It is unnecessary

Charles Lyell, Principles of Geology, vol. III, Londres, John Murray, 1830, p. 232-233.

Indusial limestone.- There is another remarkable form of frehs-water limestone in Auvergne, called « indusial », from the cases, or indusiae, of the larvae of Phryganea, great heaps of which have ben encrusted, as the lay, by hard travertin, and formed into a rock. We may often see, in in our ponds, some of the living species of these insects, covered with small freh-water shells, which they have the power of fixing to the outside of their tubular cases, in order, probably, to give them weight and strenght. It appears that, in the same manneer, a large species which swarmed in the Eocene lakes of Auvergne, was accustomed to attach to its dwelling the shells of a spiral univalve of the genus Paladina. A hundred of these minute shells are somethimes 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 tube are packed within the compass of a cubic inch, and that some single strate of this limetone are six feet thick, and may be traced over a considerable area, we many form some idea of the countless number of insects and mollusca which contributed their integuments and shells to compose this singularly constructed rock. It is unnecessary to suppose that the Phryganea lived on the spots where their cases are now found ; they made have multiplied in the shallows near the margin of the lakes, and their buoyant cases may have been drefted by a current far into the deep water.