OCR Text |
Show 232 EOCENE PERIOD· [Ch. XVII. into limestone. Sometimes only concretionary nodules abound in them; but these, by an additional quantity of calcareous matter, unite, as already noticed (p. 229), into regular beds. On each side of the basin of the Limagne, both on the east at Gannat, and on the west at Vichy, a white oolitic limestone is quarried. At Vichy, the oolite resembles our Bath stone in appearance and beauty, and, like it, is soft when first. taken from the quarry, but soon hardens on exposure to the au·. At Gannat, the stone contains land-shells and bones of quadrupeds, resembling those of the Paris gypsum. In several places in the neiO'hbourhood of Gannat, at Marculot among others, this b stone is divided by layers of clay. At Chadrat, in the hill of La Serre, the limestone is pisolitic, and in this and other respects resembles the travertin of Tivoli. It presents the same combination, of a radiated and concentric structure, and the coats of the different segments of spheroids have the same undulating surface. (See wood-cut No.5, chap. xii. vol. i.) Indusial limesfone.-Tbere is another remarkable form of fresh-water limestone in Auvergne, called 'indusia},' from the cases, or indwda:, of the larvre of Pbryganea, great heaps of which have been encrusted, as they lay, by hard travertin, and formed into a rock. We may often see, in our ponds, some of the living species of these insects, covered with small fresh-water shells, which they have the power of fixing to the outside of their tubular cases, in order, probably, to give them weight and strength. It appears that, in the same manner, a large species which swarmed in the Eocene lakes of Auvergne, was accustomed to attach to its dwelling the shells of a small spiral univalve of the genus Paludina. A hundred of these minute shells are sometimes 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. 'Vhen we consider that ten or twelve tubes are packed within the compass of a cubic inch, and that some single strata of this lin1estone are six feet thick, and may be Ch. XVII.] LACUSTRINE STRATA-AUVERONE. 233 traced over a considerable area, we may 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 Phryganere lived on the spots where their cases are now found; they may have multiplied in the shallows near the margin of the lake, and their buoyant cases may have been drifted by a current far into the deep water. The calcareous strata of the Limagne, like the other members of the lacustrine formation, are for the most part horizontal, or inc1ined at a very slight angle, but instances of local dislocation are sometimes seen. At the town of Vichy, for example, the strata dip at an angle of between 30 and 40 degrees ; in an ancient quarry behind the convent of Celestines, and near the hot spring at the same place, the beds of limestone are seen first inclined at an angle of 80°, and then vertical. 5. Gypseous marls.-More than 50 feet of thinly-laminated gypseous marls, exactly resembling those in the hill of Montmartre, at Paris, are worked for gypsum at St. Romain, on the right bank of the Allier. They rest on a series of green cypriferous marls which altemate with grits, the united thickness of this inferior group being seen, in a vertical section on the banks of the river, to exceed 250 feet. Gene'ral arrangement and origin of the fresh-water formations of Auvergne.-The relations of the different groups above described cannot be learnt by the &tudy of any one section, and he who sets out with the expectation of finding a fixed order of succession may perhaps complain that the diffel'ent parts of the basin give contradictory results. The arenaceous division, the marls and the litbestone, may all be seen in some localities to alternate with each other, yet it can by no means be affirmed that there is no order of arrangement. The sands, sandstone, and conglomerate, constitute in general a littoral group; the foliated white and green marls a contemporaneous central deposit, and the limestone is for the most part subordinate to the newer portions of the above groups. |