Henry Eley, Geology in the Garden; or the Fossils in the Flint Pebbles, Londres, Bell & Daldy, 1859, p. 137-138.
A striking proof of the latter very material fact mau be seen in the curious Indusial Limestone of these lacustrine formations. It is very well know that the larvae, or grubs, of certain flies, which frequent the banks of rivers and pools, live, while they are in that preparatory state, in the water, and protect themselves from their ennemies by constructing tubes in which they may lie concealed. The common cad, caddis, or case-worm of the angler, which turns to the stone-fly, is ofthis sort. Different species of this grub use different materials for their indusia, covers or cases; some agglutinating together vegetable matter, small sticks, or reeds; others choosing grains of sand; while a third kind prefer the dead shells of small mollusks for the purpose. It is evident that the larger of the ancient lakes of Auvergne was inhabited by a species of the latter family, and that they swarmed in it in a remarkable manner; for their cases, incrusted with calcareous matter, are seen to form there thick layers of limestone, called indusial from this strange origin, – which alternate with the more usual kinds of marl through a thickness of several hundred feet. « More than a hundred of these shells, » says Mr. Scrope, « may be counted round a single tube, and ten or twelve tubes may be found packed together irrefularly in a single cubic inche of the rock.