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.
Look at any piece of slate. All know that slate splits or cleaves
freely, in one direction only, into flat layers. Now any one would
suppose at first sight, and fairly enough, that the flat surface--the
"plane of cleavage"--was also the plane of bedding. In simpler
English we should say--The mud which has hardened into the slate was
laid down horizontally; and therefore each slate is one of the little
horizontal beds of it, perhaps just what was laid down in a single
tide. We should have a right to do so, because that would be true of
most sedimentary rocks. But it would not be true of slate. The
plane of bedding in slate has nothing to do with the plane of
cleavage. Or, more plainly, the mud of which the slate is made may
have been deposited at the sea-bottom at any angle to the plane of
cleavage. We may sometimes see the lines of the true bedding--the
lines which were actually horizontal when the mud was laid down--in
bits of slate, and find them sometimes perpendicular to, sometimes
inclined to, and sometimes again coinciding with the plane of
cleavage, which they have evidently acquired long after.
Nay, more. These parallel planes of cleavage, at each of which the
slate splits freely, will run through a whole mountain at the same
angle, though the beds through which they run may be tilted at
different angles, and twisted into curves.
Now what has made this change in the rook? We do not exactly know.
One thing is clear, that the particles of the now solid rock have
actually moved on themselves. And this is proved by a very curious
fact--which the reader, if he geologises about slate quarries much,
may see with his own eyes. The fossils in the slate are often
distorted into quaint shapes, pulled out long if they lie along the
plane of cleavage, or squeezed together, or doubled down on both
sides, if they lie across the plane. So that some force has been at
work which could actually change the shape of hard shells, very
slowly, no doubt, else it would have snapped and crumbled them.
If I am asked what that force was, I do not know. I should advise
young geologists to read what Sir Henry de la Beche has said on it in
his admirable "Geological Observer," pp. 706-725. He will find
there, too, some remarks on that equally mysterious phenomena of
jointing, which you may see in almost all the older rocks; it is
common in limestones. All we can say is, that some force has gone
on, or may be even now going on, in the more ancient rocks, which is
similar to that which produces single crystals; and similar, too, to
that which produced the jointed crystals of basalt, i.e. lava, at the
Giant's Causeway, in Ireland, and Staffa, in the Hebrides. Two
philosophers--Mr. Robert Were Fox and Mr. Robert Hunt--are of opinion
that the force which has determined the cleavage of slates may be
that of the electric currents, which (as is well known) run through
the crust of the earth. Mr. Sharpe, I believe, attributes the
cleavage to the mere mechanical pressure of enormous weights of rock,
especially where crushed by earthquakes. Professor Rogers, again,
points out that as these slates may have been highly heated, thermal
electricity (i.e. electricity brought out by heat) may have acted on
them.
One thing at least is clear. That the best slates are found among
ancient lavas, and also in rocks which are faulted and tilted
enormously, all which could not have happened without a
proportionately enormous pressure, and therefore heat; and next, that
the best slates are invariably found in the oldest beds--that is, in
the beds which have had most time to endure the changes, whether
mechanical or chemical, which have made the earth's surface what we
see it now.
Another startling fact the section of Snowdonia, and I believe of
most mountain chains in these islands, would prove--namely, that the
contour of the earth's surface, as we see it now, depends very
little, certainly in mountains composed of these elder rocks upon the
lie of the strata, or beds, but has been carved out by great forces,
long after those beds were not only laid down and hardened, but
faulted and tilted on end. Snowdon itself is so remarkable an
instance of this fact that, as it is a mountain which every one in
these happy days of excursion-trains and steamers either has seen or
can see, I must say a few more words about it.
Any one who saw that noble peak leaping high into the air, dominating
all the country round, at least upon three sides, and was told that
its summit consisted of beds much newer, not much older, than the
slate-beds fifteen hundred feet down on its north-western flank--any
one, I say, would have the right at first sight, on hearing of
earthquake faults and upheavals, to say--The peak of Snowdon has been
upheaved to its present height above and out of the lower lands
around. But when he came to examine sections, he would find his
reasonable guess utterly wrong. Snowdon is no swelling up of the
earth's crust. The beds do not, as they would in that case, slope up
to it. They slope up from it, to the north-west in one direction,
and the south-south-west in the other; and Snowdon is a mere
insignificant boss, left hanging on one slope of what was once an
enormous trough, or valley, of strata far older than itself. By
restoring these strata, in the direction of the angles, in which they
crop out, and vanish at the surface, it is found that to the north-
west--the direction of the Menai Straits--they must once have risen
to a height of at least six or seven thousand feet; and more, by
restoring them, specially the ash-bed of Snowdon, towards the south-
east--which can be done by the guidance of certain patches of it left
on other hills--it is found that south of Ffestiniog, where the
Cambrian rocks rise again to the surface, the south side of the
trough must have sloped upwards to a height of from fifteen to twenty
thousand feet, whether at the bottom of the sea, or in the upper air,
we cannot tell. But the fact is certain, that off the surface of
Wales, south of Ffestiniog a mass of solid rock as high as the Andes
has been worn down and carried bodily away; and that a few miles
south again, the peak of Arran Mowddy, which is now not two thousand
feet high, was once--either under the sea or above it--nearer ten
thousand feet.
