Page:A Treatise on Geology, volume 1.djvu/277

 CHAP. VI. The tertiary system of strata is the most recent of all the regular marine series of deposits: its relation to the existing oceans is therefore a highly interesting subject of inquiry; the more so, as, from the phenomena of alternating marine and freshwater deposits, conclusions have long since been presented by distinguished writers that particular tracts were alternately raised above and sunk below the sea. Cuvier and Brongniart proposed this hypothesis to explain the freshwater interpolations among the marine strata of Paris; and the notion has gradually become a popular part of geological speculation. The solution of an old geological problem requires far more caution than the explanation of a modern geographical phenomenon. For in regard to the older events we are seldom aware of all the essential facts, from which not only the nature of the physical agency is to be ascertained, but the measure of its force, the local centre of its effect, and the sudden or gradual, the continuous or interrupted mode of its application. The geographical relations of tertiary strata must be understood before venturing to adopt or to reject the hypothesis of partial elevation and subsidence.

Before the deposition of the tertiary system, Europe had acquired many of its marking features: the Pyrenees, Brittany, parts of Wales and Scotland, Scandinavia, the Carpathians, Apennines, the mountains of Bohemia, the Vosges, Auvergne, and other tracts, were uplifted above the sea. But these appear to have stood up like unconnected islands, round which the ocean currents passed variously into wide basins like those of the Danube, Paris, &c.; or poured into insulated bays, like what may be termed the Gulf of Bohemia. The direction, force, and materials mixed with these currents, would be materially influenced by the submarine slopes from these insulated ridges, and by other undulations in the bed of the sea; the nature and abundance of the tertiary sediments, and the organic forms which are buried in them, would be greatly dependent on the force and origin of the currents: and thus we see a reason why