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The extent of caverns is sometimes very great, and often quite incapable of being accurately determined. The more interesting of them consist of a multitude of passages, often narrow, too small, indeed, to allow a human being to penetrate, but connecting numerous large open cavities lying far within the rock. These are often more or less occupied by water, which enters and gets out of them in a manner scarcely traceable. The open spaces are sometimes large and lofty and well ventilated, but sometimes they are smaller and nearly choked. Nothing, indeed, can be ima. gined more irregular than this chain of connected cavities running perhaps for miles under the earth, at various levels, with no reference to any plan or system that we can at all trace. Almost all limestones are cavernous, but some kinds much more so than others.
Within the cavern, the walls of which are generally worn and often smoothed, as if by the passage of water, there are often sheets, columns, and pinnacles of stone which, when undimmed by the smoke of lamps and torches, are half transparent, and of the most brilliant yellowish-white appearance. These hang down from the roof, rise up from the floor, arrange themselves fantastically as curtains, tables, or festoons, and even take the forms of animals and human beings. They are of precisely the same material as the limestone walls of the cavern, but are easily seen to be of different origin.
By a little examination, it may be found that all of them have been formed in connection with the drip of water; the water, penetrating through innumerable small fissures in the limestone rock, takes up a part of the mineral, and carries it along, eating away its course, partly chemically, by dissolving the rock, and partly mechanically, by constant rubbing. When it reaches an open empty cavity where there is a current of air, the water is evaporated, and the stone left behind.
Such is the history of that variety of curious and beautiful appearances seen in caverns : the magic fountains and organs; the cathedral aisles and vaulted roofs; the drooping trees; the crouching animals; the busts; and the apparent vegetation. All these are nothing more than fantastic forms, slowly and gradually accumulated; and the wonderful things told about them are due quite as much to the fancy of the describer as to Nature herself.
The floors of such caverns are often nearly level and hard, being repetitions of the same half-crystalline material, and produced in the same way. The sheets of limestone on the floor of the cavern are sometimes called stalagmite, to distinguish them from the stalactites that drop from the roof. In the limestone floor, in the mud under it, and often in heaps not yet covered with stalagmite, there have been found in many caverns numerous bones of quadrupeds. Some of these were no doubt savage animals that had used the cavern as a den; some were certainly the prey of wolves, hyænas, and other carnivorous tenants, which they had dragged into their lair, perhaps for the benefit of their young; some again seemed to have been carried into the cave and buried there when unusual floods of water had drifted river deposits, mud, bones, and other material from a distance,
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leaving it behind in these sheltered places after the waters had retired. When the bones thus found are carefully examined and compared with those of known species, they are found to belong to races that are no longer common in the adjacent country. Thus in England and Western Europe there are bones of hyænas and large bears, of a kind of tiger, and of many other fierce carnivorous creatures, only met with at present in Asia and Africa. In Brazil, under similar circumstances, are bones of wild animals equally different from the inhabitants of the neighbouring tropical or temperate land; and in Australia there have been found remains of kangaroos and wombats much larger than, and very different from, those of the present neighbourhood.
With the carnivorous monsters in our English caverns are numerous bones of elephants and rhinoceroses, and even of hippopotamuses, mixed with fragments of rein-deer, and of a very large horned deer, long since lost sight of among existing races, as well as of large animals of the ox tribe. Every thing indicates great antiquity, and a very different climate; and yet with these strange associates are seen chiselled flints (evidently human weapons), all buried at the same time. The condition of the bones, the great proportion of some one species in each cavern, the number of bones and teeth of young individuals not at all more injured than the harder bones near them, and the fact that many of the bones of the deer and oxen are much gnawed, as if by the teeth of hyænas, in the hyæna caverns, and not at all so marked where the bones indicate that the cavern had been otherwise owned, -all prove that the caverns had long served as the dens of these wild and powerful animals.
Limestone presents itself in nature under very different aspects. Crossing England diagonally, and, owing to various causes, not much developed near the sea, the peculiar features of limestone cliffs are not much seen on our shores; but in the interior there are many fine, and some noble, specimens of limestone-cliff scenery. In the beautiful and wild valleys of the north-western part of Yorkshire, in the Peak district, and some of the river-valleys of Derbyshire, especially near Matlock and along the course of the Dove, the bold vertical faces of compact limestone rock are grand and picturesque in the extreme. Something of a similar beauty characterises the Cheddar rocks and others adjacent in the Mendips; and still more remarkable are the deep narrow gorges and richly-clothed ravines of Linton, in North Devon.
These specimens of scenery are well contrasted by the hilly parts of the middle of England, especially near Cheltenham and Bath, where a much softer rock of the same nature but very different texture presents a correspondingly different appearance. No one who did not carefully examine for himself would suppose that the hills of Gloucestershire and Derbyshire were composed of the same mineral; for it is difficult for two minerals to be more distinct from one another than are the limestones in these localities in respect to colour, hardness, compactress, and mode of resisting or yielding to the action of weather.
Almost all limestones may be found, on a little investigation, to be made up of beds of various thickness. Between the beds there is often a thin plate of some other material, or of the same material in a different form. All without exception of the limestone rocks are also more or less cracked near the surface, so that water has access to the interior; and thus it is not to be wondered at that numerous springs of water come out wherever these beds are cut off abruptly on a hill-side or cliff, and that in all limestone districts the surface is dry, while water may generally be had by digging
The minerals found between beds of limestone, or mixed up with fragments of limestone in cavities and cracks within the substance of the rock, are of great value. Almost all the lead and zinc used in commerce are found in the shape of ores or stony minerals under these circumstances; and often at first sight it would be difficult for any one not familiar with minerals to distinguish some of the most valuable of these ores from valueless stones. Even iron is found sometimes in enormous quantities mixed up with limestone, and looking so much like it, that for a long time it has been regarded as a mere variety. Tens of thousands of tons of iron-ore are now obtained annually from beds that a few years ago were looked upon only as poor and worthless limestones.
