Sand and gravel of the boulder--What they suggested--Their
consideration leads us among the more mechanical operations
of Nature--An endless succession of mutations in the economy
of the universe--Exhibited in plants--In animals--In
the action of winds and oceanic currents--Beautifully
shown by the ceaseless passage of water from land to
sea, and sea to land--This interchange not an isolated
phenomenon--How aided in its effects by a universal process
of decay going on wherever a land surface is exposed to
the air--Complex mode of Nature's operations--Interlacing
of different causes in the production of an apparently
single and simple effect--Decay of rocks--Chemical
changes--Underground and surface decomposition--Carbonated
springs--The Spar Cave--Action of rain-water--Decay of
granite--Scene in Skye--Trap-dykes--Weathered cliffs of
sandstone--Of conglomerate--Of shale--Of limestone--Caverns
of Raasay--Incident--Causes of this waste of calcareous
rocks--Tombstones.
From the blackened plants that darkened the upper layers of the
boulder, the transition was natural to the matrix in which they lay.
The whole rock consisted of a fine quartzy sand more or less distinctly
laminated, and showing in its lower parts well-rounded pebbles of
quartz, green grit, and felspathic trap. The contemplation of these
features suggested the existence of some old land with elevated ranges
of hills, and wide verdant valleys traversed by rivulets and rivers
which bore a ceaseless burden of mud, sand, and gravel, onwards to
the sea. The pebbles afforded some indication of the kind of rocks
that formed the hill-sides. Perhaps the higher grounds exhibited that
grey wrinkled appearance peculiar to the quartz districts of the
north-western Highlands, with here and there a bluff crag of felspathic
trap shooting up from among the fern-brakes of the valley, or cutting
across the channel of some mountain stream that tumbled over the pale
rock in a sheet of foam. And there may have been among these uplands
smooth undulating districts, dotted over with dark araucarian pines,
and densely clothed with a brushwood of rolling fern, but which showed
in all their ravines the green grit that formed the framework of the
country,--its beds twisted and contorted, jointed and cleaved, like
the grits and slates along the banks of many a stream, beloved by
the angler, in the classic ground of the Ettrick and the Yarrow. But
whatever may have been the special features of its scenery, there can
be no doubt of the land's existence. The carbonized plants stand up to
tell us of its strange and luxuriant vegetation. We have listened to
their story, and suffered them to lead us away into forest, and lake,
and sea, to look on the various forms of life, vegetable and animal,
which abounded in that far-distant age. We return again to the boulder,
and shall now seek to learn the lessons which the sand and pebbles have
to teach us. Their subject belongs to what is called physical geology,
and will bring before us some of the more mechanical operations of
nature, such as the slow but constant action of air, rain, and rivers,
upon hard rock, the grinding action of the waves, and the consequent
accumulation of new masses of sedimentary rock.
In all the departments of nature that come under the cognizance of man,
there is seen to be an endless succession of mutations. According to
the Samian philosopher--
"Turn wheresoe'er we may by land or sea,
There's nought around us that doth cease to be.
Each object varies but in form and hue,
Its parts exchange; hence combinations new.
And thus is Nature through her mighty frame
For ever varying, and yet still the same."
In the world of life we see how animals are sustained by a constant
series of chemical changes in their blood, every respiration of air
adding, as it were, fresh fuel to the flame of life within. In plants,
too, there is an analogous process. The atmospheric air is by them
decomposed, part of it being given off again, and part retained to
build up the organic structure. Plants withdraw mineral matter from the
soil, animals feed upon plants, and thus the earthy substances, after
having formed a part, first of rock masses, then of vegetable, and
subsequently of animal organizations, are returned again to the soil,
whence to be once more withdrawn and undergo new cycles of mutation.
