On Iso-diametric Lines, as means of representing the Distribution of Sedimentary Clay and Sandy Strata, as distinguished from Calcareous Strata, with special reference to the Carboniferous Rocks of Britain

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Abstract
§ 1. — Introduction .—A large and interesting field of inquiry is open to us in comparisons of the relative distribution of the calcareous and the truly sedimentary members of diffcrent geological formations. We have, as it appears to me, been too much in the habit of classing limestones (whether coralline, crinoidal, shelly, or oolitic) as strictly sedimentary; yet it will be found, by such comparisons as those alluded to, that the relation which is borne by sandstones and shales to limestones is one, not of similarity, but of contrast. In other words, that where we have a group of strata, as, for example, the Lower Carboniferous, composed partly of "sedimentary" and partly of calcareous members, it will generally be found that the one series is complemental of the other, and developed from opposite directions. This arises from the differences in the origin of the two classes of stratified rocks, the calcareous being essentially organic, and the "sedimentary " essentially mechanical ; so that where the forces and agencies tending to the accumulation of the latter are in active operation, these very forces and agencies are in direct antagonism to the other, and, as a result, calcareous strata are either not formed or only sparingly*. ( a .) Of these two ever-acting principles we have numerous examples both in recent and in geologic periods. If we take as an illustration the Gulf of Mexico and the West Indian Islands, we find he sediment brought down by the Mississippi forming deposits of sand and clay

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