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.