Abstract
Synopsis: A study of the attitude of columnar joints in the Dumbarton Rock basalt suggests that the plug is narrowing downwards and could represent an infilled crater. At a late stage in the volcano’s history, before the emplacement of the plug, subsidence took place along fractures developed around the crater. The narrow strip of pyroclastic rocks and sediments exposed along the north-west shore represents part of this subsided mass. Chemical analyses of the contact basalt show increasing MgO and H 2 O and decreasing CaO towards the contact. It is thought that the albitised and chloritised contact basalt is due to migration of volatiles towards these contacts.

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