Their various Forms—Natural Tunnels—The Ventanillas of Gualgayoc—Eimeo—Torgatten—Hole in the MÜrtschenstock—The Trebich Cave—Grotto of Antiparos—Vast Dimensions of the Cave of Adelsberg and of the Mammoth Cave—Discovery of Baumann’s Cave—Limestone Caves—Causes of their Excavation—Stalactites and Stalagmites—Their Origin—Variety of Forms—Marine Caves—Shetland—Fingal’s Cave—The Azure Cave—Cave under Bonifacio—Grotta di Nettuno, near Syracuse—The Bufador of Papa Luna—Volcanic Caves—The Fossa della Palomba—Caves of San Miguel—The Surtshellir.

The natural excavations which abound in many mountain chains, or on rocky shores washed by the stormy sea, are extremely various in their forms. Many are mere rents or crevices in the disruptured rocks; others wide vaults, not seldom of hall or dome-like dimensions, or long and narrow passages branching out in numerous ramifications. Not seldom the same cave alternately expands into spacious chambers, and then again contracts into narrow tunnels or galleries. The walls of many are smooth and nearly parallel; the sides of others are irregular and rugged. Many have narrow entrances and swell at greater depths into majestic proportions; while others open with wide portals, and gradually diminish in size as they penetrate into the rock. Sometimes an excavation pierces a mountain from side to side like a natural tunnel, so as to allow a passage to the light of day. Such, among others, are the numerous perforations or windows (ventanillas) in the serrated bastions of the rich silver mountain Gualgayoc in the Peruvian Andes, or the opening through one of the high peaks of the romantic island of Eimeo which rises within sight of Tahiti out of the dark blue ocean. According to a popular tradition, this hole owes its origin to Oro, the powerful god of war, who, having one day quarrelled with the minor god of Eimeo, hurled his mighty spear at him over the sea. As even gods, when losing their temper, are apt to miss their aim, the puny delinquent escaped unhurt, while the dreadful lance flew like a thunderbolt through the mountain, leaving the perforation as a lasting memorial of its passage. In Europe we likewise meet with several remarkable instances of such natural tunnels. One of the most celebrated is the grotto of Torgatten in Norway, which perforates a huge rock, 400 feet above the level of the sea. Its proportions are truly colossal, as it is no less than 900 feet long by from 80 to 100 feet broad; and the arches of its vast portals measure respectively 200 and 120 feet. Its floor is nearly horizontal, and covered with fine sand; its sides are smooth, as if they had been chiselled by the hand of man. The sea, with its numberless cliffs and white-crested breakers, appears through the immense gallery as through the tube of a gigantic telescope, and in fine sunny weather affords a spectacle of incomparable beauty.

Whoever has visited the romantic lake of WallenstÄdt, in Switzerland, will have had his attention directed to a tunnel near the summit of the almost inaccessible MÜrtschenstock, a favourite resort of the chamois. It is visible from the lake near the hamlet of MÜhlehorn, and, though of considerable dimensions, appears to the eye like a mere speck of snow on the huge grey rockwall, which towers to a height of 7,517 feet. From the 1st to the 3rd of February, at 2 o’clock in the afternoon, the inhabitants of MÜhlehorn see through this aperture the disk of the sun for the first time after a long winter.

In the structure of some caves a vertical direction predominates; as, for instance, in the Trebich Cave, three leagues from Trieste, which consists of several perpendicular shafts, connected by narrow transversal passages, and descending one after another, until finally, at a depth of more than a thousand feet, the cavern terminates in a wide vaulted space spanning a subterranean river. Such, also, is the renowned Grotto of Antiparos, into which the visitor is let down by a rope to a depth of about twenty fathoms. After reaching a tolerably even platform, he is obliged to descend another precipice, and then to proceed over slippery rocks until he finally reaches the terminal vault.

In most caverns, however, the chief direction is horizontal, either on several planes, separated from each other by more or less steep passages, or on a single level. The dimensions of caves are as various as their forms. Many are small and of inconsiderable depth—mere holes worn in the rock; while others are of a truly astonishing size, and fatigue the wondering spectator as he wanders through their lofty halls or endless galleries. The famous Cave of Adelsberg in Carniola has been explored to a distance of 1,243 fathoms from the chief entrance; and in the Mammoth Cave in Kentucky no less than 226 avenues branch out to the right and left from the main gallery, so as to form a network of subterranean passages and halls of various dimensions, whose total length has been computed at about 160 miles!