While chalk is made up almost entirely of the very small shells of animals, of which thousands would be required to bury a pin's head, most of the harder limestones are equally remarkable for fragments of shells and corals, and other hard coatings of animals which are for the most part of much larger size. Such common shells as are cast up on every seabeach will in time, and by long accumulation, occupy an important place even among rocks; for with water constantly running through them, they become at last firmly cemented together. But it is chiefly by the work of the coral animal that large mountain masses of limestone are now obtained. Coral, especially certain varieties abounding in some tropical and warm seas, consists of a curious mass of individuals, with one common gelatinous living substance, connecting them. The whole mode of growth is more like that of a plant than an animal, the individuals resembling the leaves and flowers, while the common central mass is the stem or trunk. These curious animals are able with great facility to separate from the salt water about them those few grains of limestone they require, and these are immediately replaced, because the sea washes over limestone, and quickly sucks up as much as it needs when deprived of any part by animals. There is thus a never-failing supply of lime in the ocean, and the little creatures we refer to have been pilfering in safety from the very creation of the world till now. What they take, however, they at once use, building up with it a strong and almost imperishable framework, which, so long as it is coated with living matter, does not become worn or wasted. When myriads of these individuals collect together in a mass and secrete limestone, they soon construct a wall which rises to the lowest level of low water, commencing at the bottom of the sea at some moderate depth. If every thing remains in the same state, a fringe of coral soon forms round every part of an island, or along the whole of a coast that has once been reached by the animals, except, indeed, where an interruption exists, such as a river or stream of fresh water of some magnitude entering the sea, or some extraordinary and abrupt submarine cavity prevents the advance of the coral by depriving it of a support to build upon. Such fringes of coral are well known in some parts of the world, especially within the tropics and in the warmer waters both of the Atlantic and Pacific Oceans.
But besides these fringes of coral, which when the animal dies may, if they are not washed away, become coral limestone, there are other cases in which the coral is found to extend to a very much greater depth than the larger kinds are thought able to work in, the extreme depth of coral having been found to amount to many hundreds of fathoms, whereas the larger corals are believed to die if removed even twenty fathoms deep. In some way or other it certainly seems that coral-beds can even now be prepared of great thickness, as well as covering a wide surface; and if this is the case at present, there is no reason why a similar result should not have been obtained formerly.
Many of those remarkable and extensive limestones, to which we are indebted for much of the picturesque in English scenery, were beyond doubt the composition of little animals just such as those now building coral in warm latitudes. The results may be seen and compared together, and in many cases the whole form of construction and peculiarities of the animals may be identified. We are thus bound to admit the close resemblance between the present and past manufactures of the modern coral animal, and those ancient and crystalline limestones that are now quarried for building-stone, or carefully selected and set aside as furnishing marble for some of our most elegant manufactures, or the noblest efforts of genius.
Limestones form the staple material of the flanks of the Alps, and extend in still greater abundance eastwards into Asia, scarcely any thing else being seen in the Carpathians and the Caucasus. A large part of Italy and Greece is also made of, or covered with similar rock. In these latter countries marble often replaces common limestone, and this seems owing to the vicinity of a great disturbing force, exemplified in the elevation of the mountain-chains to the north, and the frequent eruption of melted rock and steam from the important volcanoes of the south of Europe. Spain is not less remarkable for its marbles and limestone rocks than France and Italy, but they are less known, owing to the rarity (till lately) of any commercial or industrial activity in that country.
The second kind of limestone mentioned at the beginning of this Article, that kind which is known to mineralogists under the names gypsum and alabaster, is very different, in many respects, from the stones just described. It is not often in regular beds of great extent, being far more commonly found in large detached lumps. It is very much softer than any of the building stones, and either of a dead white appearance, like chalk (but not soiling a black surface), or else nearly transparent. It is
more frequently found with sandstones than limestones. Large quantities of it are obtained from Derbyshire and Nottinghamshire in England, and from deposits around Paris, and in Italy, near Florence. Egypt also abounds with it. When pure, whether white or transparent, it is often cut into vases, lamps, figures, and other ornaments, some of which are of great beauty and value; but for the most part it is burnt, and in that state sold to make plaster of Paris and other plasters by various treatment.
There is another kind of limestone quite different from either of the two described, and often so hard as to be mistaken for flint. It is found in pebbles in large quantity in the gravels of Suffolk, and in beds in some other parts of England. It consists of rolled bones and other remains of animals, and is called by chemists a phosphate of lime. When properly prepared, this stone serves as an admirable mineral manure or dressing for land for agricultural purposes.
The pebbles thus used were at one time called coprolites (abbreviated to cops), owing to an idea entertained that they consisted of petrified dung. Among them, however, we often meet with such things as earbones of whales, parts of the back-bone of whales and sharks, bones of quadrupeds, and other things that point to them as being for the most part, like the common limestones, due to animal life in its more complete form.
Such is an outline of some of the principal facts known about limestones. We may sum up the account in a few words, by saying that they are a group of stones of great usefulness to man, in almost all their varied forms, both directly and indirectly; they are widely spread, they form very picturesque and characteristic scenery, and they contain mineral wealth. Most of them are directly due to the influence of animal life at some period or other, often very remote; but they have since undergone a good deal of change, and hardly now resemble what they once must have been. Some, no doubt, are still soft and little altered; but most kinds have been converted into a substance which, though greatly altered, still shows its origin very clearly, or into a curious half-crystallised mass, which gives no clue at all by its appearance to the history of its forma