But this perpetual interchange is not confined to the vital world. We
see it in the action of winds, when heated air rises and moves in one
direction, and the colder parts sink and travel the opposite way. The
same principle is exhibited by the oceanic currents, the removal of a
body of water, from whatever cause, always necessitating the ingress of
a corresponding quantity to supply its place. But perhaps one of the
most beautiful instances of these interchanges in the whole inorganic
world is the ceaseless passage of water from the land to the sea, and
from the sea to the land. The countless thousands of rivulets, and
streams, and gigantic rivers, that are ever pouring their waters into
the great deep, do not in the least raise its level or diminish its
saltness. And why? Simply because the sea gives off by evaporation
as much water as it receives from rain and rivers. The vapour thus
exhaled ascends to the upper regions of the atmosphere, where it forms
clouds, and whence it eventually descends as rain. The larger part of
the rain probably falls upon the ocean, but a considerable amount is
nevertheless driven by winds across the land. This finds its way into
the streams, and so back again to the sea, only, however, to be anew
evaporated and sent as drizzling rain across the face of land and sea.
This interchange is constantly in progress, and seems to have been as
unvarying during past ages.
But the ceaseless passage of water between land and sea is not a mere
isolated and independent phenomenon. Like all the rest of Nature's
processes, even the simplest, it produces important and complicated
effects. And the reader may, perhaps, think it worth looking at for a
little, when he reflects that to this seemingly feeble cause we owe no
small part of our solid lands, whether as islands wasted by the sea,
or as part of vast and variegated continents, wide rolling prairies
covered with verdure and roamed over by herds of cattle, or wintry
Alpine hills lifeless and bare.
The truth of this will appear when we reflect that the moisture which
rises from the sea and falls on the land as rain, is free from any
admixture of impurities; but by the time it again reaches the sea,
after a circuit of perhaps many miles down valley and plain, it has
grown turbid and discoloured, carrying with it a quantity of mud, sand,
and drift-wood. The sediment thus transported soon sinks to the bottom,
where it eventually hardens into rock, and in course of time is raised
above the waves as part of a new land. Such I conceive to have been the
origin of the sand, gravel, and imbedded plants of our boulder. It may
be well, however, in going into the details of the subject, to take a
wider view of this interesting branch of geology, and look for a little
at the forms and modes of the decomposition of rocks, and the varied
manner in which new sedimentary accumulations are formed.
All over the world, wherever a land surface spreads out beneath the
sky, there goes on a process of degradation and decay. Hills are
insensibly crumbling into the valleys, valleys are silently eroded,
and crags that ever since the birth of man have been the landmarks of
the race, are yet slowly but surely melting away. It matters not where
the hill or plain may lie, the highest mountains of the tropics and
the frozen soil of the poles, yield each in its measure and degree
to the influence of the general law. It might seem that so universal
a process should be the result of some equally prevalent and simple
cause. But when we set ourselves to examine the matter, we find it far
otherwise. The waste of the solid lands, in place of arising from some
single general action, is found to result from a multiform chain of
causes, often local in their operation and variable in their effects.
Such an investigation affords a good illustration of the general mode
and fashion in which Nature delights to work. It shows us that what
may seem a very simple process may be in reality a very complicated
one; that in truth there exist in the world around us few if any
simple, single processes, which stand out by themselves unconnected
with any other; that, on the contrary, all become intimately linked
together, the effects of one often forming part of the chain of causes
in another, and producing by their combined action that complex yet
strikingly harmonious order that pervades all the operations of Nature.
To an extent of which Cicero never dreamed, there runs through all
the world "such an admirable succession of things that each seems
entwined with the other, and all are thus intimately linked and bound
together."[51] Man separates out these various processes, classifies
and arranges them, because from the imperfection of his mental powers
he cannot otherwise understand their effects; all would seem but
chaos and confusion. But the formal precision and the sharp lines of
demarcation exist only in his mind. They have no place in the outer
world. There we see process dove-tailing with process, and spreading
out over the material world in an endless network of cause and effect.
We feebly try to trace out these interlacing threads, but we can follow
them far in no direction. Proteus-like, they seem to change their
aspect, blending now into one form, now into another, and so eluding
our keenest pursuit.
