Grottoes and caves occur in every kind of rock, in lavas, basalt, slate, and granite, as well as in limestone, dolomite, and gypsum; for the volcanic powers are capable of rending the hardest stone, and the foaming breakers of a turbulent ocean meet with no cliff that is able ultimately to resist their never-tiring assaults. [Illustration: ENTRANCE TO THE CAVE OF ADELSBERG.] But, owing to their great fragility and to the solubility of limestone (carbonate of lime) in water containing carbonic acid, calcareous rocks are more liable than any others to be shattered and undermined, both by volcanic and aqueous causes. Its water readily absorbs carbonic acid gas. Every drop of rain that falls upon the ground necessarily contains some small portion of this gas, which, as we all know, is constantly mixed with the atmosphere, and thus becomes a solvent for chalk; more particularly if the latter, as, for instance, in the Karst Mountains of Carniola, contains some proto-carbonate of iron, which, changing into an oxide when in contact with water, yields its carbonic acid to the percolating fluid, and consequently increases its solvent powers. Hence every shower of rain that filters through the crevices of a limestone rock wears away some part of its mass; and if we consider the vast number of years over which these operations have extended, and add to their effects the transporting powers of the waters on their progress through the subterranean channels which they have excavated or enlarged, we can easily comprehend how in the course of ages whole mountains may be hollowed out.