The history of these structures is very evident. The straw-like stalactites were formed by the evaporation of the carbonic acid from the surface of each drop of water, as it accumulated in one spot, and the consequent deposit of carbonate of lime around its circumference. It could not be formed in the centre, because of the continual movement of the successive drops in falling. By a circumferential growth of this kind a small crystal tube, of the diameter of a drop, is slowly developed, which continues to lengthen until the result is one of the straw-columns, with a hole in the centre for the passage of the water, which cannot readily part with its carbonic acid till it arrives at the end of the tube. Sometimes the hole has been subsequently blocked up by calc-spar, or the general surface been covered over with successive layers, until it becomes a mass of considerable diameter. If the drop fell into a deep pool, the straw-column was continued down to the water-line; if in shallow water, or on the floor, a pedestal was built up, as is represented in the preceding figures. The crystallization going on in the pools is greater at the surface than below, because of the greater evaporation, and consequently the stalagmitic film is gradually extending over it on every side from the edges (Figs. 12, 13). As I broke my way into some of the unexplored recesses, through the thickly planted straw-shafts, and scene after scene of fairy beauty, unsullied by man, opened upon my eyes, the ringing of the fragments on the crystalline floor that accompanied almost every movement made me feel an intruder, and sorry for the destruction. In some places, where the drip was continuous, and the calcareous basin which it had built up for itself shallow, small spherical bodies of calcite were so beautifully polished by friction in the agitated water, that they deserve the name of cave-pearls from their lustre. In Fig. 16 I have represented a tiny basin with its pearly contents. Where the drip had ceased to be continuous each of these formed a nucleus for the deposit of calcite crystals, by which they were united to the bottom of the basin. [Illustration: FIG. 16.--Basin containing Cave-pearls.] [Illustration: FIG. 17.--Fungoid Structures, magnified.]