Town Geology
by
Charles Kingsley
Part 3 out of 3
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.
Back to Full Books
|