charles Lyell has, I

see, in the new edition of his "Student's Elements of Geology," begun

his book with the uppermost, that is, newest, strata, or layers; and

has gone regularly downwards in the course of the book to the lowest

or earliest strata; and I shall follow his plan.

 

I must ask you meanwhile to remember one law or rule, which seems to

me founded on common sense; namely, that the uppermost strata are

really almost always the newest; that when two or more layers,

whether of rock or earth--or indeed two stones in the street, or two

sheets on a bed, or two books on a table--any two or more lifeless

things, in fact, lie one on the other, then the lower one was most

probably put there first, and the upper one laid down on the lower.

Does that seem to you a truism? Do I seem almost impertinent in

asking you to remember it? So much the better. I shall be saved

unnecessary trouble hereafter.

 

But some one may say, and will have a right to say, "Stop--the lower

thing may have been thrust under the upper one." Quite true: and

therefore I said only that the lower one was most probably put there

first. And I said "most probably," because it is most probable that

in nature we should find things done by the method which costs least

force, just as you do them. I will warrant that when you want to

hide a thing, you lay something down on it ten times for once that

you thrust it under something else. You may say, "What? When I want

to hide a paper, say, under the sofa-cover, do I not thrust it

under?"

 

No, you lift up the cover, and slip the paper in, and let the cover

fall on it again. And so, even in that case, the paper has got into

its place first.

 

Now why is this? Simply because in laying one thing on another you

only move weight. In thrusting one thing under another, you have not

only to move weight, but to overcome friction. That is why you do

it, though you are hardly aware of it: simply because so you employ

less force, and take less trouble.

 

And so do clays and sands and stones. They are laid down on each

other, and not thrust under each other, because thus less force is

expended in getting them into place.

 

There are exceptions. There are cases in which nature does try to

thrust one rock under another. But to do that she requires a force

so enormous, compared with what is employed in laying one rock on

another, that (so to speak) she continually fails; and instead of

producing a volcanic eruption, produces only an earthquake. Of that

I may speak hereafter, and may tell you, in good time, how to

distinguish rocks which have been thrust in from beneath, from rocks

which have been laid down from above, as every rock between London

and Birmingham or Exeter has been laid down. That I only assert now.

But I do not wish you to take it on trust from me. I wish to prove

it to you as I go on, or to do what is far better for you: to put

you in the way of proving it for yourself, by using your common

sense.

 

At the risk of seeming prolix, I must say a few more words on this

matter. I have special reasons for it. Until I can get you to "let

your thoughts play freely" round this question of the superposition

of soils and rocks, there will be no use in my going on with these

papers.

 

Suppose then (to argue from the known to the unknown) that you were

watching men cleaning out a pond. Atop, perhaps, they would come to

a layer of soft mud, and under that to a layer of sand. Would not

common sense tell you that the sand was there first, and that the

water had laid down the mud on the top of it? Then, perhaps, they

might come to a layer of dead leaves. Would not common sense tell

you that the leaves were there before the sand above them? Then,

perhaps, to a layer of mud again. Would not common sense tell you

that the mud was there before the leaves? And so on down to the

bottom of the pond, where, lastly, I think common sense would tell

you that the bottom of the pond was there already, before all the

layers which were laid down on it. Is not that simple common sense?

 

Then apply that reasoning to the soils and rocks in any spot on

earth. If you made a deep boring, and found, as you would in many

parts of this kingdom, that the boring, after passing through the

soil of the field, entered clays or loose sands, you would say the

clays were there before the soil. If it then went down into

sandstone, you would say--would you not?--that sandstone must have

been here before the clay; and however thick--even thousands of feet-

-it might be, that would make no difference to your judgment. If

next the boring came into quite different rocks; into a different

sort of sandstone and shales, and among them beds of coal, would you

not say--These coal-beds must have been here before the sandstones?

And if you found in those coal-beds dead leaves and stems of plants,

would you not say--Those plants must have been laid down here before

the layers above them, just as the dead leaves in the pond were?

 

If you then came to a layer of limestone, would you not say the same?

