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.
Fig. 16.—Basin containing Cave-pearls.
Fig. 17.—Fungoid Structures, magnified.
In the principal chamber in the cave, which is very nearly free from drip, the upper surfaces of the stones and stalagmites on the floor are covered with a peculiar fungoid-like deposit of calcite, consisting of rounded bosses, attached to the general surface by a pedicle (see [Figs. 17], [18]) sometimes not much thicker than a hair. They stood close together at various levels, following the inequalities of the surface of attachment, and being on an average about 0·2 inch long. Several microscopical sections ([Fig. 17]) showed that each was formed originally on a slight elevation of the general surface, which would cause a greater evaporation of water than the surrounding portions, and therefore be covered with a greater deposit of calcite. This process would go on until the height was reached to which the water slowly passing over the general surface would no longer rise. Hence the remarkable uniformity of the height of the bosses. The evaporation is greater at the point furthest removed from the general surface, and therefore the apex is larger than the base (see [Fig. 17]). In [Figure 18] they stand as thickly together as trees in a virgin forest, and are developed in greatest vigour where the small eminences cause a greater evaporation than the small depressions, and are stoutest and strongest at the free edges. Some of the pedicles, as in the figure, present traces of erosion, the outer layers having been eaten away by acid-laden water.
Some of these singular little bosses may have been moulded on minute fungi, such as those in the cave of Ingleborough, but their presence is not revealed by the microscope.
The Black-rock Cave, near Tenby.
Fig. 18.—Fungoid Structure, Black-rock Cave.