=Swallow holes and limestone sinks.=—Above the caverns of limestone formations there are selected points where the water has descended in the largest volume, and here funnel-shaped depressions have been dissolved out from the surface of the rock. In different districts such depressions have become known as “sinks”, “swallow holes”, _entonnoirs_, and _Orgeln_. Wherever the depressions have a characteristic circular outline, there can be little doubt that they are the product of solution by the descending water, and have relatively small connections only with the subterranean caverns. They have thus naturally collected upon their bottoms the insoluble clay which was contained in the impure limestone as well as a certain amount of slope wash from the surface. Inasmuch as the clays are impervious to water, the bottoms of these swallow holes are better supplied with moisture than the surrounding rock surfaces, and by nourishing a more vigorous plant growth are strongly impressed upon the landscape (Fig. 193). [Illustration: FIG. 192.—Plan of a portion of Mammoth Cave, Kentucky (after H. C. Hovey).] [Illustration: FIG. 193.—Trees and shrubs growing luxuriantly upon the bottoms of sinks within a limestone country (after a photograph by H. T. A. de L. Hus).] Certain of the depressions above caverns are, however, less regular in outline, and their bottoms are occupied by a mass of limestone rubble. In some instances, at least, these depressions appear to be the result of local incaving of the cavern roofs. An incaving of this nature may close up an earlier gallery in the cavern and divert the cave waters to a new course. The destruction of the roofs of caverns through this process of incaving may continue until only relatively small remnants are left. From long subterranean tunnels the caves are thus transformed into subaërial rock bridges that have become known as “natural bridges.” The best-known American example is the Natural Bridge near Lexington, Virginia. Much grander natural bridges have been formed in sandstone by a totally different process, and must not be confused with these limestone remnants of caverns. =The sinter deposits.=—Just as water can dissolve the calcareous rocks with the formation of caverns, it can under other conditions deposit the material which has thus been taken into solution. Its power to hold carbonate of lime in solution is dependent upon the presence of carbonic acid gas within the water. Water charged with gas and dissolved lime carbonate is said to be “hard”, and if the gas be driven off by boiling or otherwise, the dissolved lime is thrown out of solution and deposited in a form well known to all housekeepers.