[Footnote 51: Cicero, _De Nat. Deor._ lib. i. 4. So, in Bacon's _Wisdom
of the Ancients_, under the fable "Pan or Nature:"--"The chain of
natural causes links together the rise, duration, and corruption; the
exaltation, degeneration, and workings; the processes, the effects,
and changes, of all that can any way happen to things." Such is the
philosopher's explanation of the Destinies as sisters of Pan. In no
part of his writings can the thorough practical character of Bacon's
philosophy be more conspicuously seen than in his treatment of these
ancient fables. Glancing over the titles of the different papers, you
are tempted to wonder what an intellect which could only appreciate
poetry as a mode of narrating history or as a vehicle for the teaching
of truth, will make of such fairy tales as those of Pan, Orpheus,
Proteus, Cupid, and many others. They seem like so many airy Naiads
crushed within the iron grasp of a hundred-banded Briareus. But a
perusal of those delightful pages will show that the giant has really
no malevolent intentions towards his fair prisoners; nay, that he only
wishes, by stripping them of their paint and finery, to show that,
with all their lightness and grace, they are nevertheless strong buxom
dames, of the same doughty race with himself.]
As an instance, therefore, of this remarkable interlacing of different
causes in what we call a single process, the disintegration of rocks
deserves our attention. In ordinary language, we say a stone rots
away, and its debris is washed down by the rains and streamlets, and
the process does not at first sight seem at all more complex than the
expression used to describe it; yet if we examine the subject, we shall
ere long find that there are in nature many simpler things than the
rotting away of a stone. To effect such a result, there come into play
a whole category of agencies, chemical and mechanical, so combined in
their operation, and so intimately blended in their effects, that it
becomes no easy task to tell where one set ends and another begins.
A rock is said to undergo a chemical change, when one or more of its
component parts passes from one state of combination to another--as,
for instance, when a mineral absorbs oxygen, and, from the condition
of a protoxide, changes into that of a peroxide; or when, parting with
its silicic acid, it takes an equivalent amount of carbonic acid, and
in place of a silicate becomes a carbonate. Now these, and similar
metamorphisms, are chiefly produced by water permeating through the
rocky mass, and thus no sooner does the old combination cease, than the
new one which replaces it is dissolved by the slowly filtering water,
and carried away either to greater depths, or to the surface. Every
drop of water, therefore, that finds its way through the rock, carries
away an infinitesimal portion of the mineral matter, and the stone is
consequently undergoing a continual decay. This condition of things may
go on either at some depth in the earth's crust, or on the surface.
In the former case, springs and percolating water are the agents in
effecting the change; in the latter, it is produced chiefly by rain and
streams. But wherever the process goes on, the results, unless where
counteracted by some opposite agency, are ultimately the same. It may
be of use to look at some examples of these changes, and, by dividing
them in a rough way, into underground and surface actions, we shall be
enabled to mark more clearly their effects.
A common source of the decay of rocks arises from the percolation
through them of water charged with carbonic acid. Decomposing
vegetation gives off a large amount of this gas, which is readily
absorbed by rain-water. The water sinks into the ground filtering
through cracks and fissures in the rocks, whence it afterwards
re-emerges in the form of springs. Now wherever, in its passage through
these subterranean rocks, the water meets with any carbonate, the
carbonic acid contained in the liquid immediately begins to dissolve
out the mineral matter, and carries it eventually to the surface. There
the amount of evaporation is often sufficient to cause a re-deposit of
the mineral in solution. If it be lime, a white crust gathers along
the sides of the stream, delicately enveloping grass-stalks, leaves,
twigs, snail-shells, and other objects, which it may meet with in its
progress. Such "petrifying" springs, as they are popularly termed,
occur abundantly in our limestone districts. It should be borne in
mind, however, that they only produce an incrustation round the organic
nucleus, and do not petrify it. That alone is a true petrifaction
where the substance is literally fossilized, or turned into stone.
A familiar instance of a similar chemical process may be seen under
many a bridge, and along the vaulted roofs of many an old castle.
