Abstract
Detailed investigations of volume relationships of rocks of the alkali basalt - trachyte - phonolite assemblage on Saint Helena demonstrate a decrease from basic to salic ends of the series. Constituent volume abundances, allowing for erosion, are calculated as: basalts = 70−80%, trachybasalts = 25−15%, trachyandesites = 4%, trachytes plus phonolites = 1%. Oversampling of parasitic intrusions, however, resulted in bimodal distributions of collected and analyzed rock types. Volumetric data of this type suggest that hypotheses concerning the existence of a ‘Daly gap’ be treated with extreme caution.
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