As many caves are without any visible communication with the external world, and the entrance of others is frequently narrow, and concealed behind rocks in solitary ravines on wild hill slopes or steep sea shores, far from the busy haunts of man, we cannot wonder that chance has frequently been instrumental in their discovery. Sometimes a hunter pursuing a wild animal has been led to the hidden cave in which it sought a refuge, or the workmen in a quarry have been suddenly surprised at meeting with a hollow in the rock, which opened an unexpected passage into the bowels of the mountain. The digging of wells, of cellars, of foundations, the boring for mines or Artesian wells, has often revealed the existence of unknown subterranean chambers; and so recently as 1868, one of the finest known caverns, which already attracts a number of delighted visitors, was discovered in the neighbourhood of the thriving manufacturing town of Iserlohn in Westphalia, on blasting a rock for the making of a railway. We may thus infer that a vast number of caves must still be totally unknown; many so situated that chance may one day lead to their discovery; while others are hollowed out at such vast depths in the earth-rind as to be for ever inaccessible to man.

Even of those caves which have been objects of curiosity for centuries, many have still been by no means thoroughly explored. In the year 1848 an American gentleman persuaded the guides of Baumann’s Cave in the Harz Mountains to accompany him on a voyage of discovery through parts of the cavern hitherto untrodden by man. It was no easy task to clamber over slippery rocks and deep chasms yawning into black abysses; but curiosity and the spirit of adventure kept leading them on from passage to passage and vault to vault, when suddenly the lights began to burn more dimly; and the glass of the guiding compass having been accidentally broken warned them to retrace their steps. They had been wandering for twenty-four hours in the subterranean labyrinth, and after so long an absence from the light of day, joyfully hailed the green hill slope which decks that mysterious palace of the gnomes. Franz Baumann, the first discoverer of the cavern, was less fortunate. Its tortuous windings confused the expert and intrepid miner, who lost his way in the recesses of the cave. While seeking in vain for an outlet, his sparing light went out. Three days he groped about in darkness, until at length, worn out and exhausted, he was led by a wonderful chance to the mouth of the cave. Before he died he had yet sufficient strength briefly to mention the wonders he had seen during his fatal expedition. His descendants still enjoy the privilege of serving as guides to the visitors of the cave, and never fail to relate the melancholy end of their ill-fated forefather.

Grottoes and caves occur in every kind of rock, in lavas, basalt, slate, and granite, as well as in limestone, dolomite, and gypsum; for the volcanic powers are capable of rending the hardest stone, and the foaming breakers of a turbulent ocean meet with no cliff that is able ultimately to resist their never-tiring assaults.

But, owing to their great fragility and to the solubility of limestone (carbonate of lime) in water containing carbonic acid, calcareous rocks are more liable than any others to be shattered and undermined, both by volcanic and aqueous causes. Its water readily absorbs carbonic acid gas. Every drop of rain that falls upon the ground necessarily contains some small portion of this gas, which, as we all know, is constantly mixed with the atmosphere, and thus becomes a solvent for chalk; more particularly if the latter, as, for instance, in the Karst Mountains of Carniola, contains some proto-carbonate of iron, which, changing into an oxide when in contact with water, yields its carbonic acid to the percolating fluid, and consequently increases its solvent powers. Hence every shower of rain that filters through the crevices of a limestone rock wears away some part of its mass; and if we consider the vast number of years over which these operations have extended, and add to their effects the transporting powers of the waters on their progress through the subterranean channels which they have excavated or enlarged, we can easily comprehend how in the course of ages whole mountains may be hollowed out.

