But before proceeding any further, it is necessary to inquire in what way these caverns could have been formed, in which we find accumulated so many relics of the existence of primitive man. M. Desnoyers, Librarian of the Museum of Natural History at Paris, is of opinion that these caverns are crevices of the same class as metalliferous _lodes_, only instead of containing metallic ores they must have been originally filled by the deposits of certain thermal springs. [Illustration: Fig. 19.--Theoretical Section of a Vein of Clay in the Carboniferous Limestone, _before_ the hollowing out of Valleys by diluvial Waters.] Fig. 19 represents, according to M. Desnoyers' treatise on _caverns_, one of these primordial veins in the carboniferous limestone. At the time of the diluvial inundation, these veins were opened by the impetuous action of the water. When thus cleared out and brought to the light of day, they assumed the aspect of caves, as represented in fig. 20. [Illustration: Fig. 20.--Theoretical section of the same Vein of Clay converted into a Cavern, _after_ the hollowing out of Valleys by diluvial Waters.] The European diluvial inundation was, as we know, posterior to the glacial epoch. It is also likely that caverns were sometimes produced by the falling in of portions of some of the interior strata, or that they were formerly the natural and subterranean channels of certain watercourses; many instances of this kind being now known in different countries. We must also add that it is not probable that all caverns originated in the same way; but that one or other of the several causes just enumerated must have contributed to their formation. Under the general denomination of _caverns_, all kinds of subterranean cavities are comprehended; but it will be as well to introduce several distinctions in this respect. There are, in the first place, simple clefts or crevices, which are only narrow pits deviating but slightly from the vertical. Next we have grottos (or _baumes_ as they are called in the south of France), which generally have a widely opening inlet, and are but of small extent. Lastly, we must draw a distinction between these and the real bone caverns, which consist of a series of chambers, separated by extremely narrow passages, and are often of very considerable dimensions. Some of these caverns occupy an extent of several leagues underground, with variations of level which render their exploration very difficult. They are generally very inaccessible, and it is almost always necessary to ply the pick-axe in order to clear a way from one chamber to another. In most of these grottos and caverns the ground and sides are covered with calcareous deposits, known by the name of _stalactite_ and _stalagmite_, which sometimes meet one another, forming columns and pillars which confer on some of these subterranean halls an elegance replete with a kind of mysterious charm.