of calcareous water (Fig. 20). But the deposition on the roof does not exhaust the stock of dissolved carbonate. When the drops reach the ground the same process of evaporation and precipitation continues. Little mounds of the same white substance are built up on the floor, and, if the place remain undisturbed, may grow until they unite with the stalactites from the roof, forming white pillars that reach from floor to ceiling (Fig. 19, and p. 154).

It is in limestone caverns that stalactitic growth is seen on the most colossal scale. These quiet recesses having remained undisturbed for many ages, the process of solution and precipitation has advanced without interruption until, in many cases, vast caverns have been transformed into grottoes of the most marvellous beauty. White glistening fringes and curtains of crystalline carbonate of lime, or spar, as it is popularly called, hang in endless variety and beauty of form from the roof. Pillars of every

dimension, from slender wands up to thick-ribbed columns like those of a cathedral, connect the roof and the pavement. The walls, projecting in massive buttresses and retiring into alcoves, are everywhere festooned with a grotesque drapery of stone. The floor is crowded with mounds and bosses of strangely imitative forms which recall some of the oddest shapes above ground. Wandering through such a scene, the visitor somehow feels himself to be in another world, where much of the architecture and ornament belongs to styles utterly unlike those which can be seen anywhere else.

The material composing stalactite and stalagmite is at first, as already stated, a fine white chalky pulp-like substance which dries into a white powder. But as the deposition continues, the older layers, being impregnated with calcareous water, receive a precipitation of carbonate of lime between their minute pores and crevices, and assume a crystalline structure. Solidifying and hardening by degrees, they end by becoming a compact crystalline stone (spar) which rings under the hammer.

The numerous caverns of limestone districts have offered ready shelter to various kinds of wild animals and to man himself. Some of them (Bone-Caves) have been hyæna-dens, and from under their hard floor of stalagmite the bones of hyænas and of the creatures they fed upon are disinterred in abundance. Rude human implements have likewise been obtained from the same deposits, showing that man was contemporary with animals which have long been extinct. The solvent action of underground water has thus been of the utmost service in geological history,

first, in forming caverns that could be used as retreats, and then in providing a hard incrustation which should effectually seal up and preserve the relics of the denizens left upon the cavern-floors.

Calcareous springs, issuing from limestone or other rock abounding in lime, deposit carbonate of lime as a white precipitate. So large is the proportion of mineral contained by some waters that thick and extensive accumulations of it have been formed. The substance thus deposited is known by the name of Calcareous Tufa, Calc-sinter, or Travertine. It varies in texture, some kinds being loose and crumbling, others hard and crystalline. In many places it is composed of thin layers or laminæ, of which sixty may be counted in the thickness of an inch, but bound together into a solid stone. These laminæ mark the successive layers of deposit. They are formed parallel to the surface over which the water flows or trickles, hence they may be observed not only on the flat bottoms of the pools, but irregularly enveloping the walls of the channel as far up as the dash of water or spray can reach. Rounded bosses may thus be formed above the level of the stream, and the recesses may be hung with stalactites.

The calcareous springs of Northern and Central Italy have long been noted for the large amount of their dissolved lime, the rapidity with which it is deposited, and the extensive masses in which it has accumulated. Thus at San Filippo in Tuscany, it is deposited in places at the rate of one foot in four months, and it has been piled up to a depth of at least 250 feet, forming a hill a mile and a quarter long, and a third of a mile broad. So compact are many of the Italian travertines that they have from time immemorial been extensively used as a building stone, which can be dressed and is remarkably durable. Many of the finest buildings of ancient and modern Rome have been constructed of travertine.