As the streams that flow on the surface of the earth are constantly altering their courses, thus also the subterranean waters are ever active in excavating new channels in the bosom of the rock. Finding at length new outlets on a lower level, they abandon their ancient beds, and the explorer now wanders dry-footed where once a foaming river gushed along. The Cave of Adelsberg is a remarkable example of the changes which the subterranean waters, aided by time or by the disrupting power of earthquakes, may thus bring about; for the Poik now flows beneath its galleries in the same north-easterly direction, in a channel which is for the greatest part unknown and inexplorable, so that the dry cave of the present day must evidently have been the old river-bed. But nowhere can be found such perfect, unequivocal, and abundant proofs of the action of running water in corroding and excavating new passages in a soluble rock as in the huge Mammoth Cave. The rough-hewn block in the quarry does not bear more distinct proof of the hammer and the chisel of the workman than these interminable galleries afford of its denuding and dissolving power. At Niagara we see a vast chasm evidently cut by water for seven miles, and still in progress; but we cannot see beneath the cataract the water-worn surface, nor the rounded angles of the precipice; while the frosts and rains of countless winters have reduced the walls of the chasm itself to a talus of crumbling and moss-grown rocks. But in the Mammoth Cave we see a freshness and perfection of surface such as can be found only where the destructive agencies of meteoric causes are wholly absent. Here we have the dry beds of subterranean rivers exactly as they were left thousands of years ago by the streams which flowed through them when Niagara was young. No angle is less sharp, no groove or excavation less perfect, than it was originally left when the waters were suddenly drained off by cutting their way to some lower level. The very sand and rounded pebbles, which now pave the galleries and which anciently formed the bed of the stream, have remained in many of the more distant galleries untrodden even by the foot of man. ‘The rush of ideas was strange and overpowering,’ says Professor Silliman, ‘as I stood in one of these before unvisited avenues, in which the glow of a lamp had never before shone, and considered the complex chain of phenomena which were before me. There were the delicate silicious forms of cyathophylli and encrinites protruding from the softer limestone, which had yielded to the dissolving power of the water; these carried me back to that vast and desolate ocean in which they flourished, and were entombed as the crystalline matrix was slowly cast around them, mute chroniclers of a distant epoch. Then succeeded the long periods of the upper secondary, and, these past, the slow but resistless force of the contracting sphere elevated and drained the rocky beds of the ancient ocean. The action of the meteorological causes commenced, and the dissolving power of fresh water, following the almost invisible lines of structure in the rocks, began to hollow out these winding paths slowly and yet surely.’ What a lesson for the thoughtful spectator, and how vast a prospect into the dark abysses of the past here unrolls itself before him!

After abandoning the vaults where they once collected and formed a running stream, the waters, filtering through the porous limestone, begin to ornament them with lustrous petrifactions; for whether below or above the surface of the earth, Nature ever loves to decorate her works. The moisture, charged with carbonate of lime, evaporates or parts with its free carbonic acid in coming into contact with the air of the cave; the carbonate, now no longer held in solution, precipitates and forms calcareous incrustations or excrescences, which in course of time assume every variety of fantastic shape, either hanging like icicles from the vault (stalactites), or rising in columns (stalagmites) from the floor of the cave where the dripping water deposited its spar. Sometimes stalactites and stalagmites join as they continue to grow in opposite directions, and ultimately form pillars which appear to sustain the roof.

On considering the simple physical and chemical agencies which are at work in the formation of these beautiful productions—solution, mechanical dripping, evaporation, and precipitation—a great similarity might naturally be expected in their forms; but here also Nature shows herself as a consummate artist, and with the simplest means brings forth an astonishing variety of effects. As among the leaves of a forest there are not two perfectly alike, thus also every stalactite differs from another; and the celebrated traveller Kohl affirms that every stalactital cave has its peculiar style or character of decoration. The causes to which stalactites owe their existence are indeed everywhere the same, but the circumstances under which the drops fall and evaporate are so various that in each case some new shape is produced. Thus all the infinite diversity of forms which we admire in the corals and sponges of the seas, is wonderfully repeated in the dark vaults of the subterranean world.

The variety and beauty of their colouring likewise contribute to adorn these formations. They are generally white, sometimes rivalling the purity of snow, and translucent, even when of considerable thickness, but often also green, brown, yellow, red, orange—a variety of tints which produces the most pleasing effects, and is chiefly owing to the metallic salts with which the water has been impregnated while filtering through the calcareous rock.

