Limestones and Marbles.

In a former Article an account was given of those curious and rather remarkable deposits of chalk which are so abundant and characteristic in many parts of the south and east of England. Chalk is, after all, only a particular form of limestone, although so different in some respects as to deserve special consideration; but there is much to be said of general interest about the other kinds, often known under different names, but always curious, and available for many very useful purposes. They are much more widely spread than chalk, and their value is recognised in all countries.

Limestones and marbles differ from chalk in being harder and more compact, and having a much less earthy texture. While buried for ages in the earth, they have become to a certain extent altered, by a process which, if continued long enough, would turn them into crystalline minerals. Many of the limestones, indeed, are already partly crystalline, either here and there in the bulk of the rock, or else in cavities. Crystals are often seen shot out, as it were, from the walls of a cavity, although all around there is no apparent change in the mixed fragments of shells and fine particles of sand of which the stone was evidently made up. In proportion as limestones are of closer grain, of firmer texture, and more compact, they approach to the condition called semi-crystalline, and thus they pass by successive stages into marbles, which are true crystalline limestones.

Limestone is, as every one knows, a common mineral enough in England, very large parts of the country consisting of little else. Vast quantities are also found not only in most parts of Europe, but generally throughout the world. There are, however, some large districts in which this rock is absent, and others where it is not to be obtained very near the surface; and as lime is wanted not only for building purposes, as stone or for mortar, but for mixing with and forming part of soils, the places where limestones are rare generally suffer much from the want of them. When it is remembered that all bone consists chiefly of lime, and that birds require it to form a hard coating for their eggs, the necessity of this mineral will be seen. It is useful to know where to find it, and how to recognise the varieties.

There are two or three very different minerals generally called limestones. One is the common carbonate of lime-such as Bath and Portland stones-passing into marble, and having the same composition, with the addition of some impurities. Another is the magnesian limestone, or dolomite, a mixed carbonate of lime and magnesia; also a building stone, and unfortunately very notorious, as being the stone of which the Houses of Parliament were built, concerning whose early decay there is so much discussion at present. Gypsum, or alabaster, much used in a burnt state in making plaster of Paris and other varieties of plaster, and occasionally

sculptured as an ornamental stone, is a third kind; but this is a sulphate, not a carbonate of lime, and has therefore a distinct chemical composition. The two first varieties when burnt yield a hardish compact substance, known as quick-lime; but the last yields plaster of Paris, an extremely fine powdery material, having very different properties. Quick-lime mixed with water heats, swells, and falls to powder; but plaster of Paris absorbs water, and immediately sets, and becomes permanently hard. The difference between the common and magnesian limestones and gypsum is therefore practically very essential.

Many of the limestones, such as those used for building, are of a grayish and dirty white, or cream colour, of very uniform texture, and tolerably hard; some, like those found in Derbyshire and Devonshire, are compact enough to take a high polish, but being coloured and veined are of no value for artistic purposes, though much used for furniture and decoration, for which they are well adapted. Others, again, such as the Carrara and Parian marbles, are used in sculpture, and are known as statuary marbles. These latter are of the most exquisite white tint, and show a texture like loaf-sugar, or even sometimes like virgin wax.

Of building stones there is indeed an immense variety; some of them being hard and others soft, some brittle and others tough, some full of shells, some made up of little round egg-shaped particles, like the roe of a fish (oolite or roe-stone), some of sandy grains easily separated. Most of them absorb water very readily, and in large quantity; and when exposed in buildings where the weather affects them, and where they are exposed to wet and dry, heat and cold, they are very apt to become rotten, and the sculptured and ornamental parts break off.

In England it is rare to find limestones of a dead white colour, like chalk, and at the same time extremely hard and close grained. Such limestones, however, are common enough in other countries, and are very valuable materials for construction, as they are handsome and durable, absorbing but little water. Each country possesses its own materials, and those of one district are by no means always, nor are they usually, identical with those of another.

Most of the common limestones lie in beds of moderate thickness, separated from each other by an intermediate bed of clay or rubbish, or of stone valueless for building purposes. Very often these beds lie horizontally, or nearly so, and they are almost always parallel to each other; but occasionally they are tilted at a high angle,-a position that must have been produced by some force lifting them up from below after they had been hardened; and generally in such cases the beds are broken asunder, and are more or less rotten near the point where the elevation took effect. When limestones lie on the flanks of mountains, or form mountain masses reaching to the clouds, it is not difficult to see and understand the mode of action of the force, and we may even judge of its magnitude; but when a large district is affected by moderate elevations, it is not so easy to trace the cause.

All limestones when in the earth contain a good deal of water, and they are softer and more easily chiselled when just removed from the quarry than after a few months' exposure. When left exposed to dry air, the stone dries, and a hard crust forms upon it, which resists the action of weather; but if used at once, and subject to the pressure which must act upon all stones in a building before the stone has had time to consolidate, the weather will generally have much more effect upon it.

Most of the stones in a quarry very near the top are more cracked and destroyed than those taken from some depth; so that many quarries now are completely underground, the stone being worked out from the bed within the bowels of the earth, just as coal is removed from the mine. One result of this method is, that with care the best bed and the best part of a bed of stone may be secured; but it is necessary to take precautions that the stones thus brought out are properly dried before use, as they will have undergone no chance of weathering until removed into the open air.

An old quarry, and a quarry where the stone is got in the open air, is a picturesque object enough: the steep face, the successive steps as one bed is worked in advance of another, the vegetation bursting out from all the cracks and corners, and the half-decayed weathered look of the parts where no work has been going on for some time, are all objects on which the eye rests with pleasure. In one place a huge crane is lifting large blocks to a truck, in another a puff of smoke marks where a recent blast has taken place; while the approaches to the quarry, with their rough roads and broken rails, form a contrast with the surrounding scenery which is eminently favourable to the picturesque.

Those quarries where the whole or most of the work is carried on underground and out of sight, are far less interesting. Opening often on the bank of a river or canal, nothing is to be seen but a small tunnel or entry, the wagons bringing out the stone already reduced to convenient sizes, and ready to put on the boats lying alongside. Still, even here, the eye rests with pleasure on a certain contrast of nature with art, which rarely fails to produce some effects pleasing to the lover of the picturesque. In many limestone rocks of large extent there are caverns or open spaces communicating with the outer world. Among such caverns, those of Adelsberg in Carinthia, on the road from Vienna to Trieste, and the so-called Mammoth Cave of Kentucky, in America, are the largest that have been described. The Peak Cavern in Derbyshire, the caverns in Yorkshire, Somersetshire, and South Wales, those in Sicily, Central France, and Bavaria, and others in the rock of Gibraltar, are well known as objects of curiosity, visited by strangers, and some of them afford very curious facts for consideration. These are of two kinds, one having reference to the mechanics of the cavern,-the way it was hollowed out and has been partly filled up with those wonderful appearances called stalactites, and the other to the remains of animals found frequently on and beneath the earthy floor of the cave.

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 imagined 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,

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, compactness, and mode of resisting or yielding to the action of weather.