Rhythm in sedimentation and its interpretation: with particular reference to the Carboniferous sequence

I. Introduction. Any law expressing the order in which different types of sediment are deposited is of the greatest value, not only in correlation but also in the interpretation of the sequence in terms of the physical conditions under which the sediments were laid down. In coal-bearing strata, for example, it is usually practicable to distinguish some ten to twelve different types of sediment. Most of the individual beds are only a few feet in thickness, and many are seldom thicker than an inch or two; nevertheless in the aggregate they make up some thousands of feet of coal-bearing measures and may have taken some millions of years to be deposited. It can be suggested at the outset that any law expressing the order of deposition of the rocks in such a sequence is likely to be rhythmic in character. What, however, do we mean by rhythm? Rhythm in ordinary speech applies primarily to motion. It is defined in the Concise Oxford Dictionary as ‶movement with regular succession of strong and weak elements.″ This hardly seems to cover what most people have in mind when they think of rhythm in sedimentation. Indeed the terms rhythm and cycle have been used so loosely in stratigraphical work that some definition of the meaning attached to them in this discussion is essential. Rhythm is used to mean the repeated recurrence of any distinctive type of rock in a stratigraphical sequence. Some degree of regularity in the recurrence is considered essential if such recurrence