Numerous tapering stalactites hang down from between the joints of the
masonry, resembling, so to speak, icicles of stone, often of a dazzling
whiteness. They are formed by the percolation of carbonated water
through the mortar of the joints, the carbonate of lime thus withdrawn
being re-deposited where the water reaches the air and evaporates. A
little pellicle of lime first gathers on the roof, and every succeeding
drop adds to the length of the column. In some cases, where the supply
of water is too great for the amount of evaporation, part falls on the
floor, and, being there dissipated, leaves behind a slowly-gathering
pile of lime called stalagmite. In some of the Eastern grottos, the
pillars from the roof have become united to those on the floor, forming
the most exquisite and fairy-like combinations of arch and pillar.
An example of a calcareous grotto has now become pretty familiar to
our summer tourists, under the name of the Spar Cave. It lies on an
exposed cliff-line along the western shores of Skye, against which
the surge of the Atlantic is ever breaking. You approach it from the
sea, and enter a narrow recess between two precipitous walls of rock,
open above to the sky, and washed below by the gurgling tide. Crossing
the narrow, shingly beach, you find the ground thickly covered with
herbage, while, grouped along the dark walls, are large bunches of
spleenwort, hart's-tongue fern, and other plants that love the shade.
Soon after entering the cave, all becomes sombre and cold; and the few
candles, with which the party have furnished themselves, only serve
to heighten the gloom. After scrambling on for a time across dank,
dripping rocks, and over a high bank of smooth marble, on which it is
difficult to creep, almost impossible to stand, you arrive at a deep
pool of clear, limpid water, which extends across the cave from side to
side, barring all farther passage. The scenery at this point will not
readily be forgotten. The roof towers so high that the lights are too
feeble to show it, while the walls, roughened into every form of cusp
and pinnacle, pillar and cornice, all glittering in the light, resemble
the grotto of some fairy dream. On returning again to the light of day,
if you ask the cause that has given rise to all this beauty, it will
be found a very simple one. The cleft occupied by the cave has been
once filled by a wall of igneous rock called a trap-dyke. Atmospheric
influences, aided probably by the waves, have caused the decomposition
and removal of this intruded rock, and the calcareous sandstone on
either side 'now stands up in a wall-like form. The upper part of
the dyke remains as a roof to the cave, but it has become completely
covered over with the calcareous deposits left by the carbonated water
that filters through the adjacent limy sandstone. The amount of water
is considerable, and consequently every part of the cavern--roof,
walls, and floor--has been incrusted with a white crystalline carbonate
of lime. In volcanic countries, where the springs often come to the
Surface in a highly heated state, charged, too, with various chemical
ingredients, they produce no slight amount of physical change on the
surrounding districts, and must be regarded as important geological
agents.
But perhaps the most common and widely-diffused form of decomposition,
is that produced on the surface of the earth by the action of
rain-water, in slowly dissolving out the soluble parts of rocks, and
washing away the loose, incoherent grains that remain behind. It is
hard to say whether this process is more chemical or mechanical.
The solution of the mineral matter belongs to the former class of
changes, while the removal of debris must be ranked among the latter.
The results of these combined forces form one of the most important
branches of investigation which can occupy the attention of the
physical geologist, and in contemplating them, we are at a loss whether
most to admire their magnitude, or the immense lapse of time which
they must have occupied. It may be worth while to look at the progress
of this kind of disintegration, that we may see how wide-spread and
constant is the waste that goes on over the world, and how materially
the effects of running water are by this means increased. A volume
might be written about the decay of rocks, and a most interesting one
it would be, but its authorship would devolve rather on the chemist
than the geologist.[52] We can do no more here than merely glance at
one or two illustrative examples.
[Footnote 52: A German chemist, Bischoff by name, has written two
learned volumes in which this subject is discussed (translated into
English, and published by the Cavendish Society), valuable for their
facts, but not always very safe in their deductions.]