And if you found that limestone full of shells and corals, dead, but

many of them quite perfect, some of the corals plainly in the very

place in which they grew, would you not say--These creatures must

have lived down here before the coal was laid on top of them? And

if, lastly, below the limestone you came to a bottom rock quite

different again, would you not say--The bottom rock must have been

here before the rocks on the top of it?

 

And if that bottom rock rose up a few miles off, two thousand feet,

or any other height, into hills, what would you say then? Would you

say: "Oh, but the rock is not bottom rock; is not under the

limestone here, but higher than it. So perhaps in this part it has

made a shift, and the highlands are younger than the lowlands; for

see, they rise so much higher?" Would not that be as wise as to say

that the bottom of the pond was not there before the pond mud,

because the banks round the pond rose higher than the mud?

 

Now for the soil of the field.

 

If we can understand a little about it, what it is made of, and how

it got there, we shall perhaps be on the right road toward

understanding what all England--and, indeed, the crust of this whole

planet--is made of; and how its rocks and soils got there.

 

But we shall best understand how the soil in the field was made, by

reasoning, as I have said, from the known to the unknown. What do I

mean? This: On the uplands are fields in which the soil is already

made. You do not know how? Then look for a field in which the soil

is still being made. There are plenty in every lowland. Learn how

it is being made there; apply the knowledge which you learn from them

to the upland fields which are already made.

 

If there is, as there usually is, a river-meadow, or still better, an

aestuary, near your town, you have every advantage for seeing soil

made. Thousands of square feet of fresh-made soil spread between

your town and the sea; thousands more are in process of being made.

 

You will see now why I have begun with the soil in the field; because

it is the uppermost, and therefore latest, of all the layers; and

also for this reason, that, if Sir Charles Lyell's theory be true--as

it is--then the soils and rocks below the soil of the field may have

been made in the very same way in which the soil of the field is

made. If so, it is well worth our while to examine it.

 

You all know from whence the soil comes which has filled up, in the

course of ages, the great aestuaries below London, Stirling, Chester,

or Cambridge.

 

It is river mud and sand. The river, helped by tributary brooks

right and left, has brought down from the inland that enormous mass.

You know that. You know that every flood and freshet brings a fresh

load, either of fine mud or of fine sand, or possibly some of it

peaty matter out of distant hills. Here is one indisputable fact

from which to start. Let us look for another.

 

How does the mud get into the river? The rain carries it thither.

 

If you wish to learn the first elements of geology by direct

experiment, do this: The next rainy day--the harder it rains the

better--instead of sitting at home over the fire, and reading a book

about geology, put on a macintosh and thick boots, and get away, I

care not whither, provided you can find there running water. If you

have not time to get away to a hilly country, then go to the nearest

bit of turnpike road, or the nearest sloping field, and see in little

how whole continents are made, and unmade again. Watch the rain

raking and sifting with its million delicate fingers, separating the

finer particles from the coarser, dropping the latter as soon as it

can, and carrying the former downward with it toward the sea. Follow

the nearest roadside drain where it runs into a pond, and see how it

drops the pebbles the moment it enters the pond, and then the sand in

a fan-shaped heap at the nearest end; but carries the fine mud on,

and holds it suspended, to be gradually deposited at the bottom in

the still water; and say to yourself: Perhaps the sands which cover

so many inland tracts were dropped by water, very near the shore of a

lake or sea, and by rapid currents. Perhaps, again, the brick clays,

which are often mingled with these sands, were dropped, like the mud

in the pond, in deeper water farther from the shore, and certainly in

stilt water. But more. Suppose once more, then, that looking and

watching a pond being cleared out, under the lowest layer of mud, you

found--as you would find in any of those magnificent reservoirs so

common in the Lancashire hills--a layer of vegetable soil, with grass

and brushwood rooted in it. What would you say but: The pond has

not been always full. It has at some time or other been dry enough

to let a whole copse grow up inside it?

