Amygdaloidal Basalts Research Articles

Societies and Academies

LONDON. Geological Society, June 28.—Dr. Alfred Harker, president, in the chair.-Dr. A. Smith Woodward: A new species of Edestus from the Upper Carboniferous of Yorkshire; with a geological appendix bv J. Pringle. The fossil confirms the interpretation of Edestus as a row of symphysial teeth of an Elasmobranch fish. The row of eight bilaterally symmetrical teeth, fused together, occurs at the tapering end of a pair of calcified cartilages, which evidently represent a jaw. An imperfect detached tooth probably belongs to an opposing row. The teeth are large compared with their base, and the serrated edges have been worn during life. Small Orodont teeth of the form named Campo-dus are scattered in the shale near the jaw. Markings on the Edestus teeth themselves suggest that they have been derived from the Campodus type of tooth. The specimen was obtained from shale below the Rough Rock, in the upper part of the Millstone Grit, at Brockholes, near Huddersfield.-A. Holmes: The Tertiary volcanic rocks of Mozambique. With the exception of a coastal belt of Cretaceous and Tertiary sediments, flanked on the west by later Tertiary volcanic rocks, the territory consists of a complex of gneisses and other foliated rocks, intruded upon by granites belonging to at least two different periods. From Fernao Vellosa Harbour to Mokambo Bay the junction of the sedimentary formations with the crystalline complex is faulted, and the volcanic rocks are distributed on each side of the fault. The lavas are of post-Oligocene age, and are the result of fissure-eruptions, the feeding channels being exposed as small dykes that penetrate the underlying rocks. The prevailing lavas are amygdaloidal basalts. An andesite dyke of later date occurs near the Monapo River. In the north, near the Sanhuti River, picrite-basalt, basalt, phonolite, and solvsbergite have been found, and related lavas occurring elsewhere in the area are tephritic pumice and segirine-trachyte. The “alkali” series can be closely matched by the lavas of Abyssinia, British East Africa, Reunion, and Tene-riffe. The amygdaloidal basalts of the “calc-alkali” series are similar to those of the Deccan, Arabia, and East Africa, and also to those (of late Karroo age) occurring in South Africa and Central Africa. Each of the series was probably evolved by a process of differentiation acting on a parent magma. From1 the composition of the amygdale minerals it is deduced that the parent magma of the “alkali” series was rich in carbon dioxide and under-saturated in silica; whereas that of the “calc-alkali” series was rich in water and over-saturated in silica. The radio-activity of the lavas indicates that the depth from which the parent magma came was probably between thirty-three and forty-four miles from the earth's surface.