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 Li m ague, 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 inclusice, of the larvae 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 Auvero-ne. 
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 
