On the Middle and Upper Lias of the Dorsetshire Coast

Abstract

§ I. Introduction . 1. Sir H. De la Beche's subdivision of the Dorsetshire Lias .—The only published section giving details of the Lias formation of Dorsetshire, at present in existence, is one made more than thirty years ago by the late Sir Henry De la Beche*. When I undertook the investigation of the geology of the coast in the neighbourhood of Lyme Regis, I soon found that that section, though according well with certain general facts, was owing to the rapid progress of geological knowledge, and the consequent changes in our system of classification, utterly useless to students. The divisions and subdivisions of Dela Beche were founded solely on lithological grounds, whereas those of the present day are based in great measure upon palæontological considerations. He classed all our Upper and nearly half our Middle Lias with the Inferior Oolite Sands†, and he subdivided the Lias, as then understood, accordingly. As he had classed sands with sands, and marls with the Lias, so he divided the latter into “Upper Lias Marls” (which, under the present system, include both the Middle and Lower Lias Marls), “Lias Limestones” (which correspond to a subdivision of the Lower Lias), and “Lower Lias Marls” (which are the “ Avicula contorta series,” no longer regarded even as Liassic). The section quoted gave to the entire Lias Formation of Dorsetshire a thickness of 600 feet; our present acceptation of the term includes between 1000 and 1100 feet of strata. The plan which I have adopted, in order to lay