Town Geology
by
Charles Kingsley
Part 2 out of 3
is, the beds are not exactly parallel. The Bunter had been slightly
tilted, and slightly waterworn, before the Keuper was laid on it.
It is reasonable, therefore, to suppose, that the Bunter in England
was dry land, and therefore safe from fresh deposit, through ages
during which it was deep enough beneath the sea in Germany, to have
the Muschelkalk laid down on it. Here again, then, as everywhere, we
have evidence of time--time, not only beyond all counting, but beyond
all imagining.
And now, perhaps, the reader will ask--If I am to believe that all
new land is made out of old land, and that all rocks and soils are
derived from the wear and tear of still older rocks, off what land
came this enormous heap of sands more than 5,000 feet thick in
places, stretching across England and into Germany?
It is difficult to answer. The shape and distribution of land in
those days were so different from what they are now, that the rocks
which furnished a great deal of our sandstone may be now, for aught I
know, a mile beneath the sea.
But over the land which still stands out of the sea near us there has
been wear and tear enough to account for any quantity of sand
deposit. As a single instance--It is a provable and proven fact--as
you may see from Mr. Ramsay's survey of North Wales--that over a
large tract to the south of Snowdon, between Port Madoc and Barmouth,
there has been ground off and carried away a mass of solid rock
20,000 feet thick; thick enough, in fact, if it were there still, to
make a range of mountains as high as the Andes. It is a provable and
proven fact that vast tracts of the centre of poor old Ireland were
once covered with coal-measures, which have been scraped off in
likewise, deprived of inestimable mineral wealth. The destruction of
rocks--"denudation" as it is called--in the district round Malvern,
is, I am told, provably enormous. Indeed, it is so over all Wales,
North England, and West and North Scotland. So there is enough of
rubbish to be accounted for to make our New Red sands. The round
pebbles in it being, I believe, pieces of Old Red sandstone, may have
come from the great Old Red sandstone region of South East Wales and
Herefordshire. Some of the rubbish, too, may have come from what is
now the Isle of Anglesey.
For you find in the beds, from the top to the bottom (at least in
Cheshire), particles of mica. Now this mica could not have been
formed in the sand. It is a definite crystalline mineral, whose
composition is well known. It is only found in rocks which have been
subjected to immense pressure, and probably to heat. The granites
and mica-slates of Anglesey are full of it; and from Anglesey--as
likely as from anywhere else--these thin scales of mica came. And
that is about all that I can say on the matter. But it is certain
that most of these sands were deposited in a very shallow water, and
very near to land. Sand and pebbles, as I said in my first paper,
could not be carried far out to sea; and some of the beds of the
Bunter are full of rounded pebbles. Nay, it is certain that their
surface was often out of water. Of that you may see very pretty
proofs. You find these sands ripple-marked, as you do shore-sands
now. You find cracks where the marl mud has dried in the sun: and,
more, you find the little pits made by rain. Of that I have no
doubt. I have seen specimens, in which you could not only see at a
glance that the marks had been made by the large drops of a shower,
but see also from what direction the shower had come. These delicate
markings must have been covered up immediately with a fresh layer of
mud or sand. How long since? How long since that flag had seen the
light of the sun, when it saw it once again, restored to the upper
air by the pick of the quarryman? Who can answer that? Not I.
Fossils are very rare in these sands; it is not easy to say why. It
may be that the red oxide of iron in them has destroyed them. Few or
none are ever found in beds in which it abounds. It is curious, too,
that the Keuper, which is all but barren of fossils in England, is
full of them in Wurtemberg, reptiles, fish, and remains of plants
being common. But what will interest the reader are the footprints
of a strange beast, found alike in England and in Germany--the
Cheirotherium, as it was first named, from its hand-like feet; the
Labyrinthodon, as it is now named, from the extraordinary structure
of its teeth. There is little doubt now, among anatomists, that the
bones and teeth of the so-called Labyrinthodon belong to the animal
which made the footprints. If so, the creature must have been a
right loathly monster. Some think him to have been akin to lizards;
but the usual opinion is that he was a cousin of frogs and toads.
Looking at his hands and other remains, one pictures him to oneself
as a short, squat brute, as big as a fat hog, with a head very much
the shape of a baboon, very large hands behind and small ones in
front, waddling about on the tide flats of a sandy sea, and dragging
after him, seemingly, a short tail, which has left its mark on the
sand. What his odour was, whether he was smooth or warty, what he
ate, and in general how he got his living, we know not. But there
must have been something there for him to eat; and I dare say that he
was about as happy and about as intellectual as the toad is now.
Remember always that there is nothing alive now exactly like him, or,
indeed, like any animal found in these sandstones. The whole animal
world of this planet has changed entirely more than once since the
Labyrinthodon waddled over the Cheshire flats. A lizard, for
instance, which has been found in the Keuper, had a skull like a
bird's, and no teeth--a type which is now quite extinct. But there
is a more remarkable animal of which I must say a few words, and one
which to scientific men is most interesting and significant.
Both near Warwick, and near Elgin in Scotland, in Central India, and
in South Africa, fossil remains are found of a family of lizards
utterly unlike anything now living save one, and that one is crawling
about, plentifully I believe--of all places in the world--in New
Zealand. How it got there; how so strange a type of creature should
have died out over the rest of the world, and yet have lasted on in
that remote island for long ages, ever since the days of the New Red
sandstone, is one of those questions--quite awful questions I
consider them--with which I will not puzzle my readers. I only
mention it to show them what serious questions the scientific man has
to face, and to answer, if he can. Only the next time they go to the
Zoological Gardens in London, let them go to the reptile-house, and
ask the very clever and courteous attendant to show them the
Sphenodons, or Hatterias, as he will probably call them--and then
look, I hope with kindly interest, at the oldest Conservatives they
ever saw, or are like to see; gentlemen of most ancient pedigree, who
have remained all but unchanged, while the whole surface of the globe
has changed around them more than once or twice.
And now, of course, my readers will expect to hear something of the
deposits of rock-salt, for which Cheshire and its red rocks are
famous. I have never seen them, and can only say that the salt does
not, it is said by geologists, lie in the sandstone, but at the
bottom of the red marl which caps the sandstone. It was formed most
probably by the gradual drying up of lagoons, such as are depositing
salt, it is said now, both in the Gulf of Tadjara, on the Abyssinian
frontier opposite Aden, and in the Runn of Cutch, near the Delta of
the Indus. If this be so, then these New Red sandstones may be the
remains of a whole Sahara--a sheet of sandy and all but lifeless
deserts, reaching from the west of England into Germany, and rising
slowly out of the sea; to sink, as we shall find, beneath the sea
again.
And now, as to the vast period of time--the four or five worlds, as I
called it--which elapsed between the laying down of the New Red
sandstones and the laying down of the boulder-clays.
I think this fact--for fact it is--may be better proved by taking
readers an imaginary railway journey to London from any spot in the
manufacturing districts of central England--begging them, meanwhile,
to keep their eyes open on the way.
