Page:The Journal of geology (IA journalofgeology11893univers).pdf/51

Products: Rock cores of disintegrated masses, sand, (chiefly quartz-sand), residual clays, and lime, magnesia, iron, etc., in solution.

Favorable conditions:

By comparison of the statements of favorable conditions for rock-breaking, rock-decay and transportation it becomes apparent that breaking and decay are favored by opposite conditions in nearly all respects, while breaking and transportation are most efficient under like conditions. But breaking promotes decay, and decay aids transportation, by reducing the size of the particles to be decomposed and carried, and the maximum effect of erosion is probably attained when rock-breaking is active among greater elevations, and rock-decay and transportation are both proceeding energetically on lower slopes.^[1]

The amount of material furnished by erosion is an important consideration in reference to the rate of accumulation of sediments over a given area, and is a condition not to be overlooked in comparing thickness of deposits with the lapse of geological ages.

Shingle, gravel, sand, clay and silt are products of erosion of rock masses. They are produced either by mechanical breaking or by chemical disintegration. These two sub-processes of the general process of erosion are favored by unlike conditions. Those conditions which render breaking most efficient are unfavorable to immediate disintegration; and those conditions which promote disintegration limit breaking. Breaking, the reduction of a rock mass to small pieces, is usually