If I am asked whither is all that enormous mass of rock--millions of
tons--gone? Where is it now? I know not. But if I dared to hazard
a guess, I should say it went to make the New Red sandstones of
England.
The New Red sandstones must have come from somewhere. The most
likely region for them to have come from is from North Wales, where,
as we know, vast masses of gritty rock have been ground off, such as
would make fine sandstones if they had the chance. So that many a
grain of sand in Chester walls was probably once blasted out of the
bowels of the earth into the old Silurian sea, and after a few
hundreds of thousands of years' repose in a Snowdonian ash-bed, was
sent eastward to build the good old city and many a good town more.
And the red marl--the great deposit of red marl which covers a wide
region of England--why should not it have come from the same quarter?
Why should it not be simply the remains of the Snowdon Slate? Mud
the slate was, and into mud it has returned. Why not? Some of the
richest red marl land I know, is, as I have said, actually being made
now, out of the black slates of Ilfracombe, wherever they are
weathered by rain and air. The chemical composition is the same.
The difference in colour between black slate and red marl is caused
simply by the oxidation of the iron in the slate.
And if my readers want a probable cause why the sandstones lie
undermost, and the red marl uppermost--can they not find one for
themselves? I do not say that it is the cause, but it is at least a
causa vera, one which would fully explain the fact, though it may be
explicable in other ways. Think, then, or shall I think for my
readers?
Then do they not see that when the Welsh mountains were ground down,
the Silurian strata, being uppermost, would be ground down first, and
would go to make the lower strata of the great New Red Sandstone
Lowland; and that being sandy, they would make the sandstones? But
wherever they were ground through, the Lower Cambrian slates would be
laid bare; and their remains, being washed away by the sea the last,
would be washed on to the top of the remains of the Silurians; and so
(as in most cases) the remains of the older rock, when redeposited by
water, would lie on the remains of the younger rock. And do they not
see that (if what I just said is true) these slates would grind up
into red marl, such as is seen over the west and south of Cheshire
and Staffordshire and far away into Nottinghamshire? The red marl
must almost certainly have been black slate somewhere, somewhen. Why
should it not have been such in Snowdon? And why should not the
slates in the roof be the remnants of the very beds which are now the
marl in the fields?
And thus I end my story of the slates in the roof, and these papers
on Town Geology. I do so, well knowing how imperfect they are:
though not, I believe, inaccurate. They are, after all, merely
suggestive of the great amount that there is to be learnt about the
face of the earth and how it got made, even by the townsman, who can
escape into the country and exchange the world of man for the world
of God, only, perhaps, on Sundays--if, alas! even then--or only once
a year by a trip in a steamer or an excursion train. Little, indeed,
can he learn of the planet on which he lives. Little in that
direction is given to him, and of him little shall be required. But
to him, for that very reason, all that can be given should be given;
he should have every facility for learning what he can about this
earth, its composition, its capabilities; lest his intellect, crushed
and fettered by that artificial drudgery which we for a time miscall
civilisation, should begin to fancy, as too many do already, that the
world is composed mainly of bricks and deal, and governed by acts of
parliament. If I shall have awakened any townsmen here and there to
think seriously of the complexity, the antiquity, the grandeur, the
true poetry, of the commonest objects around them, even the stones
beneath their feet; if I shall have suggested to them the solemn
thought that all these things, and they themselves still more, are
ordered by laws, utterly independent of man's will about them, man's
belief in them; if I shall at all have helped to open their eyes that
they may see, and their ears that they may hear, the great book which
is free to all alike, to peasant as to peer, to men of business as to
men of science, even that great book of nature, which is, as Lord
Bacon said of old, the Word of God revealed in facts--then I shall
have a fresh reason for loving that science of geology, which has
been my favourite study since I was a boy.
Footnotes:
{1} See "Nature," No. XXV. (Macmillan & Co.)
{2} These Lectures were delivered to the members of the Natural
Science Class at Chester in 1871.
{3} See a most charming paper on "The Physics of Arctic Ice," by Dr.
Robert Brown of Campster, published in the Quarterly Journal of the
Geological Society, June, 1870. This article is so remarkable, not
only for its sound scientific matter, but for the vividness and
poetic beauty of its descriptions, that I must express a hope that
the learned author will some day enlarge it, and publish it in a
separate form.
{4} See Lyell, "Antiquity of Man," p. 294 et seq.