In every place where the broken strata present their edges on abrupt surfaces, there fall down to their base, every spring, and even after every storm, fragments of their materials, which are rounded by rolling upon each other. These collected heaps gradually assume an inclination determined by the laws of cohesion, and thus form, at the bottom of the cliff, taluses, of greater or less elevation, according as the fragments which have fallen are more or less abundant. These taluses constitute the sides of the valleys in all elevated, mountainous regions, and are covered with a rich vegetation, whenever the fragments from the upper parts begin to fall less abundantly; but their want of solidity subjects themselves also to slips, when they are undermined by rivulets. On these occasions, towns, and rich and populous districts, are sometimes buried under the ruins of a mountain; the courses of rivers are interrupted, and lakes are formed in places which were before the abodes of fertility and cheerfulness. Fortunately these great slips happen but seldom, and the principal use of those hills of debris, is to furnish materials for the ravages of torrents.
About This Book
The essay assembles geological observations and fossil evidence to reconstruct Earth's successive changes, arguing that strata and petrified remains record numerous abrupt revolutions of the surface that caused mass extinctions and replacement of faunas. It examines how current agencies—erosion, slips, alluvial deposition, coastal cliffs, stalactites, lithophyte growths, incrustations, and volcanic activity—operate, and distinguishes their slow effects from the sudden events inferred in the rock record. It uses stratigraphic sequences and fossil assemblages to date relative episodes and to argue that many major revolutions preceded the appearance of existing life forms, offering a systematic account of Earth's physical and organic history.