We must now consider the condition under which caves become filled up with various deposits. If the velocity of the stream in a water-cave be lessened, the silt, sand, or pebbles it was hurrying along will be dropped, and may ultimately block up the entire watercourse. In bringing this to pass, however, the carbonate of lime in the water plays a most important part. If the excess of carbonic acid by which it is held in solution be lost by evaporation, it immediately reassumes its crystalline form, and shoots over the surface of the pool like plates of ice, or is deposited in loose botryoidal masses at their sides and on their bottoms; and, since the atmospheric water very generally percolates through the crannies in the rock, the sides and roof of the channel, above the level of the water, are adorned with a stony drapery of every conceivable shape. The rate at which this accumulation takes place depends upon the free access of air necessary for evaporation, and is therefore variable,--as in the case of the Ingleborough cave. In all the caves which I have examined there is a free current of air. If a water-channel becomes blocked up by either or both these causes, the joints and fissures in the rock offer an outlet to the drainage, more or less free, at a lower level, as in the Ingleborough cave, Poole’s cave, near Buxton, and many others. Sometimes, however, owing to the increased rain-fall, or to the obstruction of the lower channels, the water re-excavates the old passages, as we shall see to have been the case with the famous caverns of Kent’s Hole and Brixham. In the summer of 1872, a sudden rain-fall not merely opened out for itself a new passage into a swallow-hole close to Gaping Gill, on the flanks of Ingleborough, but forced its way out through the old entrance of the Ingleborough cave, breaking up the calcareous breccia, and removing the large stones in its course. A cave obviously may become dry, either by the drainage passing along a lower level, or by the elevation of the district by subterranean energy. After it has been forsaken by the stream, the particles brought down by the atmospheric water percolating through the joints, tend to fill it up on the surface, and these may be either of clay, loam, or sand. These actions may be studied in this country in the well-known caves of Ingleborough, Buxton, Cheddar, Wookey Hole, and a great many others in Derbyshire, Yorkshire, Staffordshire, Durham, Cumberland, and Wales. _The Cave of Caldy._ [Illustration: FIG. 9.--A View in the Fairy Chamber, Caldy.] [Illustration: FIG. 10.--Stalagmites in the Fairy Chamber, Caldy.] [Illustration: FIG. 11.--The Fairy Chamber, Caldy.]