And here I must say that I wish folks in general would keep their
eyes a little more open when they travel by rail. When I see young
people rolling along in a luxurious carriage, their eyes and their
brains absorbed probably in a trashy shilling novel, and never lifted
up to look out of the window, unconscious of all that they are
passing--of the reverend antiquities, the admirable agriculture, the
rich and peaceful scenery, the like of which no country upon earth
can show; unconscious, too, of how much they might learn of botany
and zoology, by simply watching the flowers along the railway banks
and the sections in the cuttings: then it grieves me to see what
little use people make of the eyes and of the understanding which God
has given them. They complain of a dull journey: but it is not the
journey which is dull; it is they who are dull. Eyes have they, and
see not; ears have they, and hear not; mere dolls in smart clothes,
too many of them, like the idols of the heathen.
But my readers, I trust, are of a better mind. So the next time they
find themselves running up southward to London--or the reverse way--
let them keep their eyes open, and verify, with the help of a
geological map, the sketch which is given in the following pages.
Of the "Black Countries"--the actual coal districts I shall speak
hereafter. They are in England either shores or islands yet
undestroyed, which stand out of the great sea of New Red sandstone,
and often carry along their edges layers of far younger rocks, called
now Permian, from the ancient kingdom of Permia, in Russia, where
they cover a vast area. With them I will not confuse the reader just
now, but will only ask him to keep his eye on the rolling plain of
New Red sands and marls past, say, Birmingham and Warwick. After
those places, these sands and marls dip to the south-east, and other
rocks and soils appear above them, one after another, dipping
likewise towards the south-east--that is, toward London.
First appear thin layers of a very hard blue limestone, full of
shells, and parted by layers of blue mud. That rock runs in a broad
belt across England, from Whitby in Yorkshire, to Lyme in
Dorsetshire, and is known as Lias. Famous it is, as some readers may
know, for holding the bones of extinct monsters--Ichthyosaurs and
Plesiosaurs, such as the unlearned may behold in the lake at the
Crystal Palace. On this rock lie the rich cheese pastures, and the
best tracts of the famous "hunting shires" of England.
Lying on it, as we go south-eastward, appear alternate beds of sandy
limestone, with vast depths of clay between them. These "oolites,"
or freestones, furnish the famous Bath stone, the Oxford stone, and
the Barnack stone of Northamptonshire, of which some of the finest
cathedrals are built--a stone only surpassed, I believe, by the Caen
stone, which comes from beds of the same age in Normandy. These
freestones and clays abound in fossils, but of kinds, be it
remembered, which differ more and more from those of the lias
beneath, as the beds are higher in the series, and therefore nearer.
There, too, are found principally the bones of that extraordinary
flying lizard, the Pterodactyle, which had wings formed out of its
fore-legs, on somewhat the same plan as those of a bat, but with one
exception. In the bat, as any one may see, four fingers of the hand
are lengthened to carry the wing, while the first alone is left free,
as a thumb: but in the Pterodactyle, the outer or "little" finger
alone is lengthened, and the other four fingers left free--one of
those strange instances in nature of the same effect being produced
in widely different plants and animals, and yet by slightly different
means, on which a whole chapter of natural philosophy--say, rather,
natural theology--will have to be written some day.
But now consider what this Lias, and the Oolites and clays upon it
mean. They mean that the New Red sandstone, after it had been dry
land, or all but dry land (as is proved by the footprints of animals
and the deposits of salt), was sunk again beneath the sea. Each
deposit of limestone signifies a long period of time, during which
that sea was pure enough to allow reefs of coral to grow, and shells
to propagate, at the bottom. Each great band of clay signifies a
long period, during which fine mud was brought down from some wasting
land in the neighbourhood. And that land was not far distant is
proved by the bones of the Pterodactyle, of Crocodiles, and of
Marsupials; by the fact that the shells are of shallow-water or shore
species; by the presence, mixed with them, of fragments of wood,
impressions of plants, and even wing-shells of beetles; and lastly,
if further proof was needed, by the fact that in the "dirt-bed" of
the Isle of Portland and the neighbouring shores, stumps of trees
allied to the modern sago-palms are found as they grew in the soil,
which, with them, has been covered up in layers of freshwater shale
and limestone. A tropic forest has plainly sunk beneath a lagoon;
and that lagoon, again, beneath the sea.
And how long did this period of slow sinking go on? Who can tell?
The thickness of the Lias and Oolites together cannot be less than a
thousand feet. Considering, then, the length of time required to lay
down a thousand feet of strata, and considering the vast difference
between the animals found in them, and the few found in the New Red
sandstone, we have a right to call them another world, and that one
which must have lasted for ages.
After we pass Oxford, or the Vale of Aylesbury, we enter yet another
world. We come to a bed of sand, under which the freestones and
their adjoining clays dip to the south-east. This is called commonly
the lower Greensand, though it is not green, but rich iron-red. Then
succeeds a band of stiff blue clay, called the Gault, and then
another bed of sand, the upper Greensand, which is more worthy of the
name, for it does carry, in most places, a band of green or
"glauconite" sand. But it and the upper layers of the lower
Greensand also, are worth our attention; for we are all probably
eating them from time to time in the form of bran.
It had been long remarked that certain parts of these beds carried
admirable wheatland; it had been remarked, too, that the finest hop-
lands--those of Farnham, for instance, and Tunbridge--lay upon them:
but that the fertile band was very narrow; that, as in the Surrey
Moors, vast sheets of the lower Greensand were not worth cultivation.
What caused the striking difference?
My beloved friend and teacher, the late Dr. Henslow, when Professor
of Botany at Cambridge, had brought to him by a farmer (so the story
ran) a few fossils. He saw, being somewhat of a geologist and
chemist, that they were not, as fossils usually are, carbonate of
lime, but phosphate of lime--bone-earth. He said at once, as by an
inspiration, "You have found a treasure--not a gold-mine, indeed, but
a food-mine. This is bone-earth, which we are at our wits' end to
get for our grain and pulse; which we are importing, as expensive
bones, all the way from Buenos Ayres. Only find enough of them, and
you will increase immensely the food supply of England, and perhaps
make her independent of foreign phosphates in case of war."
His advice was acted on; for the British farmer is by no means the
stupid personage which townsfolk are too apt to fancy him. This bed
of phosphates was found everywhere in the Greensand, underlying the
Chalk. It may be traced from Dorsetshire through England to
Cambridge, and thence, I believe, into Yorkshire. It may be traced
again, I believe, all round the Weald of Kent and Sussex, from Hythe
to Farnham--where it is peculiarly rich--and so to Eastbourne and
Beachey Head; and it furnishes, in Cambridgeshire, the greater part
of those so-called "coprolites," which are used perpetually now for
manure, being ground up, and then treated with sulphuric acid, till
they become a "soluble super-phosphate of lime."
So much for the useless "hobby," as some fancy it, of poking over old
bones and stones, and learning a little of the composition of this
earth on which God has placed us.
