The facts that the grains composing most sedimentary rocks show more or less sorting, that this sorting is not uniform, either in degree or kind, and that the rocks are known to have been laid down by various agents, naturally leads to the question: To what extent may it be possible to determine, from the mechanical constitution of a layer of sedimentary rock, what conditions and processes affected its deposition? The term sedimentary rock is here used in preference to clastic rock, for not only do most deposits which were once sediments show some sorting, but the grains composing many of the so-called chemical deposits, particularly the limestones, seem to have been transported and sized to a greater extent than generally realized. One is deeply impressed with this fact, particularly when observing the sea bottom in diving apparatus. The lime-depositing waters around Florida and Yucatan, for example, sweep the deposits . . .