Among the mineral substances that most readily yield to the action of
the weather, are the silicates and the carbonates. The rocks containing
the former belong in large measure to what we call the igneous class,
such as the granites and traps; while those containing the latter
form the bulk of our useful stones, such as limestone and sandstone.
The removal of alkaline silicates is due to their conversion into
carbonates, which are readily soluble in water. Rain falling on a rock
in which they are largely present, dissolves a small portion, and
carries it into the soil or into streams, and thence to the ocean.
Every shower in this way withdraws a minute amount of mineral matter,
and tends to leave the harder insoluble grains of the rock standing out
on the surface in the form of a loose pulverulent crust, easily washed
away. The debris thus formed, where allowed to accumulate, makes an
excellent kind of soil known to the Scottish farmers as "rotten rock."
The tourist who has visited any of our granitic districts, such as the
south-western parts of Cornwall, the rugged scenery of Arran, or the
hills of the Aberdeenshire Highlands, must be familiar with some of the
forms of waste which the rocks of these regions display. Mouldering
blocks, poised sometimes on but a slender base, and eaten away into the
most fantastic shapes, abound in some localities, while in other parts,
as for instance at the summit of Goatfell in Arran, the rock weathers
into a sort of rude masonry, and stands out in its nakedness and ruin
like some crumbling relic of Cyclopean art. In other districts, as in
Skye and in the adjoining island of Raasay, the granitic hills are
of a still more mouldering material. Their summits, white and bald,
sometimes rise to a height of fully two thousand feet above the sea,
while down their sides are spread long reddish-yellow tracks of debris
intermingled with patches of stunted herbage. Every winter adds to the
waste, and lengthens the lines of rubbish. Some of these hills form a
good field wherein to study the disintegration of granitic rocks, such,
for instance, as Beinn na Cailleaich, that rises from the shores of
Broadford Bay. Around the eastern base of that mountain there stretches
a flat moory district, with a few protruding blocks that have rolled
down into the plain. The earlier part of the ascent lies over a region
of metamorphic limestone, where the grey weathered masses of the
calcareous rock, often like groups of mouldering tombstones, are seen
protruding in considerable numbers through the rich soft grass and the
scanty brushwood of hazel and fern. Leaving this more verdant zone, we
enter a district of brown heath that slowly grows in desolation as we
ascend. Huge blocks of syenite--a granitic rock of which the upper part
of the mountain entirely consists--cumber the soil in every direction,
and gradually increase in numbers till the furze can scarcely find a
nestling-place, and is at last choked altogether. Then comes a scene
of utter desolation. Grey masses of rock of every form and size are
piled upon each other in endless confusion. Some of them lie buried in
debris, others tower above each other in a rude sort of masonry, while
not a few perched on the merest point seem but to await the storms of
another winter to hurl them down into the plain. The ascent of such a
region is no easy task, and must not unfrequently be performed on hands
and knees. But once at the top, the view is enough to compensate a
tenfold greater exertion. Far away to the west, half sunk in the ocean,
lie the isles of Eigg, Coll, and Tiree, with the nearer mountains of
Rum. North-west, are the black serrated peaks of the Coolins, that
stand out by themselves in strange contrast with every other feature
of the landscape. Northward, stretches the great range of syenitic
hills, with the sea and the northern Hebrides beyond. Away to the east,
across the intervening strait, lie the hills of the mainland, with
all their variety of form and outline, and all their changing tints,
as the chequered light and shade glide athwart the scene. Southward,
the eye rests on the grey wrinkled hills of Sleat, and far over along
the line where earth and sky commingle, are the mountains of Morven,
stretching westwards till they end in the bold weather-beaten headland
of Ardnamurchan, beyond which lies the blue boundless ocean. The top of
Beinn na Cailleaich is flat and smooth, surmounted in the centre by a
cairn. Tradition tells that beneath these stones there rest the bones
of the nurse of a Norwegian princess. She had accompanied her mistress
to "the misty hills of Skye," and eventually died there. But the love
of home continued strong with her to the end, for it was her last
request that she might be buried on the top of Beinn na Cailleaich,
that the clear northern breezes, coming fresh from the land of her
childhood, might blow over her grave.