How to explain the presence of this vast mass of animal matter, in
one or two thin bands right across England, I know not. That the
fossils have been rolled on a sea-beach is plain to those who look at
them. But what caused so vast a destruction of animal life along
that beach, must remain one of the buried secrets of the past.
And now we are fast nearing another world, which is far younger than
that coprolite bed, and has been formed under circumstances the most
opposite to it. We are nearing, by whatever rail we approach London,
the escarpment of the chalk downs.
All readers, surely, know the white chalk, the special feature and
the special pride of the south of England. All know its softly-
rounded downs, its vast beech woods, its short and sweet turf, its
snowy cliffs, which have given--so some say--to the whole island the
name of Albion--the white land. But all do not, perhaps, know that
till we get to the chalk no single plant or animal has been found
which is exactly like any plant or animal now known to be living.
The plants and animals grow, on the whole, more and more like our
living forms as we rise in the series of beds. But only above the
chalk (as far as we yet know) do we begin to find species identical
with those living now.
This in itself would prove a vast lapse of time. We shall have a
further proof of that vast lapse when we examine the chalk itself.
It is composed--of this there is now no doubt--almost entirely of the
shells of minute animalcules; and animalcules (I use an unscientific
word for the sake of unscientific readers) like these, and in some
cases identical with them, are now forming a similar deposit of mud,
at vast depths, over the greater part of the Atlantic sea-floor.
This fact has been put out of doubt by recent deep-sea dredgings. A
whole literature has been written on it of late. Any reader who
wishes to know it, need only ask the first geologist he meets; and if
he has the wholesome instinct of wonder in him, fill his imagination
with true wonders, more grand and strange than he is like to find in
any fairy tale. All I have to do with the matter here is, to say
that, arguing from the known to the unknown, from the Atlantic deep-
sea ooze which we do know about, to the chalk which we do not know
about, the whole of the chalk must have been laid down at the bottom
of a deep and still ocean, far out of the reach of winds, tides, and
even currents, as a great part of the Atlantic sea-floor is at this
day.
Prodigious! says the reader. And so it is. Prodigious to think that
that shallow Greensand shore, strewed with dead animals, should sink
to the bottom of an ocean, perhaps a mile, perhaps some four miles
deep. Prodigious the time during which it must have lain as a still
ocean-floor. For so minute are the living atomies which form the
ooze, that an inch, I should say, is as much as we can allow for
their yearly deposit; and the chalk is at least a thousand feet
thick. It may have taken, therefore, twelve thousand years to form
the chalk alone. A rough guess, of course, but one as likely to be
two or three times too little as two or three times too big. Such,
or somewhat such, is the fact. It had long been suspected, and more
than suspected; and the late discoveries of Dr. Carpenter and Mr.
Wyville Thompson have surely placed it beyond doubt.
Thus, surely, if we call the Oolitic beds one new world above the New
Red sandstone, we must call the chalk a second new world in like
wise.
I will not trouble the reader here with the reasons why geologists
connect the chalk with the greensands below it, by regular
gradations, in spite of the enormous downward leap, from sea-shore to
deep ocean, which the beds seem (but only seem) to have taken. The
change--like all changes in geology--was probably gradual. Not by
spasmodic leaps and starts, but slowly and stately, as befits a God
of order, of patience, and of strength, have these great deeds been
done.
But we have not yet done with new worlds or new prodigies on our way
to London, as any Londoner may ascertain for himself, if he will run
out a few miles by rail, and look in any cutting or pit, where the
surface of the chalk, and the beds which lie on it, are exposed.
On the chalk lie--especially in the Blackheath and Woolwich district-
-sands and clays. And what do they tell us?
Of another new world, in which the chalk has been lifted up again, to
form gradually, doubtless, and at different points in succession, the
shore of a sea.
But what proof is there of this?
The surface of the chalk is not flat and smooth, as it must have been
when at the bottom of the sea. It is eaten out into holes and
furrows, plainly by the gnawing of the waves; and on it lie, in many
places, large rolled flints out of chalk which has been destroyed,
beds of shore-shingle, beds of oysters lying as they grew, fresh or
brackish water-shells standing as they lived, bits of lignite (fossil
wood half turned to coal), and (as in Katesgrove pits at Reading)
leaves of trees. Proof enough, one would say, that the chalk had
been raised till part of it at least became dry land, and carried
vegetation.
And yet we have not done. There is another world to tell of yet.
For these beds (known as the Woolwich and Reading beds) dip under
that vast bed of London clay, four hundred and more feet thick, which
(as I said in my last chapter) was certainly laid down by the estuary
of some great tropic river, among palm-trees and Anonas, crocodiles
and turtles.
Is the reader's power of belief exhausted?
If not: there are to be seen, capping almost every high land round
London, the remains of a fifth world. Some of my readers may have
been to Ascot races, or to Aldershot camp, and may recollect the
table-land of the sandy moors, perfectly flat atop, dreary enough to
those to whom they are not (as they have long been to me) a home and
a work-field. Those sands are several hundred feet thick. They lie
on the London clay. And they represent--the reader must take
geologists' word for it--a series of beds in some places thousands of
feet thick, in the Isle of Wight, in the Paris basin, in the volcanic
country of the Auvergne, in Switzerland, in Italy; a period during
which the land must at first have swarmed with forms of tropic life,
and then grown--but very gradually--more temperate, and then colder
and colder still; till at last set in that age of ice, which spread
the boulder pebbles over all rocks and soils indiscriminately, from
the Lake mountains to within a few miles of London.
For everywhere about those Ascot moors, the top of the sands has been
ploughed by shore-ice in winter, as they lay a-wash in the shallow
sea; and over them, in many places, is spread a thin sheet of ice
gravel, more ancient, the best geologists think, than the boulder and
the boulder-clay.
If any of my readers ask how long the period was during which those
sands of Ascot Heath and Aldershot have been laid down, I cannot
tell. But this we can tell. It was long enough to see such changes
in land and sea, that maps representing Europe during the greater
part of that period (as far as we can guess at it) look no more like
Europe than like America or the South Sea Islands. And this we can
tell besides: that that period was long enough for the Swiss Alps to
be lifted up at least 10,000 feet of their present height. And that
was a work which--though God could, if He willed it, have done it in
a single day--we have proof positive was not done in less than ages,
beside which the mortal life of man is as the life of the gnat which
dances in the sun.
And all this, and more--as may be proved from the geology of foreign
countries--happened between the date of the boulder-clay, and that of
the New Red sandstone on which it rests.
IV. THE COAL IN THE FIRE
My dear town-dwelling readers, let me tell you now something of a
geological product well known, happily, to all dwellers in towns, and
of late years, thanks to railroad extension, to most dwellers in
country districts: I mean coal.
