The rocks laid down under water form the larger and more important part of the sedimentary series.

I. Mechanical Deposits

These have resulted from the accumulation of debris derived from the destruction of preexisting rocks, carried in mechanical suspension by moving water, whether waves, currents, or streams, and dropped when the velocity of the moving water was no longer sufficient to carry them. The study of the dynamical processes has already taught us that such accumulations are forming to-day in all kinds of bodies of water, and an examination of the rocks will show that similar accumulations have been made since the beginning of recorded geological time. Mineralogically, the mechanical deposits are of two principal kinds, the siliceous and the argillaceous. The sorting power of water has been sufficient to separate them more or less completely, though we find mixtures of the two in all proportions.