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Societies and Academies

    Naturevolume 113pages699700 (1924) | Download Citation

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    LONDON. Geological Society, April 9.—Dr. J. W. Evans, president, in the chair.—H. H. Thomas and A. H. Cox: The volcanic series of Roch, Trefgarn, and Sealyham (Pembrokeshire). The area dealt with lies in Northern Pembrokeshire, and stretches from Roch on the west, across the valley of the River Cleddau, to Ambleston on the east, a distance of some 9 miles. The rocks for the greater part strike approximately east and west, and the district is dominated by an east-and-west elevated tract formed of rhyolitic lavas and ashes (Roch Series). The. lower ground on the south and east is occupied by rocks of Upper Cambrian and Lower Ordovician age, and in the latter two distinct volcanic series have been recognised and mapped to which the names Trefgarn and Sealyham respectively have been applied., The Roch rhyolitic series seems to be of a pre-Cambrian age.—L. Hawkes: On an olivine-rhyolite from eastern Iceland. The acid mass described is exposed in the cliffs of Hamar-fjord, south-eastern Iceland, and is known as the “Rauthaskritha “; it has hitherto been regarded as an intrusion of dyke or stock form. It is suggested that the rhyolite was a lava-flow which filled a de pression in the basaltic plateau, and was afterwards submerged beneath succeeding flows of basalt. There is evidence of the coexistence of extreme acid and basic magmas throughout the Tertiary cycle of igneous activity in Iceland. Phenocrysts of an iron-rich olivine are distributed evenly throughout the flow. The olivine-bearing acid rocks include representatives of many rock-types. It is concluded that, in the early stages of cooling, the iron orthosilicate is in equilibrium with magmas containing a considerable amount of free silica.

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    https://doi.org/10.1038/113699a0

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