All these wonderful plays of Nature, in which form and colour contribute to delight the eye or to charm the fancy of the spectator, are, however, still less interesting than the reflections suggested by the slow growth of stalactites in general, and the enormous size which some of them attain.

Inscriptions seventy or eighty years old appear covered only with a thin translucent coat of sinter, and in the Cave of Adelsberg names scratched in the walls more than six centuries ago are still perfectly legible. How many ages must, then, have passed before such colossal stalagmites could have been formed as, for instance, in the Australian cavern explored by Mr. Woods,^[13] or in the Cave of Corneale, near Trieste, where we find one of these formations measuring fifty feet in circumference, and another rising thirty-five feet above the ground, with a trunk as massive as that of an old oak. The ruins of Thebes, or the rock-temples of Ipsamboul, appear almost as works of the present day when compared with those amazing monuments of time. But, while meditating on their colossal dimensions, the mind is necessarily carried still further back, and wanders through the countless ages which the filtering waters, collecting into subterranean streams, required for hollowing out the vast cavities on whose floor those gigantic stalagmites were subsequently deposited. An epoch of still older date presents itself when the limestone rocks, now pierced with vast stalactital caverns, were first slowly forming at the bottom of the primeval sea by the accumulation of countless exuviÆ of zoophytes, star-fishes, and foraminifera, and after growing into strata many hundred feet thick, were then forced upwards by plutonic powers, and became portions of the dry land. Nor is this the end of the vast perspective, for changes still more remote loom in the fathomless distance. The mind grows giddy while thus plunging into the abyss of time, and, in spite of the ideas of sublimity awakened by such meditations, feels a painful sense of its incapacity to conceive a plan of such infinite extent.

While on land the running or filtering waters restlessly pursue their work of excavation, the tumultuous waves of the ocean impress on every rocky shore the seal of their tremendous power. As, day after day, and year after year, the billows strike against the cliffs that oppose their progress, they undermine their foundations, scoop out wide portals in their projecting headlands, and hollow out deep caverns. Here also water appears as the beautifying element, decorating inanimate nature with picturesque forms; and the sea nowhere exhibits more romantic scenes than on the bold coasts against which her waves have been beating for many a millennium. During the calm ebb tide seals are often seen sunning themselves at the entrance of the oceanic grottoes, while cormorants stand before them as guardians of the dark galleries beyond; the waves murmur in softer strains, and the screeching sea-mew glides with his silvery pinions through the tranquil air; but when the stormy flood batters against the coast, the billows rush into the caverns, scaring all animal life away, and no voice is heard but that of the ocean.

Our coasts abound in beauties such as these, particularly on the wild shores of Shetland or the stormy Hebrides—

Along the coast of the mainland of Shetland and the neighbouring islets of Bressay and Noss, cape follows upon cape, consisting of bold cliffs hollowed into caverns, or divided into pillars and arches of fantastic appearance, by the constant action of the waves. As the voyager passes the most northerly of these headlands, and turns into the open sea, the scenes become yet more sublime. Rocks, upwards of three or four hundred feet in height, present themselves in colossal succession, sinking perpendicularly into the sea, which is very deep, even within a few fathoms of their base. All these huge precipices abound with caves, many of which run much farther into the rock than the boldest islander has ever ventured to penetrate. One of these marine excavations, called ‘The Orkneyman’s Harbour,’ is remarkable for the circumstance of an Orkney vessel having once run in there to escape a French privateer. Sir Walter Scott, who visited this interesting spot, found the entrance lofty enough to admit his six-oared boat without striking the mast, but a sudden turn in the direction of the cave would have consigned him to utter darkness if he had gone in further. The dropping of the sea fowl and cormorants into the water from the sides of the cavern, when disturbed by his approach, had something in it wild and terrible.

The shores of Caithness and of Sutherland, and of many of the islets in the Highland seas, likewise exhibit many wonderful specimens of the fantastic architecture of the ocean; but pre-eminent above all in grandeur and renown is Fingal’s Cave.