Coal, as of course you know, is commonly said to be composed of
vegetable matter, of the leaves and stems of ancient plants and
trees--a startling statement, and one which I do not wish you to take
entirely on trust. I shall therefore spend a few pages in showing
you how this fact--for fact it is--was discovered. It is a very good
example of reasoning from the known to the unknown. You will have a
right to say at first starting, "Coal is utterly different in look
from leaves and stems. The only property which they seem to have in
common is that they can both burn." True. But difference of mere
look may be only owing to a transformation, or series of
transformations. There are plenty in nature quite as great, and
greater. What can be more different in look, for instance, than a
green field of wheat and a basket of loaves at the baker's? And yet
there is, I trust, no doubt whatsoever that the bread has been once
green wheat, and that the green wheat has been transformed into
bread--making due allowance, of course, for the bone-dust, or gypsum,
or alum with which the worthy baker may have found it profitable to
adulterate his bread, in order to improve the digestion of Her
Majesty's subjects.
But you may say, "Yes, but we can see the wheat growing, flowering,
ripening, reaped, ground, kneaded, baked. We see, in the case of
bread, the processes of the transformation going on: but in the case
of coal we do not see the wood and leaves being actually transformed
into coal, or anything like it."
Now 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
which the richest hothouse in England was poor and small. But if the
sinking process which was going on continued a few hundred years, all
that huge mass of wood and leaf would be sunk beneath the swamp, and
covered up in mud washed down from the mountains, and sand driven in
from the sea; to form a bed many feet thick, of what would be first
peat, then lignite, and last, it may be, coal, with the stems of
killed trees standing up out of it into the new mud and sand-beds
above it, just as the Sigillariae and other stems stand up in the
coal-beds both of Britain and of Nova Scotia; while over it a fresh
forest would grow up, to suffer the same fate--if the sinking process
went on--as that which had preceded it.
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.
Thus, I think, we have got first from the known to the unknown; from
a tropic coral island back here to the limestone hills of Great
Britain; and I did not speak at random when I said that I was not
leading you away as far as you fancied by several thousand miles.
Examine any average limestone quarry from Bristol to Berwick, and you
will see there all that I have been describing; that is, all of it
which is not soft animal matter, certain to decay. You will see the
lime-mud hardened into rock beds; you will see the shells embedded in
it; you will see the corals in every stage of destruction; you will
see whole layers made up of innumerable fragments of Crinoids--no
wonder they are innumerable, for, it has been calculated, there are
in a single animal of some of the species 140,000 joints--140,000
bits of lime to fall apart when its soft parts decay. But is it not
all there? And why should it not have got there by the same process
by which similar old coral beds get up the mountain sides in the West
Indies and elsewhere; namely, by the upheaving force of earthquakes?
When you see similar effects, you have a right to presume similar
causes. If you see a man fall off a house here, and break his neck;
and some years after, in London or New York, or anywhere else, find
another man lying at the foot of another house, with his neck broken
in the same way, is it not a very fair presumption that he has fallen
off a house likewise?
You may be wrong. He may have come to his end by a dozen other
means: but you must have proof of that. You will have a full right,
in science and in common sense, to say--That man fell off the house,
till some one proves to you that he did not.
In fact, there is nothing which you see in the limestones of these
isles--save and except the difference in every shell and coral--which
you would not see in the coral-beds of the West Indies, if such
earthquakes as that famous one at St. Thomas's, in 1866, became
common and periodic, upheaving the land (they needs upheave it a very
little, only two hundred and fifty feet), till St. Thomas's, and all
the Virgin Isles, and the mighty mountain of Porto Rico, which looms
up dim and purple to the west, were all joined into dry land once
more, and the lonely coral-shoal of Anegada were raised, as it would
be raised then, into a limestone table-land, like that of Central
Ireland, of Galway, or of County Clare.
But you must clearly understand, that however much these coralline
limestones have been upheaved since they were formed, yet the sea-
bottom, while they were being formed, was sinking and not rising.
This is a fact which was first pointed out by Mr. Darwin, from the
observations which he made in the world-famous Voyage of the Beagle;
and the observations of subsequent great naturalists have all gone to
corroborate his theory.
It was supposed at first, you must understand, that when a coral
island rose steeply to the surface of the sea out of blue water,
perhaps a thousand fathoms or more, that fact was plain proof that
the little coral polypes had begun at the bottom of the sea, and, in
the course of ages, built up the whole island an enormous depth.
But it soon came out that that theory was not correct; for the coral
polypes cannot live and build save in shallow water--say in thirty to
forty fathoms. Indeed, some of the strongest and largest species
work best at the very surface, and in the cut of the fiercest surf.
And so arose a puzzle as to how coral rock is often found of vast
thickness, which Mr. Darwin explained. His theory was, and there is
no doubt now that it is correct, that in these cases the sea-bottom
is sinking; that as it sinks, carrying the coral beds down with it,
the coral dies, and a fresh live crop of polypes builds on the top of
the houses of their dead ancestors: so that, as the depression goes
on, generation after generation builds upwards, the living on the
dead, keeping the upper surface of the reef at the same level, while
its base is sinking downward into the abyss.
Applying this theory to the coral reef of the Pacific Ocean, the
following interesting facts were made out:
That where you find an Island rising out of deep water, with a ring
of coral round it, a little way from the shore--or, as in Eastern
Australia, a coast with a fringing reef (the Flinders reef of
Australia is eleven thousand miles long)--that is a pretty sure sign
that that shore, or mountain, is sinking slowly beneath the sea.
That where you find, as you often do in the Pacific, a mere atoll, or
circular reef of coral, with a shallow pond of smooth water in the
centre, and deep sea round, that is a pretty sure sign that the
mountain-top has sunk completely into the sea, and that the corals
are going on building where its peak once was.
And more. On working out the geography of the South Sea Islands by
the light of this theory of Mr. Darwin's, the following extraordinary
fact has been discovered:
That over a great part of the Pacific Ocean sinking is going on, and
has been going on for ages; and that the greater number of the
beautiful and precious South Sea Islands are only the remnants of a
vast continent or archipelago, which once stretched for thousands of
miles between Australia and South America.
Now, applying the same theory to limestone beds, which are, as you
know, only fossil coral reefs, we have a right to say, when we see in
England, Scotland, Ireland, limestones several thousand feet thick,
that while they were being laid down as coral reef, the sea-bottom,
and probably the neighbouring land, must have been sinking to the
amount of their thickness--to several thousand feet--before that
later sinking which enabled several hundred feet of millstone grit to
be laid down on the top of the limestone.
This millstone grit is a new and a very remarkable element in our
strange story. From Derby to Northumberland it forms vast and lofty
moors, capping, as at Whernside and Penygent, the highest limestone
hills with its hard, rough, barren, and unfossiliferous strata.
Wherever it is found, it lies on the top of the "mountain," or
carboniferous limestone. Almost everywhere, where coal is found in
England, it lies on the millstone grit. I speak roughly, for fear of
confusing my readers with details. The three deposits pass more or
less, in many places, into each other: but always in the order of
mountain limestone below, millstone grit on it, and coal on that
again.
