On the earth’s surface, ranging from mountain crests to oceanic deeps, are wide gradations in geologic process and organic environment, but the line which most sharply draws division between two worlds of process and of life is the strand. In the study of the sediments, holding as they do the record of earth history and organic evolution, a fundamental question is therefore whether the strata were laid down in contact with the air or beneath the level of the sea. If fossils are present they commonly give an answer, but the absence of fossils from many formations leaves the problems of origin to be solved by other methods of attack. It is important that the criteria which are used for such purposes should be always subject to scrutiny in order that inherited errors may be detected and further progress made toward refinements of discrimination—refinements which though concerning small details may yet result in a . . .