IKDUSIAL LIMESTONE. 875 



inhabit our ponds and ditches. The body of the animal is 

 enclosed in two cases, like a mollusc within a bivalve shell 

 (fig. 265), its principal organs of motion con- 

 sisting of antennae, and feet, which protrude 

 from between the valves. These shields, it 

 frequently sheds, and we may judge to what 

 an extent, and during how long a period 

 of time, these creatures continued to cast 

 their shells in the waters of the lake, when 

 we learn that beds of limestone, one hun- 

 dred feet in thickness, are in an essential FIG. 265.^Fossii 



, * , , . ft , -I shell of Cypiis, 



degree formed ol the exuviae ol these crus- magnified, 

 taceans, which divide the limestone into 

 laminae, no thicker than the leaves of a book. 



A large and extinct species of caddis-worm, which abounded 

 in the ancient lakes of Auvergne, was accustomed to cover 

 its case with the shells of a small species of Paludina. Yast 

 beds of the limestone of this region termed indusial are 

 essentially composed of these remains. "When, we consider 

 that ten or twelve of these cases may be packed within the 

 space of a cubic inch, and that single beds of this lime- 

 stone may be traced over an area several miles in extent, 

 we may form some idea of the countless numbers of insects 

 and molluscs, whose remains have contributed to form this 

 remarkable rock, and of the incalculable period of time 

 required for its deposition. 



These indusial limestones form but a portion of the strata 

 of the district. The various beds present an alternate series 

 of volcanic and lacustrine deposits, an accumulation of clays, 

 sands, and breccias, beds of gypsum, and freshwater lime- 

 stone, containing lacustrine and terrestrial organic remains, 

 with intrusions of lava, scoriae, and basalts, to an extent so 

 enormous as to indicate no less than six successive eras of 

 alternate activity and repose, each of vast duration, and 

 tending, in the aggregate, immensely to increase the geolo- 

 gical antiquity of these formations. 



The beds of freshwater limestone contain, in addition to 

 vegetable remains, and land and freshwater shells, teeth and 

 bones of the Palaotkerium, Anoplotherium, deer, ox, martin, 

 dog, &c. ; whilst in the overlying deposits of sand and diluvial 

 gravel are embedded the teeth and bones of the elephant, 