Now what does its presence prove? What but this? That after the
great coral reefs which spread over Somersetshire and South Wales,
around the present estuary of the Severn,--and those, once perhaps
joined to them, which spread from Derby to Berwick, with a western
branch through North-east Wales,--were laid down--after all this, I
say, some change took place in the sea-bottom, and brought down on
the reefs of coral sheets of sand, which killed the corals and buried
them in grit. Does any reader wish for proof of this? Let him
examine the "cherty," or flinty, beds which so often appear where the
bottom of the millstone grit is passing into the top of the mountain
limestone--the beds, to give an instance, which are now quarried on
the top of the Halkin Mountain in Flintshire, for chert, which is
sent to Staffordshire to be ground down for the manufacture of china.
He will find layers in those beds, of several feet in thickness, as
hard as flint, but as porous as sponge. On examining their cavities
he will find them to be simply hollow casts of innumerable joints of
Crinoids, so exquisitely preserved, even to their most delicate
markings, that it is plain they were never washed about upon a beach,
but have grown where, or nearly where, they lie. What then, has
happened to them? They have been killed by the sand. The soft parts
of the animals have decayed, letting the 140,000 joints (more or
less) belonging to each animal fall into a heap, and be imbedded in
the growing sand-rock; and then, it may be long years after, water
filtering through the porous sand has removed the lime of which the
joints were made, and left their perfect casts behind.
So much for the millstone grits. How long the deposition of sand
went on, how long after it that second deposition of sands took
place, which goes by the name of the "gannister," or lower coal-
measures, we cannot tell. But it is clear, at least, that parts of
that ancient sea were filling up and becoming dry land. For coal, or
fossilised vegetable matter, becomes more and more common as we
ascend in the series of beds; till at last, in the upper coal-
measures, the enormous wealth of vegetation which grew, much of it,
where it is now found, prove the existence of some such sheets of
fertile and forest-clad lowland as I described in my last paper.
Thousands of feet of rich coral reef; thousands of feet of barren
sands; then thousands of feet of rich alluvial forest--and all these
sliding into each other, if not in one place, then in another,
without violent break or change; this is the story which the lime in
the mortar and the coal on the fire, between the two, reveal.
VI. THE SLATES ON THE ROOF
The slates on the roof should be, when rightly understood, a pleasant
subject for contemplation to the dweller in a town. I do not ask him
to imitate the boy who, cliff-bred from his youth, used to spend
stolen hours on the house-top, with his back against a chimney-stalk,
transfiguring in his imagination the roof-slopes into mountain-sides,
the slates into sheets of rock, the cats into lions, and the sparrows
into eagles. I only wish that he should--at least after reading this
paper--let the slates on the roof carry him back in fancy to the
mountains whence they came; perhaps to pleasant trips to the lakes
and hills of Cumberland, Westmoreland, and North Wales; and to
recognise--as he will do if he have intellect as well as fancy--how
beautiful and how curious an object is a common slate.
Beautiful, not only for the compactness and delicacy of its texture,
and for the regularity and smoothness of its surface, but still more
for its colour. Whether merely warm grey, as when dry, or bright
purple, as when wet, the colour of the English slate well justifies
Mr. Ruskin's saying, that wherever there is a brick wall and a slate
roof there need be no want of rich colour in an English landscape.
But most beautiful is the hue of slate, when, shining wet in the
sunshine after a summer shower, its blue is brought out in rich
contrast by golden spots of circular lichen, whose spores, I presume,
have travelled with it off its native mountains. Then, indeed, it
reminds the voyager of a sight which it almost rivals in brilliancy--
of the sapphire of the deep ocean, brought out into blazing intensity
by the contrast of the golden patches of floating gulf-weed beneath
the tropic sun.
Beautiful, I say, is the slate; and curious likewise, nay, venerable;
a most ancient and elaborate work of God, which has lasted long
enough, and endured enough likewise, to bring out in it whatsoever
latent capabilities of strength and usefulness might lie hid in it;
which has literally been--as far as such words can apply to a thing
inanimate--
Heated hot with burning fears,
And bathed in baths of hissing tears,
And battered by the strokes of doom
To shape and use.
And yet it was at first naught but an ugly lump of soft and shapeless
ooze.
Therefore, the slates to me are as a parable, on which I will not
enlarge, but will leave each reader to interpret it for himself. I
shall confine myself now to proofs that slate is hardened mud, and to
hints as to how it assumed its present form.
That slate may have been once mud, is made probable by the simple
fact that it can be turned into mud again. If you grind tip slate,
and then analyse it, you will find its mineral constituents to be
exactly those of a fine, rich, and tenacious clay. The slate
districts (at least in Snowdon) carry such a rich clay on them,
wherever it is not masked by the ruins of other rocks. At
Ilfracombe, in North Devon, the passage from slate below to clay
above, may be clearly seen. Wherever the top of the slate beds, and
the soil upon it, is laid bare, the black layers of slate may be seen
gradually melting--if I may use the word--under the influence of rain
and frost, into a rich tenacious clay, which is now not black, like
its parent slate, but red, from the oxidation of the iron which it
contains.
But, granting this, how did the first change take place?
It must be allowed, at starting, that time enough has elapsed, and
events enough have happened, since our supposed mud began first to
become slate, to allow of many and strange transformations. For
these slates are found in the oldest beds of rocks, save one series,
in the known world; and it is notorious that the older and lower the
beds in which the slates are found, the better, that is, the more
perfectly elaborate, is the slate. The best slates of Snowdon--I
must confine myself to the district which I know personally--are
found in the so-called "Cambrian" beds. Below these beds but one
series of beds is as yet known in the world, called the "Laurentian."
They occur, to a thickness of some eighty thousand feet, in Labrador,
Canada, and the Adirondack mountains of New York: but their
representatives in Europe are, as far as is known only to be found in
the north-west highlands of Scotland, and in the island of Lewis,
which consists entirely of them. And it is to be remembered, as a
proof of their inconceivable antiquity, that they have been upheaved
and shifted long before the Cambrian rocks were laid down
"unconformably" on their worn and broken edges.
Above the "Cambrian" slates--whether the lower and older ones of
Penrhyn and Llanberris, which are the same--one slate mountain being
worked at both sides in two opposite valleys--or the upper and newer
slates of Tremadoc, lie other and newer slate-bearing beds of
inferior quality, and belonging to a yet newer world, the "Silurian."
To them belong the Llandeilo flags and slates of Wales, and the
Skiddaw slates of Cumberland, amid beds abounding in extinct fossil
forms. Fossil shells are found, it is true, in the upper Cambrian
beds. In the lower they have all but disappeared. Whether their
traces have been obliterated by heat and pressure, and chemical
action, during long ages; or whether, in these lower beds, we are
actually reaching that "Primordial Zone" conceived of by M. Barrande,
namely, rocks which existed before living things had begun to people
this planet, is a question not yet answered. I believe the former
theory to be the true one. That there was life, in the sea at least,
even before the oldest Cambrian rocks were laid down, is proved by
the discovery of the now famous fossil, the Eozoon, in the Laurentian
limestones, which seems to have grown layer after layer, and to have
formed reefs of limestone as do the living coral-building polypes.
