Introduction

Abstract

The rocks included in this division are those which have been formed by the chemical or mechanical activity of the agents of denudation on pre-existing rocks, and which have been deposited at ordinary temperatures and pressures from suspension or solution in water or air. They are the products of the secular decay and disintegration of the rocks of the earth’s crust. The broken debris, and most of the dissolved matter arising from these processes, are transported by wind and water, and deposited in the hollows of the land surface and of the sea floor. Some of the material is left as a residual mantle within the area in which the reactions take place; and while the land waste as a whole may be conceived as on the march to its ultimate resting-place, the sea, much of it remains long enough on the land to build up massive formations several thousands of feet thick. The material carried in suspension is deposited when the velocity of the transporting medium is checked or its physical condition otherwise changed; the dissolved materials are either precipitated directly by some change in the physical or chemical conditions of the media, or indirectly by the vital activities of animals and plants. The secondary rocks have been accumulated under a great variety of conditions, and consequently show great variations in mineral and chemical composition and in texture.