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.
[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.