We know no more as yet. But all that we do know points downwards,
downwards still, warning us that we must dig deeper than we have dug
as yet, before we reach the graves of the first living things.
Let this suffice at present for the Cambrian and Laurentian rocks.
The Silurian rocks, lower and upper, which in these islands have
their chief development in Wales, and which are nearly thirty-eight
thousand feet thick; and the Devonian or Old Red sandstone beds,
which in the Fans of Brecon and Carmarthenshire attain a thickness of
ten thousand feet, must be passed through in an upward direction
before we reach the bottom of that Carboniferous Limestone of which I
spoke in my last paper. We thus find on the Cambrian rocks forty-
five thousand feet at least of newer rocks, in several cases lying
unconformably on each other, showing thereby that the lower beds had
been upheaved, and their edges worn off on a sea-shore, ere the upper
were laid down on them; and throughout this vast thickness of rocks,
the remains of hundreds of forms of animals, corals, shells, fish,
older forms dying out in the newer rocks, and new ones taking their
places in a steady succession of ever-varying forms, till those in
the upper beds have become unlike those in the lower, and all are
from the beginning more or less unlike any existing now on earth.
Whole families, indeed, disappear entirely, like the Trilobites,
which seem to have swarmed in the Silurian seas, holding the same
place there as crabs and shrimps do in our modern seas. They vanish
after the period of the coal, and their place is taken by an allied
family of Crustaceans, of which only one form (as far as I am aware)
lingers now on earth, namely, the "King Crab," or Limulus, of the
Indian Seas, a well-known animal, of which specimens may sometimes be
seen alive in English aquaria. So perished in the lapse of those
same ages, the armour-plated or "Ganoid" fish which Hugh Miller made
so justly famous--and which made him so justly famous in return--
appearing first in the upper Silurian beds, and abounding in vast
variety of strange forms in the old Red Sandstone, but gradually
disappearing from the waters of the world, till their only
representatives, as far as known, are the Lepidostei, or "Bony
Pikes," of North America; the Polypteri of the Nile and Senegal; the
Lepidosirens of the African lakes and Western rivers; the Ceratodus
or Barramundi of Queensland (the two latter of which approach
Amphibians), and one or two more fantastic forms, either rudimentary
or degraded, which have lasted on here and there in isolated stations
through long ages, comparatively unchanged while all the world is
changed around them, and their own kindred, buried like the fossil
Ceratodus of the Trias beneath thousands of feet of ancient rock,
among creatures the likes whereof are not to be found now on earth.
And these are but two examples out of hundreds of the vast changes
which have taken place in the animal life of the globe, between the
laying down of the Cambrian slates and the present time.
Surely--and it is to this conclusion I have been tending throughout a
seemingly wandering paragraph--surely there has been time enough
during all those ages for clay to change into slate.
And how were they changed?
I think I cannot teach my readers this more simply than by asking
them first to buy Sheet No. LXXVIII. S.E. (Bangor) of the Snowdon
district of the Government Geological Survey, which may be ordered at
any good stationer's, price 3s.; and study it with me. He will see
down the right-hand margin interpretations of the different colours
which mark the different beds, beginning with the youngest (alluvium)
atop, and going down through Carboniferous Limestone and Sandstone,
Upper Silurian, Lower Silurian, Cambrian, and below them certain
rocks marked of different shades of red, which signify rocks either
altered by heat, or poured out of old volcanic vents. He will next
see that the map is covered with a labyrinth of red patches and
curved lines, signifying the outcrop or appearance at the surface of
these volcanic beds. They lie at every conceivable slope; and the
hills and valleys have been scooped out by rain and ice into every
conceivable slope likewise. Wherefore we see, here a broad patch of
red, where the back of a sheet of Lava, Porphyry, Greenstone, or what
not is exposed; there a narrow line curving often with the curve of
the hill-side, where only the edge of a similar sheet is exposed; and
every possible variety of shape and attitude between these two. He
will see also large spaces covered with little coloured dots, which
signify (as he will find at the margin) beds of volcanic ash. If he
look below the little coloured squares on the margin, he will see
figures marking the strike, or direction of the inclination of the
beds--inclined, vertical, horizontal, contorted; that the white lines
in the map signify faults, i.e. shifts in the strata; the gold lines,
lodes of metal--the latter of which I should advise him strongly, in
this district at least, not to meddle with: but to button up his
pockets, and to put into the fire, in wholesome fear of his own
weakness and ignorance, any puffs of mining companies which may be
sent him--as one or two have probably been sent him already.
Furnished with which keys to the map, let him begin to con it over,
sure that there is if not an order, still a grand meaning in all its
seeming confusion; and let him, if he be a courteous and grateful
person, return due thanks to Professor Ramsay for having found it all
out; not without wondering, as I have often wondered, how even
Professor Ramsay's acuteness and industry could find it all out.
When my reader has studied awhile the confusion--for it is a true
confusion--of the different beds, he will ask, or at least have a
right to ask, what known process of nature can have produced it? How
have these various volcanic rocks, which he sees marked as Felspathic
Traps, Quartz Porphyries, Greenstones, and so forth, got intermingled
with beds which he is told to believe are volcanic ashes, and those
again with fossil-bearing Silurian beds and Cambrian slates, which he
is told to believe were deposited under water? And his puzzle will
not be lessened when he is told that, in some cases, as in that of
the summit of Snowdon, these very volcanic ashes contain fossil
shells.
The best answer I can give is to ask him to use his imagination, or
his common sense; and to picture to himself what must go on in the
case of a submarine eruption, such as broke out off the coast of
Iceland in 1783 and 1830, off the Azores in 1811, and in our day in
more than one spot in the Pacific Ocean.
A main bore or vent--or more than one--opens itself between the
bottom of the sea and the nether fires. From each rushes an enormous
jet of high-pressure steam and other gases, which boils up through
the sea, and forms a cloud above; that cloud descends again in heavy
rain, and gives out often true lightning from its under side.
But it does more. It acts as a true steam-gun, hurling into the air
fragments of cold rock rasped off from the sides of the bore, and
fragments also of melted lava, and clouds of dust, which fall again
into the sea, and form there beds either of fine mud or of breccia--
that is, fragments of stone embedded in paste. This, the reader will
understand, is no fancy sketch, as far as I am concerned. I have
steamed into craters sawn through by the sea, and showing sections of
beds of ash dipping outwards and under the sea, and in them boulders
and pebbles of every size, which had been hurled out of the crater;
and in them also veins of hardened lava, which had burrowed out
through the soft ashes of the cone. Of those lava veins I will speak
presently. What I want the reader to think of now is the immense
quantity of ash which the steam-mitrailleuse hurls to so vast a
height into the air, that it is often drifted many miles down to
leeward. To give two instances: The jet of steam from Vesuvius, in
the eruption of 1822, rose more than four miles into the air; the jet
from the Souffriere of St. Vincent in the West Indies, in 1812,
probably rose higher; certainly it met the N.E. trade-wind, for it
poured down a layer of ashes, several inches thick, not only on St.