 

And if you found--as you will actually find along some English

shores--under the sand hills, perhaps a bed of earth with shells and

bones; under that a bed of peat; under that one of blue silt; under

that a buried forest, with the trees upright and rooted; under that

another layer of blue silt full of roots and vegetable fibre; perhaps

under that again another old land surface with trees again growing in

it; and under all the main bottom clay of the district--what would

common sense tell you? I leave you to discover for yourselves. It

certainly would not tell you that those trees were thrust in there by

a violent convulsion, or that all those layers were deposited there

in a few days, or even a few years; and you might safely indulge in

speculations about the antiquity of the aestuary, and the changes

which it has undergone, with which I will not frighten you at

present.

 

It will be fair reasoning to argue thus. You may not be always right

in your conclusion, but still you will be trying fairly to explain

the unknown by the known.

 

But have Rain and Rivers alone made the soil?

 

How very much they have done toward making it you will be able to

judge for yourselves, if you will read the sixth chapter of Sir

Charles Lyell's new "Elements of Geology," or the first hundred pages

of that admirable book, De la Beche's "Geological Observer;" and

last, but not least, a very clever little book called "Rain and

Rivers," by Colonel George Greenwood.

 

But though rain, like rivers, is a carrier of soil, it is more. It

is a maker of soil, likewise; and by it mainly the soil of an upland

field is made, whether it be carried down to the sea or not.

 

If you will look into any quarry you will see that however compact

the rock may be a few feet below the surface, it becomes, in almost

every case, rotten and broken up as it nears the upper soil, till you

often cannot tell where the rock ends and the soil begins.

 

Now this change has been produced by rain. First, mechanically, by

rain in the shape of ice. The winter rain gets into the ground, and

does by the rock what it has done by the stones of many an old

building. It sinks into the porous stone, freezes there, expands in

freezing, and splits and peels the stone with a force which is slowly

but surely crumbling the whole of Northern Europe and America to

powder.

 

Do you doubt me? I say nothing but what you can judge of yourselves.

The next time you go up any mountain, look at the loose broken stones

with which the top is coated, just underneath the turf. What has

broken them up but frost? Look again, as stronger proof, at the

talus of broken stones--screes, as they call them in Scotland;

rattles, as we call them in Devon--which lie along the base of many

mountain cliffs. What has brought them down but frost? If you ask

the country folk they will tell you whether I am right or not. If

you go thither, not in the summer, but just after the winter's frost,

you will see for yourselves, by the fresh frost-crop of newly-broken

bits, that I am right. Possibly you may find me to be even more

right than is desirable, by having a few angular stones, from the

size of your head to that of your body, hurled at you by the frost-

giants up above. If you go to the Alps at certain seasons, and hear

the thunder of the falling rocks, and see their long lines--moraines,

as they are called--sliding slowly down upon the surface of the

glacier, then you will be ready to believe the geologist who tells

you that frost, and probably frost alone, has hewn out such a peak as

the Matterhorn from some vast table-land; and is hewing it down

still, winter after winter, till some day, where the snow Alps now

stand, there shall be rolling uplands of rich cultivable soil.

 

So much for the mechanical action of rain, in the shape of ice. Now

a few words on its chemical action.

 

Rain water is seldom pure. It carries in it carbonic acid; and that

acid, beating in shower after shower against the face of a cliff--

especially if it be a limestone cliff--weathers the rock chemically;

changing (in case of limestone) the insoluble carbonate of lime into

a soluble bicarbonate, and carrying that away in water, which,

however clear, is still hard. Hard water is usually water which has

invisible lime in it; there are from ten to fifteen grains and more

of lime in every gallon of limestone water. I leave you to calculate

the enormous weight of lime which must be so carried down to the sea

every year by a single limestone or chalk brook. You can calculate

it, if you like, by ascertaining the weight of lime in each gallon,

and the average quantity of water which comes down the stream in a

day; and when your sum is done, you will be astonished to find it one

not of many pounds, but probably of many tons, of solid lime, which

you never suspected or missed from the hills around. Again, by the

time the rain has sunk through the soil, it is still less pure. It

carries with it not only carbonic acid, but acids produced by

decaying vegetables--by the roots of the grasses and trees which grow

above; and they dissolve the cement of the rock by chemical action,

especially if the cement be lime or iron. You may see this for

yourselves, again and again. You may see how the root of a tree,

penetrating the earth, discolours the soil with which it is in

contact. You may see how the whole rock, just below the soil, has

often changed in colour from the compact rock below, if the soil be

covered with a dense layer of peat or growing vegetables.

