The Silurian volcanic rocks of the Mendip Hills, Somerset; and the Tortworth area, Gloucestershire, England

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Abstract
SUMMARY: Field, petrographic and chemical studies on the Silurian volcanic rocks of the Mendip Hills show that there are probably 15 or more rock units in the series including andesite and rhyodacite lavas, rhyodacite tuffs, agglomerates, and a dolerite dyke. The predominant rock type is rhyodacite which may be as much as 80 percent of the volcanics. Volcanics of Silurian age from the Tortworth area, Gloucestershire, are of latite-andesite composition.The Mendip rocks have been deuterically altered. Calcite-quartz-laumontite veins are common in fractures in these rocks. The agglomerates are particularly susceptible to weathering and some bombs are extensively altered to clays. Twelve rocks were chemically analysed for 36 elements each. No anomalous base metal concentrations were found in the volcanics although Pb, Zn, and Cu mineralisation is known in the area. K/Rb varies from 202 to 909 in these calc-alkaline rocks.