Vincent itself, but on Barbadoes, eighty miles to windward, and
therefore on all the sea between. Now let us consider what that
represents--a layer of fine mud, laid down at the bottom of the
ocean, several inches thick, eighty miles at least long, and twenty
miles perhaps broad, by a single eruption. Suppose that hardened in
long ages (as it would be under pressure) into a bed of fine grained
Felstone, or volcanic ash; and we can understand how the ash-beds of
Snowdonia--which may be traced some of them for many square miles--
were laid down at the bottom of an ancient sea.
But now about the lavas or true volcanic rocks, which are painted (as
is usual in geological maps) red. Let us go down to the bottom of
the sea, and build up our volcano towards the surface.
First, as I said, the subterranean steam would blast a bore. The
dust and stones, rasped and blasted out of that hole would be spread
about the sea-bottom as an ash-bed sloping away round the hole; then
the molten lava would rise in the bore, and flow out over the ashes
and the sea-bottom--perhaps in one direction, perhaps all round.
Then, usually, the volcano, having vented itself, would be quieter
for a time, till the heat accumulated below, and more ash was blasted
out, making a second ash-bed; and then would follow a second lava
flow. Thus are produced the alternate beds of lava and ash which are
so common.
Now suppose that at this point the volcano was exhausted, and lay
quiet for a few hundred years, or more. If there was any land near,
from which mud and sand were washed down, we might have layers on
layers of sediment deposited, with live shells, etc., living in them,
which would be converted into fossils when they died; and so we
should have fossiliferous beds over the ashes and lavas. Indeed,
shells might live and thrive in the ash-mud itself, when it cooled,
and the sea grew quiet, as they have lived and thriven in Snowdonia.
Now suppose that after these sedimentary beds are laid down by water,
the volcano breaks out again--what would happen?
Many things: specially this, which has often happened already.
The lava, kept down by the weight of these new rocks, searches for
the point of least resistance, and finds it in a more horizontal
direction. It burrows out through the softer ash-beds, and between
the sedimentary beds, spreading itself along horizontally. This
process accounts for the very puzzling, though very common case in
Snowdon and elsewhere, in which we find lavas interstratified with
rocks which are plainly older than those lavas. Perhaps when that is
done the volcano has got rid of all its lava, and is quiet. But if
not, sooner or later, it bores up through the new sedimentary rocks,
faulting them by earthquake shocks till it gets free vent, and begins
its layers of alternate ash and lava once more.
And consider this fact also: If near the first (as often happens)
there is another volcano, the lava from one may run over the lava
from the other, and we may have two lavas of different materials
overlying each other, which have come from different directions. The
ashes blown out of the two craters may mingle also, and so, in the
course of ages, the result may be such a confusion of ashes, lavas,
and sedimentary rocks as we find throughout most mountain ranges in
Snowdon, in the Lake mountains, in the Auvergne in France, in Sicily
round Etna, in Italy round Vesuvius, and in so many West Indian
Islands; the last confusion of which is very likely to be this:
That when the volcano has succeeded--as it did in the case of Sabrina
Island off the Azores in 1811, and as it did, perhaps often, in
Snowdonia--in piling up an ash cone some hundred feet out of the sea;
that--as has happened to Sabrina Island--the cone is sunk again by
earthquakes, and gnawn down at the same time by the sea-waves, till
nothing is left but a shoal under water. But where have all its vast
heaps of ashes gone? To be spread about over the bottom of the sea,
to mingle with the mud already there, and so make beds of which, like
many in Snowdon, we cannot say whether they are of volcanic or of
marine origin, because they are of both.
But what has all this to do with the slates?
I shall not be surprised if my readers ask that question two or three
times during this paper. But they must be kind enough to let me tell
my story my own way. The slates were not made in a day, and I fear
they cannot be explained in an hour: unless we begin carefully at
the beginning in order to end at the end. Let me first make my
readers clearly understand that all our slate-bearing mountains, and
most also of the non-slate-bearing ones likewise, are formed after
the fashion which I have described, namely, beneath the sea. I do
not say that there may not have been, again, and again, ash-cones
rising above the surface of the waves. But if so, they were washed
away, again and again, ages before the land assumed anything of its
present shape; ages before the beds were twisted and upheaved as they
are now.
And therefore I beg my readers to put out of their minds once and for
all the fancy that in any known part of these islands craters are to
be still seen, such as exist in Etna, or Vesuvius, or other volcanoes
now at work in the open air.
It is necessary to insist on this, because many people hearing that
certain mountains are volcanic, conclude--and very naturally and
harmlessly--that the circular lakes about their tops are true
craters. I have been told, for instance, that that wonderful little
blue Glas Llyn, under the highest cliff of Snowdon, is the old crater
of the mountain; and I have heard people insist that a similar lake,
of almost equal grandeur, in the south side of Cader Idris, is a
crater likewise.
But the fact is not so. Any one acquainted with recent craters would
see at once that Glas Llyn is not an ancient one; and I am not
surprised to find the Government geologists declaring that the Llyn
on Cader Idris is not one either. The fact is, that the crater, or
rather the place where the crater has been, in ancient volcanoes of
this kind, is probably now covered by one of the innumerable bosses
of lava.
For, as an eruption ceases, the melted lava cools in the vents, and
hardens; usually into lava infinitely harder than the ash-cone round
it; and this, when the ash-cone is washed off, remains as the highest
part of the hill, as in the Mont Dore and the Cantal in France, and
in several extinct volcanoes in the Antilles. Of course the lava
must have been poured out, and the ashes blown out from some vents or
other, connected with the nether world of fire; probably from many
successive vents. For in volcanoes, when one vent is choked, another
is wont to open at some fresh point of least resistance among the
overlying rocks. But where are these vents? Buried deep under
successive eruptions, shifted probably from their places by
successive upheavings and dislocations; and if we wanted to find them
we should have to quarry the mountain range all over, a mile deep,
before we hit upon here and there a tap-root of ancient lava,
connecting the upper and the nether worlds. There are such tap-
roots, probably, under each of our British mountain ranges. But
Snowdon, certainly, does not owe its shape to the fact of one of
these old fire vents being under it. It owes its shape simply to the
accident of some of the beds toward the summit being especially hard,
and thus able to stand the wear and tear of sea-wave, ice, and rain.
Its lakes have been formed quite regardless of the lie of the rocks,
though not regardless of their relative hardness. But what forces
scooped them out--whether they were originally holes left in the
ground by earthquakes, and deepened since by rain and rivers, or
whether they were scooped out by ice, or by any other means, is a
question on which the best geologists are yet undecided--decided only
on this--that craters they are not.
As for the enormous changes which have taken place in the outline of
the whole of the mountains, since first their strata were laid down
at the bottom of the sea: I shall give facts enough, before this
paper is done, to enable readers to judge of them for themselves.
The reader will now ask, naturally enough, how such a heap of beds as
I have described can take the shape of mountains like Snowdon.