Sir Walter Scott, who twice visited this celebrated grotto (in 1810 and 1814), pronounced it above all description sublime. ‘The stupendous columns and side-walls, the depth and strength of the ocean with which the cavern is filled, the variety of tints formed by stalactites dropping and petrifying between the pillars, and resembling a sort of chasing of yellow or cream-coloured marble, filling the interstices of the roof—the corresponding variety below, where the ocean rolls over a red, and in some places a violet-coloured rock, the basis of the basaltic pillars—the dreadful noise of those august billows, so well corresponding with the grandeur of the scene—are all circumstances elsewhere unparalleled.’

In the Azure Cave of Capri, the Mediterranean possesses a marine grotto rivalling Fingal’s Cave in celebrity, and no less wonderful in its peculiar style of beauty. As the roof of its narrow entrance rises only a few feet above the level of the sea, it is probable that no human eye had ever been delighted with its charms before 1826, when it was accidentally discovered by two Prussian artists who were swimming in the neighbourhood. After passing the low portal the cave widens to grand proportions, 125 feet long and 145 feet broad, and except a small landing-place on a projecting rock at the further end, its precipitous walls are on all sides bathed by the influx of the waters, which, in that sea, are so clear that the smallest objects may be distinctly seen on the bottom of the deep basin, the most beautiful bathing-place a mermaid might wish for. All the light that enters the grotto must first penetrate the whole depth of the waters before it can be reflected into the cave, and it thus acquires so blue a tinge, from the large body of clear water through which it has passed, that the walls of the cavern are illumined by a radiance of the purest azure. Had Byron known of the existence of this magic cave, Childe Harold would surely have devoted some of his most brilliant stanzas to its praise.

In many other parts of the Mediterranean the limestone rocks that fringe its shores have been worn into magnificent caverns, less singular, indeed, than the fairy grot of Capri, but still of rare and wonderful beauty. Such, among others, is the Antro di Nettuno, in the island of Sardinia, about twelve miles from the small seaport of Alghero.^[14]

Exceedingly picturesque caverns have also been worn by the chafing waters in the chalk cliffs under Bonifacio, in the island of Corsica. Their entrances festooned with hanging boughs, they penetrate far into the interior of the rocks, and the water percolating through their vaulted roofs has formed stalactites of fantastic shapes. The boat glides through the arched entrance, and the glaring sunshine without is replaced by cool and grateful shade. Fishes are flitting in the clear water; limpid streams oozing through the rocks form crystal basins with pebbly bottoms; and the channels from the blue sea, flowing over the chalk, become cerulean. Poetic fancy has never pictured anything more enchanting than these lovely caves.

The rocky coast of Sicily is likewise hollowed out with numerous marine grottoes, which, though rarely noticed by travellers, may well be ranked amongst the greatest natural beauties of the island. One of the most remarkable is the Grotta di Nettuno, near Syracuse, which, in calm weather, admits a boat to a considerable distance. Its rugged vaults rise to a height of about twenty or thirty feet, and are covered with stalactites wherever the water does not reach. There is no landing-place, and throughout the whole cave the water is as deep as in the open sea beyond. Nothing can be more charming than to look back from the dark recesses of the grotto upon the bright sunshine without, and to listen to the soothing murmurs of the clear waves as they ripple against the rocky walls. The atmosphere is so pure in this delicious climate that not a trace of fog or mist obscures even the remotest parts of the cave, and the serene daylight falling through the entrance renders even its deepest shadows translucent. Here a lover of nature might linger for hours enjoying the most delicious coolness, and watching the charming effects of light and shade in their ever-varying play.

On many rocky shores the ocean has worn out subterranean channels in the cliffs, against which it has been beating for ages, and then frequently emerges in water-spouts, or fountains, from the opposite end. Thus in the Skerries, one of the Shetland Islands, a deep chasm or inlet, which is open overhead, is continued underground, and then again opens to the sky in the middle of the island. When the tide is high, the waves rise up through this inland aperture, with a noise like the blowing of a whale.

