Note:  Do not rely on this information. It is very old.

Stalactiteand Stalagmite

Stalactite and Stalagmite, the names arbitrarily applied respectively to icicle-like mineral deposits formed by the evaporation of a dripping solution, and to crystalline deposits of similar origin in layers on a more level surface below. Various minerals occur in stalactitic form, such as chalcedony, iron-pyrites, and baryte, whilst kidney-iron and malachite are practically stalagmitic; but the most familiar example of the process lines of joint on roofs of limestone caverns and the layers of the same composition on their floors. As each drop gathers on the roofs and begins to evaporate and lose carbonic acid, the excess of carbonate which it can no longer retain is deposited round its edges as a ring. Drop succeeding drop lengthens the original ring into a long pendent tube, which, by subsequent deposit outside, becomes thickened and may after reaching the floor, be indefinitely increased in diameter. At first the calcareous deposit is soft; but it becomes crystalline, fibrous crystals radiating outwards from the central tube. The further evaporation of the water which drips on to a comparatively level floor forms the more distinctly crystalline stalagmite, which rises from its regular layers into ridges and pinnacles under the rows of stalactites. The rate at which the deposits take place depends upon rainfall, porosity, etc., and cannot be used as a safe measure of time.