They are formed by the percolation of carbonated water through the mortar of the joints, the carbonate of lime thus withdrawn being re-deposited where the water reaches the air and evaporates. A little pellicle of lime first gathers on the roof, and every succeeding drop adds to the length of the column. In some cases, where the supply of water is too great for the amount of evaporation, part falls on the floor, and, being there dissipated, leaves behind a slowly-gathering pile of lime called stalagmite. In some of the Eastern grottos, the pillars from the roof have become united to those on the floor, forming the most exquisite and fairy-like combinations of arch and pillar. An example of a calcareous grotto has now become pretty familiar to our summer tourists, under the name of the Spar Cave. It lies on an exposed cliff-line along the western shores of Skye, against which the surge of the Atlantic is ever breaking. You approach it from the sea, and enter a narrow recess between two precipitous walls of rock, open above to the sky, and washed below by the gurgling tide. Crossing the narrow, shingly beach, you find the ground thickly covered with herbage, while, grouped along the dark walls, are large bunches of spleenwort, hart's-tongue fern, and other plants that love the shade. Soon after entering the cave, all becomes sombre and cold; and the few candles, with which the party have furnished themselves, only serve to heighten the gloom. After scrambling on for a time across dank, dripping rocks, and over a high bank of smooth marble, on which it is difficult to creep, almost impossible to stand, you arrive at a deep pool of clear, limpid water, which extends across the cave from side to side, barring all farther passage. The scenery at this point will not readily be forgotten. The roof towers so high that the lights are too feeble to show it, while the walls, roughened into every form of cusp and pinnacle, pillar and cornice, all glittering in the light, resemble the grotto of some fairy dream. On returning again to the light of day, if you ask the cause that has given rise to all this beauty, it will be found a very simple one. The cleft occupied by the cave has been once filled by a wall of igneous rock called a trap-dyke. Atmospheric influences, aided probably by the waves, have caused the decomposition and removal of this intruded rock, and the calcareous sandstone on either side 'now stands up in a wall-like form. The upper part of the dyke remains as a roof to the cave, but it has become completely covered over with the calcareous deposits left by the carbonated water that filters through the adjacent limy sandstone. The amount of water is considerable, and consequently every part of the cavern--roof, walls, and floor--has been incrusted with a white crystalline carbonate of lime. In volcanic countries, where the springs often come to the Surface in a highly heated state, charged, too, with various chemical