Look at any sea cliff in which the strata are twisted and set on
slope. There are hundreds of such in these isles. The beds must
have been at one time straight and horizontal. But it is equally
clear that they have been folded by being squeezed laterally. At
least, that is the simplest explanation, as may be proved by
experiment. Take a number of pieces of cloth, or any such stuff; lay
them on each other and then squeeze them together at each end. They
will arrange themselves in folds, just as the beds of the cliff have
done. And if, instead of cloth, you take some more brittle matter,
you will find that, as you squeeze on, these folds will tend to snap
at the points of greatest tension or stretching, which will be of
course at the anticlinal and synclinal lines--in plain English, the
tops and bottoms of the folds. Thus cracks will be formed; and if
the pressure goes on, the ends of the layers will shift against each
other in the line of those cracks, forming faults like those so
common in rocks.
But again, suppose that instead of squeezing these broken and folded
lines together any more, you took off the pressure right and left,
and pressed them upwards from below, by a mimic earthquake. They
would rise; and as they rose leave open space between them. Now if
you could contrive to squeeze into them from below a paste, which
would harden in the cracks and between the layers, and so keep them
permanently apart, you would make them into a fair likeness of an
average mountain range--a mess--if I may make use of a plain old
word--of rocks which have, by alternate contraction and expansion,
helped in the latter case by the injection of molten lava, been
thrust about as they are in most mountain ranges.
That such a contraction and expansion goes on in the crust of the
earth is evident; for here are the palpable effects of it. And the
simplest general cause which I can give for it is this: That things
expand as they are heated, and contract as they are cooled.
Now I am not learned enough--and were I, I have not time--to enter
into the various theories which philosophers have put forward, to
account for these grand phenomena.
The most remarkable, perhaps, and the most probable, is the theory of
M. Elie de Beaumont, which is, in a few words, this:
That this earth, like all the planets, must have been once in a state
of intense heat throughout, as its mass inside is probably now.
That it must be cooling, and giving off its heat into space.
That, therefore, as it cools, its crust must contract.
That, therefore, in contracting, wrinkles (for the loftiest mountain
chains are nothing but tiny wrinkles, compared with the whole mass of
the earth), wrinkles, I say, must form on its surface from time to
time. And that the mountain chains are these wrinkles.
Be that as it may, we may safely say this. That wherever the
internal heat of the earth tends (as in the case of volcanoes)
towards a particular spot, that spot must expand, and swell up,
bulging the rocks out, and probably cracking them, and inserting
melting lava into those cracks from below. On the other hand, if the
internal heat leaves that spot again, and it cools, then it must
contract more or less, in falling inward toward the centre of the
earth; and so the beds must be crumpled, and crushed, and shifted
against each other still more, as those of our mountains have been.
But here may arise, in some of my readers' minds, a reasonable
question--If these upheaved beds were once horizontal, should we not
be likely to find them, in some places, horizontal still?
A reasonable question, and one which admits of a full answer.
They know, of course, that there has been a gradual, but steady,
change in the animals of this planet; and that the relative age of
beds can, on the strength of that known change, be determined
generally by the fossils, usually shells, peculiar to them: so that
if we find the same fashion of shells, and still more the same
species of shells, in two beds in different quarters of the world,
then we have a right to say--These beds were laid down at least about
the same time. That is a general rule among all geologists, and not
to be gainsaid.
Now I think I may say, that, granting that we can recognise a bed by
its fossils, there are few or no beds which are found in one place
upheaved, broken, and altered by heat, which are not found in some
other place still horizontal, unbroken, unaltered, and more or less
as they were at first.
From the most recent beds; from the upheaved coral-rocks of the West
Indies, and the upheaved and faulted boulder clay and chalk of the
Isle of Moen in Denmark--downwards through all the strata, down to
that very ancient one in which the best slates are found, this rule,
I believe, stands true.
It stands true, certainly, of the ancient Silurian rocks of Wales,
Cumberland, Ireland, and Scotland.
For, throughout great tracts of Russia, and in parts of Norway and
Sweden, Sir Roderick Murchison discovered our own Silurian beds,
recognisable from their peculiar fossils. But in what state? Not
contracted, upheaved, and hardened to slates and grits, as they are
in Wales and elsewhere: but horizontal, unbroken, and still soft,
because undisturbed by volcanic rooks and earthquakes. At the bottom
of them all, near Petersburg, Sir Roderick found a shale of dried mud
(to quote his own words), "so soft and incoherent that it is even
used by sculptors for modelling, although it underlies the great mass
of fossil-bearing Silurian rocks, and is, therefore, of the same age
as the lower crystalline hard slates of North Wales. So entirely
have most of these eldest rocks in Russia been exempted from the
influence of change, throughout those enormous periods which have
passed away since their accumulation."
Among the many discoveries which science owes to that illustrious
veteran, I know none more valuable for its bearing on the whole
question of the making of the earth-crust, than this one magnificent
fact.
But what a contrast between these Scandinavian and Russian rocks and
those of Britain! Never exceeding, in Scandinavia, a thousand feet
in thickness, and lying usually horizontal, as they were first laid
down, they are swelled in Britain to a thickness of thirty thousand
feet, by intruded lavas and ashes; snapt, turned, set on end at every
conceivable angle; shifted against each other to such an extent,
that, to give a single instance, in the Vale of Gwynnant, under
Snowdon, an immense wedge of porphyry has been thrust up, in what is
now the bottom of the valley, between rocks far newer than it, on one
side to a height of eight hundred, on the other to a height of
eighteen hundred feet--half the present height of Snowdon. Nay, the
very slate beds of Snowdonia have not forced their way up from under
the mountain--without long and fearful struggles. They are set in
places upright on end, then horizontal again, then sunk in an
opposite direction, then curled like sea-waves, then set nearly
upright once more, and faulted through and through, six times, I
believe, in the distance of a mile or two; they carry here and there
on their backs patches of newer beds, the rest of which has long
vanished; and in their rise they have hurled back to the eastward,
and set upright, what is now the whole western flank of Snowdon, a
mass of rock which was then several times as thick as it is now.
The force which thus tortured them was probably exerted by the great
mass of volcanic Quartz-porphyry, which rises from under them to the
north-west, crossing the end of the lower lake of the Llanberris; and
indeed the shifts and convulsions which have taken place between them
and the Menai Straits are so vast that they can only be estimated by
looking at them on the section which may be found at the end of
Professor Ramsay's "Geological Survey of North Wales." But anyone
who will study that section, and use (as with the map) a little
imagination and common sense, will see that between the heat of that
Porphyry, which must have been poured out as a fluid mass as hot,
probably, as melted iron, and the pressure of it below, and of the
Silurian beds above, the Cambrian mud-strata of Llanberris and
Penrhyn quarries must have suffered enough to change them into
something very different from mud, and, therefore, probably, into
what they are now--namely, slate.
And now, at last, we have got to the slates on the roof, and may
disport ourselves over them--like the cats.
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