A familiar feature of many calcareous springs deserves notice. here. The precipitation of calc-sinter is not always due merely to evaporation. In many cases, where the proportion of carbonate of lime in solution is so small that under ordinary circumstances no precipitation of it would take place, large masses of it have been deposited in a peculiar fibrous form. On examination, this precipitation will be found to be caused by the action of plants, particularly bog-mosses which, decomposing the carbonic acid in the water, cause the lime-carbonate to be deposited along their stems and leaflets. The plants are thus incrusted with sinter

which, preserving their forms, looks as if it were composed of heaps of moss turned into stone. Hence the name of petrifying springs often given to waters where this process is to be seen. There is, however, no true petrifaction or conversion of the actual substance of the plants into stone. The fibres are merely incrusted with travertine, inside of which they eventually die and decay. But as the plants continue to grow outward, they increase the sinter by fresh layers, while the inner and dead parts of the mass are filled up and solidified by the deposit of the precipitate within their cavities.

A growing accumulation of travertine presents a special

interest to the geologist from the fact that it offers exceptional facilities for the preservation of remains of the plants and animals of the neighbourhood. Leaves from the surrounding trees and shrubs are blown into pools or fall upon moist surfaces where the precipitation of lime is actively going on (Fig. 21). Dead insects, snail-shells, birds, small mammals, and other denizens of the district may fall or be carried into similar positions. These remains may be rapidly enclosed within the stony substance before they have time to decay, and even if they should afterwards moulder into dust, the sinter enclosing them retains the mould of their forms, and thus preserves for an indefinite period the record of their former existence.

Chalybeate Springs.—A second but less abundant deposit from springs is found in regions where the rocks below ground contain decomposing sulphide of iron (p. 137). Water percolating through such rocks and oxidising the sulphur of that mineral, forms sulphate of iron (ferrous sulphate) which it removes in solution. The presence of any notable quantity of this sulphate is at once revealed by the marked inky taste of the water and by the yellowish-brown precipitate on the sides and bottom of the channel. Such water is termed Chalybeate. When it mixes with other water containing dissolved carbonates (which are so generally present in running water), the sulphate is decomposed, the sulphuric acid passing over to the lime or alkali of the carbonate, while the iron takes up oxygen and falls to the bottom as a yellowish-brown precipitate (limonite, p. 129). This interchange of combinations, with the consequent precipitation of iron-oxide, may continue for a considerable distance from the outflow of the chalybeate water. Nearest the source the deposit of hydrated ferric oxide or ochre is thickest. It encloses leaves, stems, and other organic remains, and preserves moulds or casts of their forms. It also cements the loose sand and shingle of a riverbottom into solid rock.

Siliceous Springs.—One other deposit from spring-water may be enumerated here. In volcanic regions, hot springs (geysers) rise to the surface which, besides other mineral ingredients, contain a considerable proportion of silica (p. 117). This substance is deposited as Siliceous Sinter round the vents whence the water is discharged, where it forms a white stone rising into mounds and terraces with fringes and bunches of coral-like growth. Where many springs have risen in the same district, their respective sheets of sinter may unite, and thus extensive tracts are buried under the deposit. In Iceland, for example, one of the sheets is said to be two leagues long, a quarter of a league wide, and a hundred feet thick. In the Yellowstone Park of North America, many valleys are floored over with heaps of sinter, and in New Zealand other extensive accumulations of the same material are to be found. It is obvious that, like travertine, siliceous sinter may readily entomb and preserve a record of the plants and animals that lived at the time of its deposition.

Summary. The underground circulation of water produces changes that leave durable records in geological history. These changes are of two kinds. (1) Landslips are caused, by which the forms of cliffs, hills, and mountains are permanently altered.

Vast labyrinths of subterranean tunnels, galleries, and caverns are dissolved out of calcareous rocks, and openings are made from these passages up to the surface whereby rivers are engulfed. Many of the caves thus hollowed out have served as dens of wild beasts and dwelling-places for man, and the relics of these inhabitants have been preserved under the stalagmite of the floors. (2) An enormous quantity of mineral matter is brought up to the surface by springs. Most of the solutions are conveyed ultimately to the sea where they partly supply the substances required by the teeming population of marine plants and animals. But, under favourable circumstances, considerable deposits of mineral matter are made by springs, more especially in the form of travertine, siliceous sinter, and ochre. In these deposits the remains of terrestrial vegetation, also of insects, birds, mammals, and other animals, are not infrequently preserved, and remain as permanent memorials of the life of the time when they flourished.