STALACTITES are pendent masses formed where water containing mineral solutions drops very slowly from an elevation. (Gr. from araXao-o-etv, to drip.) They are seen, for example, beneath bridges, arches and old buildings where water percolating through the joints of the masonry has dissolved very small quantities of the lime present in the cement and mortar between the stones. On exposure to the air part of the water evaporates and the solution of calcium carbonate becomes su persaturated; a deposit of this substance ensues and as the drop continues to fall from the same spot a small column of white calcite very slowly grows downwards in a vertical direction from the roof. In the same way stalactites of ice (icicles) are produced in frosty weather as the water dropping from eaves of buildings, elevations, branches of trees, etc., very gradually, freezes. Other minerals often occur in stalactitic growths ; thus we find in mines and in the cavities of mineral veins stalactites of limonite, opal, chalcedony and gibbsite. These are never of great size, usually not more than 2 or Sin. in length, and probably the method of origin is exactly the same as that of the larger and more com mon stalactites of ice and calcite.
The conditions essential to the perfect development of stalac tites appear to be (I) a very slow trickle of water from a fissure; (2) regular evaporation; (3) absence of disturbance, such as cur rents of air. Hence, ice stalactites form most readily on calm cold nights, and stalactites of ice or calcite are seen in greatest per fection in the interior of caves.
In limestone caves stalactites form in abundance as glimmering white columns covered with a thin film of water. The great caves, such as those of Postumia (in Italy), Jenolan (Australia), the Mammoth Cave (Kentucky), the Causses district in France, and the grottoes of Belgium, are divided into chambers which are richly festooned with stalactites, and fanciful names are given to various groups according to their similarity to different objects, natural or artificial. Ice caves of considerable size occur in the Arctic and Antarctic regions, and are draped with ice stalactites often wonderfully like those of limestone caves.
Where the water drops upon the floor of one of these caves evaporation still goes on and if the air be perfectly still the drop will always land on the same place and a pillar of deposit, called a stalagmite, will rise vertically, till in course of time it meets and joins with the stalactite above. As the stalactites thicken, they
assume tapering forms with irregular surfaces. Large stalactites may be 3 or 4f t. thick, but in that case they are usually due to coalescence of adjacent ones. Single stalactites 2ft. in diameter are not rare. From data obtained by measurement of the rate of growth at the present day it has been estimated that as much as 200,000 years may have elapsed since certain thick stalactites began to grow. Caves are of great antiquity but there is, of course, no certainty that the conditions have remained uniform. Sir Archibald Geikie records that stalactites I2in. in diameter had formed beneath a bridge in Edinburgh which was I oo years old; in caves, however, the rate of formation is rarely so great as this. Inscriptions on stalactites in the Adelsberg cave after 3o years had been covered with a scarcely perceptible film of new deposit. In one of the Moravian caves a stalactite, about as thick as a goose quill, was broken across in 188o, and in 1891 it had grown 3 or 4cm. ; from careful observations it has been calculated that one of these stalactites, 7f t. long, may have been formed in 4,000 years. The stalagmitic crust on the floor of caves is usually mixed with blocks which have fallen from the roof, sand, mud and gravel carried in by floods, and the bones of animals and men which have inhabited the cave if it had an accessible entrance.
Stalactites also occur in the interior of the lava caves found in Hawaii, Samoa, etc. Often the upper surface of a lava flow has cooled to form a crust, while the interior is still perfectly fluid, and it sometimes happens that the liquid basalt has made its escape, leaving great cavities below the hollow roof of the lava. The interior of these caves is covered with a black shining film of glassy basalt, and black stalactites of lava hang downwards. Their surface is sometimes changed to brown or red by the action of the acid vapours which filled the cave after the lava retired. These stalactites are tubular, with bluntly rounded ends, and probably their mode of growth is somewhat analogous to that of icicles.