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.
Fig. 9.—A View in the Fairy Chamber, Caldy.
Fig. 10.—Stalagmites in the Fairy Chamber, Caldy.
Fig. 11.—The Fairy Chamber, Caldy.
Among the most beautiful stalactite caverns in this country is that on the island of Caldy, immediately opposite to Tenby in Pembrokeshire, discovered some years ago in the limestone cliff, and explored by Mr. Ayshford Sanford and the Rev. H. H. Winwood, in 1866, and subsequently by the writer in 1871 and 1872. On creeping through a narrow entrance with an outlook to the sea on a precipitous side of a quarry, a passage leads to a chamber of considerable horizontal extent, the bottom being covered with silt, on which stand pedestals of dripstone from an inch to two feet in length, each rising from a thin calcareous crust which does not altogether conceal the silt below. From it a low entrance leads into a fairy-like chamber, the floor consisting of a rich red, crystalline pavement, perfectly horizontal, and studded here and there with round bosses ([Figs. 9], [10], [11]), either red or snow-white. From the roof hang stalactites offering the same beautiful contrast of colours, forming a delicate canopy of tassels, or passing downwards to the floor and constituting slender shafts about three feet long, and about the diameter of straws. Each of these is hollow, translucent, and more or less traversed by water, and in some places each stood next its fellow, almost as close as the straws in a cornfield. Sometimes the shaft stands on a cone ([Fig. 11]) of dripstone, more or less raised above the floor. Small pools of water occupy hollows in the pavement, each lined with glittering crystals of calcite ([Fig. 12]), which are slowly shooting over the surface, and converting some of the open hollows into bottle-shaped cavities ([Fig. 13]). Their sides and bottoms are covered with a crystalline growth of singular beauty, of which an idea may be formed by [woodcut 14], which represents the edge. Where the drip happened to fall into a shallow pool, it gradually built up for itself a cone, on the lower portion of which the varying water-level is marked by horizontal rings of crystals ([Fig. 15]), and the normal waterline by the upper horizontal plate. Sometimes these were united to the roof by a slender straw-shaft. In [Figure 11] the original shaft has been broken away, and as the direction of the drip has slightly shifted, a new one gradually descended, until finally it became cemented to the side of the cone.