I have already alluded to the wasting away of a trap-dyke. This
decomposition arises from the same cause as among the granites--the
solution, and removal of the silicates. All these trap-rocks are
igneous, and seem to have risen from below through open fissures and
rents. As they contain a large percentage of felspar--the same mineral
that gives to many granites their mouldering character--they may be
seen exhibiting every form and stage of decay. Often they stand out in
prominent relief from some cliff of soft shale, with a brown surface,
picturesquely roughened into spherical masses of all sizes, that give
to the rock somewhat the appearance of a hardened pile of ammunition
in which ponderous shells lie intermingled with round shot, grape, and
canister. Each of the concretionary balls when examined is found to
exfoliate in concentric pellicles like the coats of an onion, and you
may sometimes peal off a considerable number before arriving at the
central core, which consists of the hard rock still undecomposed. In
this case the process of degradation is aided by the decay of another
mineral called augite, which contains a variable percentage of iron,
and imparts the peculiar yellowish-brown tint to the weathered rock.
Trap-dykes may also be seen in a still more wasted condition, where,
in place of protruding from a cliff-line, they recede to some depth
and give rise to deep clefts and fissures. An instance of this kind
has been referred to in the case of the Spar Cave, and many others may
be seen along the same coast-line. The shore there for miles is formed
of a low cliff of white calcareous sandstone, fissured by innumerable
perpendicular clefts of greater or less width, and sometimes only a
yard or two apart. Each of these has once been filled by a dyke of
trap, which originally rose up in a melted state, and after having
solidified into a compact stony mass, began to yield to the process of
decay. In all these and similar cases, the primary cause of the waste
lies in the decomposition of the felspar. Rain-water acts in removing
the soluble portions, and the harder grains that remain, deprived of
the cementing matrix, ere long crumble down and are washed off by the
rains. In this way the rock insensibly moulders away, every frost
loosening its structure, and every shower carrying away part of its
substance.
Among the many objects of interest along a rocky coast some of the more
striking are certainly to be found in the curious and often grotesque
forms assumed by the weathered cliffs. Above high-water mark and thus
away from the dash of the waves, we can often trace the progress of
decay among such sedimentary rocks as sandstones and conglomerates.
Worn into holes and scars, projecting cusps and tapering pinnacles, or
eaten away into the rude semblance of a human form, headless perchance,
or into the shape of a huge table poised on a narrow pedestal, the
rock affords an endless variety of aspects and a continual source of
pleasure. If we chance to light upon any building constructed out of
the sandstone of such cliffs, it is worth noting that the removal of
the stone has not deprived it of its mouldering qualities; nay, that
houses erected within the memory of people still living already begin
to wear an aspect of venerable antiquity. I remember meeting with an
interesting example in the case of an old castle built on a similar
rocky coast-line. It stood on a little ness or promontory of dull red
stone, washed on all sides save one by the wild sea. The walls, of
which but a fragment remained, were built of a dark red sandstone; but
the lapse of centuries had told sadly on their masonry. The stones
rose over each other tier upon tier, corroded sometimes into holes
and hollows, sometimes into a close honey-combed surface, but the
mortar that had been used to cement them together still stood firm and
protruded from between the tiers to show, by no doubtful or ambiguous
sign, how silently yet how surely the wasting forces had been at
work. The scutcheon over the only remaining gateway had been carved
out of another kind of stone of a lighter colour and harder texture,
and so its grim lions looked nearly as fresh and formidable as when
first raised to the place of honour which they still occupied. In
this case, as before, the decomposition was owing to the presence of
a considerable proportion of soluble matter, which the rains of four
centuries had carried away along with the loosened incoherent sandy
grains.
