1. The geological periods mark the phases of the general aspect of the globe by the succession of its transformations. But with the exception of the deluge period, which bears the marks of sudden commotions, all others have been accomplished slowly, and without sudden transition. During all the times that the constituent elements of the globe have been seeking their true positions, changes have been general. Once the base consolidated, only partial modifications are produced upon the surface.
2. Besides general revolutions, the Earth has undergone a great many local perturbations, which have changed the aspect of certain countries. As for the others, two causes have led to them, - fire and water.
By fire; sometimes by volcanic eruptions, which have desolated whole districts, turning villages and their inhabitants into beds of ashes; by earthquakes, or by uprisings of the solid Earth crust, making the water flow back upon the lower countries, or by depressions of this same crust in certain places over greater or less extents, where the waters have been precipitated, leaving other territories bare. Thus islands have appeared on the bosom of the ocean, while others have disappeared; thus portions of continents have separated, and formed islands; thus arms of land in the ocean, becoming dry, have united islands to continents.
By water; - sometimes by overflows, or the retreat of the sea from certain shores; by uprisings, which have arrested the course of the water, lakes have been formed; by overflows and inundations, or the alluvia formed at the mouth of rivers. This alluvial deposit, by obliging the sea to flow back, has created new countries: such is the origin of the Delta of the Nile, in Lower Egypt; of the Delta of the Rhone, or Camargue.