Societies and Academies

Abstract

LONDON. Geological Societv, December 6.—Prof. W. W. Watts, F.R.S., president, in the chair.—Dr. T. F. Sibly: The faulted inlier of Carboniferous Limestone at Upper Vobster (Somerset). The Upper Vobster inlier lies rather less than a mile to the north of the main outcrop of the Carboniferous Limestone of the Mendips. This inlier has been dissected by quarrying operations. The northern and eastern portions are concealed by a covering of Lias, but its width from north to south is little, if at all, greater than 400 yards, while the east-and-west extent of the Carboniferous Limestone is about 1100 yards. The author has arrived at the following conclusions:—The inlier is a lenticular mass of Carboniferous Limestone, grits, and shales, superimposed upon the overfolded strata of the Coal Measures by thrust- movements. It comprises a northern limestone mass and a southern limestone mass, separated by a grit-and-shale mass. The beds of the grit-and-shale mass are in faulted relation to the Carboniferous Limestone. On the northern side, the adjacent beds of limestone represent part of the Seminula zone; on the southern side, the adjacent beds belong to the lower Dibunophyllum zone. In the northern limestone mass, Vobster Quarry exposes more than 500 feet of Seminula beds, overfolded towards the north-west. In the southern limestone mass the strata are locally overfolded northwards. The beds of the grit-and-shale mass comprise quartzites assigned to the Millstone Grit. They also include shales, with intercalated fine-grained sandstones. Possibly this mass includes the lowest beds of the Coal Measures, in addition to a portion of the Millstone Grit. In sections of Carboniferous Limestone, signs of the stresses to which the strata have been subjected are evident. The beds are often distorted, while slickensides and calcite-veins are developed. The occurrence of a lamellibranch fauna at the top of the Seminula zone is recorded.J. Romanes: Geology of a part of Costa Rica. The part of Costa Rica to the west of San Jose as far as the Pacific coast is dealt with. San Jose is situated in a valley sloping westwards, and drained by the Rio Grande and its tributaries. The northern boundary of this valley is the chain of recent volcanoes which rise from its floor, while on the south the ground rises abruptly to form the Cerro Candelaria. In this range of mountains are exposures of limestone, marl, &c., together with igneous rocks. An examination of exposures has failed to produce any Cretaceous fossils, while the occurrence of numbers of Balani points to a Tertiary age for the beds. As this limestone stretches across the Atlantic-Pacific watershed, it yields evidence of an interoceanic connection in this area in Tertiary times. Of the igneous rocks, the most interesting feature is the presence of many boulders of monzonite, indicating a plutonic mass in these mountains. The surface of the valley is composed of a thick series of andesitic lavas. On the Pacific coast at Barranca and Manzanilla fossiliferous Tertiary beds are described. These are all marine ashes, and in the Manzanilla district appear to rest unconformably on an older limestone formation. The boulder-clays of Costa Rica are normal river deposits, though, locally, landslides and spheroidal weathering have played an important part.