232 EOCENE PERIOD. [Ch. XVII. 



into limestone. Sometimes only concretionary nodules abound 

 in them ; but these, by an additional quantity of calcareous 

 matter, unite, as already noticed (p. 229), into regular beds. 



On each side of the basin of the Limagne, both on the east 

 at Gannat, and on the west at Vichy, a white oolitic limestone 

 is quarried. At Vichy, the oolite resembles our Bath stone in 

 appearance and beauty, and, like it, is soft when first taken 

 from the quarry, but soon hardens on exposure to the air. At 

 Gannat, the stone contains land-shells and bones of quadrupeds, 

 resembling those of the Paris gypsum. In several places in 

 the neighbourhood of Gannat, at Marculot among others, this 

 stone is divided by layers of clay. 



At Chadrat, in the hill of La Serre, the limestone is pisolitic, 

 and in this and other respects resembles the travertin of Tivoli. 

 It presents the same combination, of a radiated and concentric 

 structure, and the coats of the different segments of spheroids 

 have the same undulating surface. (See wood-cut No. 5, chap, 

 xii. vol. i.) 



Indusial limestone. There is another remarkable form of 

 fresh- water limestone in Auvergne, called ( indusial,' from the 

 cases, or indusice, of the larva? of Phryganea, great heaps of 

 which have been encrusted, as they lay, by hard travertin, and 

 formed into a rock. We may often see, in our ponds, some 

 of the living species of these insects, covered with small 

 fresh-water shells, which they have the power of fixing to the 

 outside of their tubular cases, in order, probably, to give them 

 weight and strength. It appears that, in the same manner, a 

 large species which swarmed in the Eocene lakes of Auvergne, 

 was accustomed to attach to its dwelling the shells of a 

 small spiral univalve of the genus Paludina. A hundred of 

 these minute shells are sometimes seen arranged around one 

 tube, part of the central cavity of which is still occasionally 

 empty, the rest being filled up with thin concentric layers of 

 travertin. When we consider that ten or twelve tubes are 

 packed within the compass of a cubic inch, and that some 

 single strata of this limestone are six feet thick, and may be 



