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.
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.
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.