On an Olivine-Dacite in the Tertiary Volcanic Series of Eastern Iceland: the Rauthaskritha, Hamarfjord

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  • LHLeonard Hawkes
    Leonard Hawkes
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Abstract
The flow of dacite to be described crops out in the face of the cliff bordering the north of Hamarfjord—one of the deep incisions into the Tertiary basalt-plateau of South-Eastern Iceland. A mass of acid rock occurring in a district entirely built up of dark basaltic lavas—it attracts attention by reason of the light colours of its different lithological members, and its unusually abundant screeformation. It is known as the ‘Rauthaskritha’, or ‘red scree’, and has been described by all geologists who have visited the district. C. W. Paijkull examined this occurrence of ’trachyt‘, and regarded it as a dyke, giving a diagrammatic section in illustration (2). The dyke is shown more than half a mile thick cutting through the basaltic series, and terminated above by one of the basalts. A glassy facies is observed at the two vertical margins and also on the top. The mass exhibits contorted banding, and is cut by dolerite-dykes which fail to penetrate the overlying basalts. The country-rocks are quite undisturbed, and no suggestion is put forward as to how room was made for this remarkable intrusion. Other writers, Th. Thoroddsen (12) and A. Helland (6), have admitted the intrusive nature of the rock. After making a detailed examination of this occurrence in the course of three summer visits, I have come to the conclusion that it represents a lava-flow. These are regularly bedded plateau-basalts with interbasaltic partings of red lateritic deposits, dipping 5° to 7° westwards. Magnificent sections from 2000 to 3000 feet

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