The hill of La Roche, in the Puy de Jussat, presents a face of a variegated quartzose grit of nearly seven hundred feet in thickness. At Chamalières, near Clermont, the same deposit is equally well exposed. Green and white foliated marls are very abundant, attaining a thickness sometimes of six to seven hundred feet, and consisting chiefly throughout this immense depth of the shells of _Cypris_, a genus which comprises several species, some of which are recent, and still existing in the waters of our stagnant pools and ditches. The structure of these beds, in this volcanic region, is as remarkable as the materials of their composition. The strata divide into plates thin as paper, which are piled up into laminated masses of several hundred feet, of various colors, but the white and green prevailing, and the whole sometimes covered by rocky currents of trachytic or basaltic lava. Gypseous marls, similar to those of Montmartre, have also contributed to the silting up of the lakes, where, as at St. Romain, they are worked, and extensively used for ornamental purposes. A remarkable deposit occurs among the series, termed the _indusial limestone_, from the circumstance of its containing the cases or _inducia_ of a tubular-form species of insects; a creature that not only assisted individually toward the increment of the rock, but possessed the power, like its existing analogues, of attaching to its body a load of shelly molluscs, in some cases no less than a hundred of these minute shells being arranged around one tube, while ten or twelve tubes are packed within the compass of a cubic inch. Some beds of this limestone are six feet thick, and may be traced over a considerable area, showing the countless number of insects and molluscs which contributed their integuments and shells to compose this singularly constructed rock. The fibular coralline rocks of the Keelan islands bear some faint resemblance to these ancient organic deposits, where the insects build from beneath, and gradually mount to the surface of the ocean when their work is done, and they perish. The _Phryganeæ_ of the tertiary age enjoyed their brief hour in the sunshine, fulfilled their destiny, sank into the waters, and contributed to form rocks over their bottom. They weaved not, like the existing races of builders, their own shroud, though the materials in which they are entombed are mainly of their own construction—concretionary plates of the finest texture, and indestructible as marble.