DENUDATION is the removal of solid matter by water in motion, whether of rivers or of the waves and currents of the sea, and the consequent laying bare of some inferior rock. The rate of abrasion depends upon the velocity of the current, and the nature of the solid materials through which it flows; these two causes equally affect the deposi tion of the abraded matter, for the carrying power of the water varies with its velocity and with the weight of the particles. The heavier debris—large stones and gravel—are carried short distances, and deposited generally in masses; the finer particles are con veyed even by a slow current to great distances, and scattered in thin layers over ex tensive districts. All deposition, except in the case of showers of volcanic ashes, is the sign of a superficial waste going on contemporaneously, and to an equal extent, else where, the gain at one point being equal to the loss at another. 'No new material has been used in the formation of the sedimentary rocks. The degradation and abrasion of igneous rocks provided the materials of the earliest strata; these in their turn were fre quently abraded and re-deposited, under new conditions, and with the remains of a newer fauna and flora. Thus the crust of the globe Las not actually increased' in thick
ness, for whenever it acquires density in one place, it becomes thinner in another. The changes that have been effected by D., and the amount of matter thus transported, are difficult to imagine. In districts where faults occur, the surface has been smoothed, and the uptilted ends have been washed away. These faults sometimes extend over several hundred sq.m., and the dislocations, had they remained unaltered, would have produced mountains with precipitous escarpments of different heights, reaching occa sionally to a thousand feet. But D. not only leveled the surface of the earth; in regions with horizantal stratification it has produced inequalities, hollowing out valleys of D., and often carrying away the whole of the superficial strata, leaving mountains here and there which show, by the direction and succession of their beds, that the strata of which they arc composed were at one time continuous.