 

But there is another force at work, and quite as powerful as rain and

rivers, making the soil of alluvial flats. Perhaps it has helped,

likewise, to make the soil of all the lowlands in these isles--and

that is, the waves of the sea.

 

If you ever go to Parkgate, in Cheshire, try if you cannot learn

there a little geology.

 

Walk beyond the town. You find the shore protected for a long way by

a sea-wall, lest it should be eaten away by the waves. What the

force of those waves can be, even on that sheltered coast, you may

judge--at least you could have judged this time last year--by the

masses of masonry torn from their iron clampings during the gale of

three winters since. Look steadily at those rolled blocks, those

twisted stanchions, if they are there still; and then ask yourselves-

-it will be fair reasoning from the known to the unknown--What effect

must such wave-power as that have had beating and breaking for

thousands of years along the western coasts of England, Scotland,

Ireland? It must have eaten up thousands of acres--whole shires, may

be, ere now. Its teeth are strong enough, and it knows neither rest

nor pity, the cruel hungry sea. Give it but time enough, and what

would it not eat up? It would eat up, in the course of ages, all the

dry land of this planet, were it not baffled by another counteracting

force, of which I shall speak hereafter.

 

As you go on beyond the sea-wall, you find what it is eating up. The

whole low cliff is going visibly. But whither is it going? To form

new soil in the aestuary. Now you will not wonder how old harbours

so often become silted up. The sea has washed the land into them.

But more, the sea-currents do not allow the sands of the aestuary to

escape freely out to sea. They pile it up in shifting sand-banks

about the mouth of the aestuary. The prevailing sea-winds, from

whatever quarter, catch up the sand, and roll it up into sand-hills.

Those sand-hills are again eaten down by the sea, and mixed with the

mud of the tide-flats, and so is formed a mingled soil, partly of

clayey mud, partly of sand; such a soil as stretches over the greater

part of all our lowlands.

 

Now, why should not that soil, whether in England or in Scotland,

have been made by the same means as that of every aestuary.

 

You find over great tracts of East Scotland, Lancashire, Norfolk,

etc., pure loose sand just beneath the surface, which looks as if it

was blown sand from a beach. Is it not reasonable to suppose that it

is? You find rising out of many lowlands, crags which look exactly

like old sea-cliffs eaten by the waves, from the base of which the

waters have gone back. Why should not those crags be old sea-cliffs?

Why should we not, following our rule of explaining the unknown by

the known, assume that such they are till someone gives us a sound

proof that they are not; and say--These great plains of England and

Scotland were probably once covered by a shallow sea, and their soils

made as the soil of any tide-flat is being made now?

 

But you may say, and most reasonably "The tide-flats are just at the

sea-level. The whole of the lowland is many feet above the sea; it

must therefore have been raised out of the sea, according to your

theory: and what proofs have you of that?"

 

Well, that is a question both grand and deep, on which I shall not

enter yet; but meanwhile, to satisfy you that I wish to play fair

with you, I ask you to believe nothing but what you can prove for

yourselves. Let me ask you this: suppose that you had proof

positive that I had fallen into the river in the morning; would not

your meeting me in the evening be also proof positive that somehow or

other I had in the course of the day got out of the river? I think

you will accept that logic as sound.

 

Now if I can give you proof positive, proof which you can see with

your own eyes, and handle with your own hands, and alas! often feel

but too keenly with your own feet, that the whole of the lowlands

were once beneath the sea; then will it not be certain that, somehow

or other, they must have been raised out of the sea again?

 

And that I propose to do in my next paper, when I speak of the

pebbles in the street.

 

Meanwhile I wish you to face fairly the truly grand idea, which all I

have said tends to prove true--that all the soil we see is made by

the destruction of older soils, whether soft as clay, or hard as

rock; that rain, rivers, and seas are perpetually melting and

grinding up old land, to compose new land out of it; and that it must

have been doing so, as long as rain, rivers, and seas have existed.

"But how did the first land of all get made?" I can only reply: A

natural question: but we can only answer that, by working from the

known to the unknown. While we are finding