Conglomerate or pudding-stone has often a picturesque outline in its
decay, more especially if its included fragments have a considerable
range of size. Large tracts Of this rock exist in various parts of
Britain, particularly in Scotland, where the basement beds of the Old
Red Sandstone consist of a coarse conglomerate, sometimes several
thousand feet in thickness. Such enormous masses form the scenery of a
large part of East Lothian, and are found in detached patches across
into Peeblesshire and Lanark. In the north, too, the neighbourhood of
Inverness and other parts of the same district display conspicuous
conglomerate hills. Unless where laid bare by streams or by the action
of breakers, the contour of these hills is rounded and tame, with a
scanty covering of short scrubby grass and very few protruding bosses
of rock. But where a mountain torrent has cut its way down the hill
side, the ravine thus formed exhibits broken walls and pillars of
rock made up of rounded balls of every shade and size, cemented by a
dark-red or green paste. The cementing material is sometimes clay,
sometimes lime, and its variable nature gives rise to a corresponding
inequality in the amount and form of decomposition. Where the rounded
pebbles are bound together by clay, rains act with rapidity in washing
away the cement, and the component balls fall out by degrees, leaving
a cliff strangely roughened by protruding knobs, and eaten away into
clefts and hollows. When the pebbles are held in a crystallized matrix
of lime, they usually remain longer together, and may sometimes be
seen standing up in the form of detached rugged pillars that defy
all regularity of size or outline, and remind one of a sort of rude
grotto-work. Such irregularities become still more marked where to the
action of the rains there has been added the spray of the ocean. A
coast-line of conglomerate, where the rock rises into cliffs, is always
a romantic one; caves, pillars, and ruined walls, all in the same rough
grotto style, meet us at every step. Here, too, we can mark the varying
effect of the waves upon the lower portions of the rock, eating it into
cavernous holes and leaving rugged projecting pinnacles to which the
mottled colours of the included pebbles give an additional and peculiar
effect.
A cliff of shale seldom shows much of the picturesque, though often a
good deal of the ruinous. The rock is easily undermined by streams,
and a shale ravine usually exhibits in consequence either heaps of
crumbling rubbish, or, where the stream comes past with a more rapid
current, perpendicular walls, jointed and laminated, but without much
variety of outline. Such cliffs, however, merit the careful attention
of the observer, for from their friability they are most easily
decomposed and washed down by streams, to form new accumulations of
similar soft argillaceous matter. A shale coast-line sometimes shows
cliffs of considerable altitude, as in some parts of Skye and Pabba,
where the Lias shales may be seen piled over each other often to a
height of seventy or eighty feet, and spreading out along the shore as
low flat reefs and skerries, brown with algæ at their seaward ends,
and showing on the higher slopes of the beach the characteristic
fossils of the Lias--_ammonites_, _belemnites_, and _gryphææ_--crowded
together by hundreds. The action of the decomposing forces has operated
more effectually on the soft material of the shale than on the hard
crystalline lime of the included shells, so that the latter stand out
in relief from the dull-brown surface of the rock, and from their
numbers and prominence form one of the most marked features of the
coast-line.
Probably few have ever visited a limestone district without marking
the manner in which that rock yields to the action of the elements,
whether in an inland part of the country where rivers have cut deep
gullies through the rock, or along some exposed shore where the stone
has been wasted by a still ruder assailant. An exposed cliff of hard
homogeneous limestone weathers into deep clefts and holes; the entire
surface assumes a pitted appearance, somewhat like a sandy beach after
a showier of rain, and the planes of stratification, or lines formed
by the parallel junction of the beds are often worn away until the
rock looks not unlike a piece of old masonry, in which the mortar has
decayed and dropped out, leaving the angles of the stones to get wasted
and rounded by the action of the weather. In many districts, too, where
the rock is richly fossiliferous, the broken joints of encrinites along
with corals and shells may be seen crowded together by myriads, their
hard skeletons protruding from the wasted rock in such a way as to show
that the stone can contain very little else. By this means we often
learn that a limestone bed is nothing but an old sea-bottom, where
the calcareous sediment was mainly derived from broken stone-lilies,
corals, and shells, though if we break off a piece of the rock the
internal fracture may show very little or no trace of any organic
structure. And hence if the geologist would form an accurate conception
of the origin and structure of many of the stratified rocks, he must
study them not in hand specimens neatly trimmed and arranged along the
shelves and drawers of a cabinet, nor even in the ponderous blocks
daily exhumed by the quarryman, but along some surf-beaten cliff-line
or down some precipitous ravine where the rock for centuries has been
exposed to the wear and tear of the elements.
