Eocene lakes

Charles LyellA manual of elementary geology: or, the ancient changes of the earth and its inhabitants as illustrated by geologiacl monuments, Londres, John Murray, 1855, p. 202.

…figured in the annexed cut, which beeongs to a species very abundant in England, has covered its case with shells of a small Planorbis. In the same manner a large species of caddis-worm which swarmed in the Eocene lakes of Auvergne was accustomed to attach to its dwelling the shells of a small spiral univalve of the genre Paludina. A hundred of theses minute shells are sometimes seen arranged around one tube, part of the central cavity of which is often empty, the rest being filled up with thin concentric layers of travertin. The cases have been thrown together confusedly and often lie, as in fig. 180., at right angles one to the other. 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 6 feet thick, and may be traced over a considerable area, we may form some idea of the countless number of insects and mollusca which constructed rock. It is unneccessary to suppose that  the Phryganeae lived on the spots where their cases are now found ; they may have multiplied in theshallows near the margin of the lake, or in the stream by which it was fed, and their cases may been drifted by a current far into the deep water.

In the summer of 1837, when examining in company with Dr. Beck, a small lake near Copenhagen, I had an opportunity of witnessing a beautiful exemplification of the manner in which the tubular cases of Auvergne were probably accumulated. This lake, called the Fuure-Soe, occuring in the inetrior of Seeland, is about twenty English miles in circumference, and in some parts 200 feet in depth. Round the shallow borders an abundant crop of reeds and rushes may be observed, covered with the indusiae..