On Abnormal Conditions of Secondary Deposits when connected with the Somersetshire and South Wales Coal-basins; and on the Age of the Sutton and Southerndown Series

Abstract

Abstract T he author first describes the geological constitution of the Mendip Hills, which, in his opinion, were upheaved by the intrusion of a basaltic dyke (now noticed for the first time) during the period of the Upper Trias. The Mendip chain proved an island barrier to the incursion of the deeper sea-deposits of the south, and on it lived the Microlestes and other terrestrial animals. Along the south side of this barrier shore-deposits were formed, the “Carboniferous Limestone” constituting the floor of the ocean at that time. He then institutes a comparison between the Rhætic and Liassic formations within and those without the Somersetshire coal-basin. The thickness of the beds, from the Trias to the Inferior Oolite, is stated to be, outside the coal-field, 3320 feet, whilst inside it is only 169 feet. These results were obtained from an examination of numerous sections, which are described in detail by the author. After considering the horizontal deposits beyond the Mendips, and the unconformable conditions within its coal-basin, the author discusses the abnormal conditions which are presented by deposits of the same age when they are intimately connected with the “Carboniferous Limestone.” In the Charterhouse Lead-mine a deposit of clay 12 feet in thickness, and containing Liassic shells, is stated to occur at a depth of 260 feet in the Carboniferous Limestone. Among the organic remains, three species of terrestrial shells, referable to the genera Helix, Vertigo , and Proserpina , and a Charaseed, were discovered. The author concludes by pointing out the peculiarities presented by the Liassic strata in Glamorganshire, with special reference to the stratigraphical position of the Sutton Stone and the Conglomerates of Brocastle &c.