Page:A Treatise on Geology, volume 1.djvu/337

 CHAP. VI. among living tribes, the diagnosis of species is far from clear, what errors may not be incurred by pronouncing a verdict on the imperfect evidence of a few fragments of detached fossil bones?

Raised Beaches.—These curious phenomena, first brought prominently forward by M. Brongniart, are a part of the great system of "Pleistocene" deposits, which includes the erratics and drifts. They demonstrate, that within a comparatively modern period, certainly since the actual seas were filled with yet existing mollusca, the beds of these seas have been subject to elevation and depression, so that, in particular places, large quantities of shells attached to their parent rocks, or mixed with the pebbles and sand of their native beaches, have been raised 10, 20, 100, or several hundred feet above high water mark. Within the reach of history, slight displacements of the relative level of land and sea have taken place, as the temple of Serapis near Puzzuoli, Lisbon, Port Royal, are supposed to prove. But these phenomena, connected with local earthquakes and volcanic eruption, are small and limited in comparison with the class of facts noticed above; which appeared to M. Brongniart of so general a character as to justify a supposition that the ocean waters had everywhere suffered a depression of level, even since the creation of existing races of mollusca, and the establishment of the main features of physical geography, though anterior to historic times. To this view of M. Brongniart it is, apparently, a fatal objection, that the levels at which the raised beaches appear, above the sea are extremely varied, even on points of the coast of the same country, and much more when we compare distant coasts; whereas, upon his view of a general lowering of the surface of the sea by one depression of the crust of the globe (affaissement de la