Limestones and other calcareous formations are liable to more than
ordinary decay, for, as we have seen above, percolating rain-water
constantly carries away mineral matter from their subterranean
portions. Accordingly, in some parts of the country, as for instance in
Yorkshire, the interior of such rocks has been eaten away into great
caverns by this form of decomposition.
Some remarkable examples occur in the island of Raasay, one of the
north-eastern Hebrides. Its eastern margin shoots up from the sea to a
height of over 900 feet, the cliff-line being formed of a calcareous
grit as perpendicular as a wall, and fissured by deep chasms and rents.
The narrow table-land between the edges of this cliff and an abrupt
ridge that rises behind, is perforated by innumerable holes and clefts,
into which if a stone be thrown it may be heard for several seconds
rumbling far below. The edges of these pitfalls are often fringed
with ferns, rushes, and long grass, so as to be nearly hidden, and it
requires no little caution to traverse this elevated region in safety.
Innumerable sheep have been lost by falling into the subterranean
abysses, and even the wary natives seem to have sometimes lost their
footing. A story is told of a woman who had crossed to the other side
of the island for the purchase of some commodities, and returning
by the high grounds had got nearly within sight of her own cottage,
when by some unlucky accident she took a false step and instantly
disappeared. Unfortunately her errand had been performed alone, so that
some time elapsed ere she was missed. The search continued unremitting
for two days, but no trace of the missing traveller could be found. At
last on the third day her figure was seen creeping slowly along the
road not many hundred yards from her own door. It appeared that she had
first slid down a sheer height of about fifty feet, when her further
passage was intercepted by the sides of the fissure. During the earlier
part of her confinement she strove hard to re-ascend the chasm, and it
was not until, the effort seeming fruitless, she had begun to resign
herself to despair, that a glimmering of light from below induced her
to attempt a descent. This proved no easy matter, and occupied many
weary hours of labour and suspense; but at length she succeeded in
worming herself to the bottom, and crawled out more dead than alive
only a little way from her home. There still stand perched on some of
these precipitous cliffs the remains of a few villages, the inhabitants
of which were accustomed to tether their children to the soil, whence
one of the hamlets received in Gaelic the soubriquet of Tethertown.
Many a valuable commodity disappeared by rolling over the cliff, and
I have been assured that it was no unfrequent occurrence for a pot of
potatoes capsized at the doorway to tumble down the slope and make no
stop until safely esconced at the sea-bottom.
The process whereby these fissures and caverns originate is the same as
that noticed already in the Spar Cave. Water containing an impregnation
of carbonic acid filters down through cracks and fissures of the
calcareous rock, dissolving out in its passage a portion of the lime
which it eventually carries back to the surface, and either deposits
there or transports into streams, and thence to the sea. Thus atom
by atom is removed wherever the percolating water reaches, until in
the course of ages an irregular cavern of greater or less extent is
produced. The decomposition of limestone at the surface results from
the same kind of action, that of carbonated water. Every shower of
rain insensibly carries away a fraction of the constituent parts of
the rock, so that the size and form of detached blocks as well as of
exposed cliffs is constantly changing. How often do we see the same
decay going on with a melancholy rapidity among the exposed marble
tombstones of our churchyards. In a few years the tablet gets worn
and furrowed as though it had stood there for centuries. Eventually,
too, the inscription becomes effaced, and perhaps ere the bones of the
deceased have mouldered away and mingled with their kindred dust, the
epitaph that recorded for the admiration of posterity his many virtues
and his vigorous talents, has faded from the stone--often, alas! only
too fit an emblem of how speedily the memory of the dead may fade away
out of the land of the living.