Societies and Academies

Abstract

LONDON. Geological Society, June 10.—L. R. Cox: The fauna of the basal shell-bed of the Portland Stone of the Isle of Portland. On the western coast of the Isle of Portland the basal bed of the Portland Stone is a highly fossiliferous shelly limestone, on the surface of which fossils weather out in an extremely good state of preservation. The specimens described were collected by Lieut.-Col. R. H. Cunnington and include about 8o species of mollusca, of which 18 lamellibranchs and 9 gastropods are new to science, and several others have not before been recorded from Great Britain.—H. L. Hawkins: Echinoidea from the Portland Stone and the Purbeck Beds. Before last year only one species (“Echinobrissus” brodiei Wright) was known from the Portland Stone. A species of “Hemicidaris from the sands was the only other echinoid recognised in the British Portlandian. The work of Lieut.-Col. Cunnington has revealed three specimens of “E.” brodiei in the basement-bed of the Portland Stone (and one from the over lying Whit-Bed); and material for the study of four other species, with indication of a sixth. Prof. Hawkins has also found Hemicidaris purbeckensis Forbes in the Middle Purbeck Series of Durlston Bay, near Swanage, which was collected from that locality about 75 years ago but not since, and specimens of an apparently new form referable to “Pseudodiadema” sensu latissimo. The irregular distribution of echinoids in these and other Jurassic strata may be due to the known tendency of echinoids to live in restricted clusters (comprising several species of similar ecological quality), which seem to migrate wholesale in successive generations.—E. Spencer: On some occurrences of spherulitic siderite and other carbonates in sediments. The spherulites occur in association with fine-grained sediments of carbonaceous, muddy, or silty type, often with comminuted plant-tissue, and are fairly uniform in size locally. The deposits seem to be of freshwater origin and devoid of calcareoi shelly remains; the carbonate material in most cases consists of nearly pure siderite. The occluded sediment is similar to that in which the spherulites are embedded; where “zoning” of the sediment occurs, it is subordinate to radial structure. The spherulites probably formed from iron-carbonate solutions held within the gradually settling and consolidating sediment. The re actions resulting from the presence in sediments of humate compounds, salt, calcium carbonate, etc., are considered. The iron compounds present in solution in fresh water were probably adsorbed by the fine-grained and partly colloidal sediments, and carried down with them during deposition. Super-saturation would result from the settling and flocculation of the sediment, and from the gradual upward expulsion of the more readily diffused water-molecules. Crystallisation would then commence at a number of centres simultaneously.