S. R. Pattinson, « The volcanoes of Auvergne », Londres, The Leisure hour A Family Journal of Instruction and Recreation, vol. 16, 1867, p. 583.
The freshwater light marls are thick enough, in some places, to form decided banks and cliffs like the chalk; but, unlike the latter, they are usually apparently stratified either by leaf-like partings or by layers of rough limestone. These two circumstances afford a marvellous insight into the composition of the masses, and furnish striking proofs of the enormously long period during which the deposition was going on, which formed this belt of clay. A magnifying glass will show that the foliated partings in the marls are due to the minute shields of the cypris, the little beetle-like creature which enlivens our pools. Layers of these, mingled with myriads of small snail-like shells, alternate with the substances of the marls, to the extend of twenty in an inch ; and the marls are five hundred feet thick. Each thin layer must have formed a platform for the disporting and the sunny life of these tiny creatures. But the rough limestones is even more curious still ; for it is absolutely made up of the tiny ends of sticks and straws of caddis-worm cases. On the banks of any pond or brook in the summer, a heap of empty caddis-worm cases may often be seen, drifted in by an eddy, and landed in the mud. Imagine these to have become cemented by an infusion of limewater, and you have at once the indusial limestone of Auvergne. If you narrowly examine the common caddis case, you will see small shells worked into the tubes, like the grotesque shell ornaments sold at watering-places. So is it with the fossil cases. A hundred minute shells of bulimus, or paludina, may be counted on a single case. There are absolute rocks, and beds of this stone several times repeated, spread over many square miles of the Limagne.