CHAPTER VIII.

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.

Fromthe 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 frameFor ever varying, and yet still the same."

"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. Theatmospheric 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 producesimportant 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 arisingfrom 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-tailingwith 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.

[51]Cicero,De Nat. Deor.lib. i. 4. So, in Bacon'sWisdom 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 atsome 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 carbonatedwater 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, butits 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.

[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. Theirsummits, 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, stretchesthe 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, whichconsists 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 anybuilding 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 conspicuousconglomerate 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, andgryphææ—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 oftenlearn 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 ofsome 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 percolatingwater 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.

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