We have now to inquire what becomes of the material which is derived from the decomposition and disintegration of the rocks. At the present time, it is estimated, about one-half of the waste of the land is carried directly into the sea, while the remainder is arrested in its journey and deposited upon the land. It must be remembered, however, that when the sea advances over the land, these deposits are, to a large extent, rapidly worked over by the waves and converted into marine deposits. The accessible rocks of the earth's crust are more largely composed of marine deposits than of those laid down in other ways, yet the non-marine sedimentary rocks are also extensively represented. It is only quite lately that the importance of this latter class of rocks Has been appreciated. Furthermore, their importance is not merely quantitative, but lies also in the help which they give in the determination of ancient land surfaces, lake beds, river channels, ice-fields, and the like. It is therefore necessary to study all the methods by which rock reconstruction is effected, on however small a scale.
The most natural primary division of the sedimentary accumulations is into the marine and the continental, including in the latter the deposits which are made upon the land, or in such bodies of water as are not parts of the sea. Between these two principal classes there is a transitional series, consisting of deposits laid down in bodies of salt water which are in tidal connection with the sea, such as estuaries, almost closed bays and sounds, or seas, like the Baltic which are partly brackish, as well as the littoral, or seashore, which by the movement of the tides is alternately a land surface and a sea-bottom. These distinctions are sufficiently obvious, yet they are not always easy to apply, especially in the absence of fossils; hence great differences of opinion continually arise concerning the interpretation of certain rock masses.