Similar phenomena occur on the south side of the Mauritius,^[15] on the north coast of Newfoundland, near Huatulco on the Mexican coast of the Pacific, and near Peniscola, in Spain, where a cave, through whose roof at storm tides the sea bursts with a terrific noise, has received the name of the ‘Bufador, or the water-spout of Pope Luna,’ the family name of Benedict XIII., who, having been deposed by the Councils of Pisa and Constance, retired to the small Spanish town where he was born. As the chief occupation of the holy father in exile was to vent a continuous torrent of curses and excommunications upon his numerous enemies, it is probable that this circumstance caused his name to be given to the noisy but harmless Bufador.

Though water, aided by time, is probably the chief excavating power, there can be no doubt that the action of subterranean fire has likewise produced, and still produces, many hollows in the hard crust of the earth. Wherever a volcano has been piled up to the skies, the matter ejected from its vents must necessarily have left a void behind, and given rise to corresponding cavities in the space beneath. The shock of an earthquake must frequently rend asunder deep-seated rocks, and the slow upheaval of considerable tracts of land can hardly be supposed to take place without the formation of hollows and crevices.

When the lavas poured forth during an eruption are in a liquid state, they do not form on cooling a compact homogeneous mass, but generally exhibit a porous, spongy texture, due to the bubbles of the vapour generated through, or entangled in, their mass. These bubbles frequently unite in larger volumes, which, influenced by their elasticity and inferior specific gravity, rise towards the surface of the lava as it flows on, and, when sufficiently powerful, raise its crust in dome-like or conical protuberances, which not seldom burst open at the summit, or crack at the sides. The hollows thus formed are often so large as to entitle them to the name of caves.

According to Sir Charles Lyell, the sudden conversion into steam of lakes or streams of water, overwhelmed by a fiery current, may perhaps explain the formation of many of the extensive underground passages or caverns which form a common feature in the structure of a volcano. Great volumes of vapour, thus produced, may force their way through liquid lava, already coated over externally with a solid crust, and may cause the sides of such passages, as they harden, to assume a very irregular outline. The famous cave on Etna, called the Fossa della Palomba, which opens near Nicolosi, not far from Monte Rossi, has not improbably been thus formed. After reaching the bottom of a hollow 625 feet in circumference at its mouth, and 78 feet deep, the explorer enters another dark cavity, and then others in succession, sometimes descending precipices by means of ladders. At length the vaults terminate in a great gallery ninety feet long, and from fifteen to fifty feet broad, beyond which there is still a passage never yet visited, so that the whole extent of the Fossa remains unknown.

The volcanic caves of Punta Delgada in the Island of San Miguel, one of the Azores, are still more grand. Their entrance is through a narrow crevice, which soon, however, expands into an enormous hall, whose vault even the strongest torch-light is unable to illumine. In one spot, an opening in the floor shows that the lava, which is here but a foot thick, forms the roof of a second cave, situated below the first, into which even the boldest explorer has never ventured, but the noise of stones cast into the abyss proves it to be of considerable size. The first-mentioned cave leads into another, the width of which is estimated by Webster^[16] at 120 feet; but the height he was unable to measure. Gradually this cave becomes narrower and lower, until, about 450 feet from the entrance, it terminates in a low vault. Black lava stalactites everywhere hang down from the roof, and the floor is so covered with sharp-sided blocks of the same volcanic material that walking among them becomes a task of the greatest difficulty.

While a lava-stream is flowing along, a slag-crust forms on its surface, inclosing the internal fluid matter as in a canal. But when the supply of fresh lava from the vent diminishes or entirely ceases, the still liquid interior at the central part of the current continuing for some time to flow on, often leaves behind hollow gutters, arched over by a thin and brittle roof—so thin sometimes as to yield to the weight of a person stepping on them. Such vaulted roofs have pseudo-stalactitic projections left by the subsidence of the liquid, and are coated with a glossy varnish. Sometimes, very large caverns are thus formed beneath the surface of a lava-stream, and even rival in their extent and windings the caves worn by water in limestone rocks. The famous Surtshellir,^[17] situated near Kalmanstunga in Iceland, is a remarkable instance of a vast lava excavation owing its origin to this cause. It has very appropriately been named after Surt, the prince of darkness and fire, of the ancient Scandinavian mythology; for this gloomy deity could not possibly have chosen a fitter residence than its vast and dismal halls, once glowing with subterranean fires, and now the seat of perpetual darkness.