Abstract

Dr. D. H. Matthews: The depth, and even the existence of the blocks on the section that you have fitted to the gravity profile depends critically on the density contrast assumed. This is not great: —0.05 gm cm−3. It is presumably based on measurement of the density of handspecimens of the exposed rocks. Was the density of the exposed mica granite really identical to that of the volcanic rocks (ignimbrites) ?

Dr Almond replied that multiple hand specimens of both mica granite and ignimbrite gave the same average density of 2.60 gm cm−3. If measurements made on ring-dyke microgranites and rhyolite lavas were included the density range was still only from 2.59 gm cm−3 to 2.61 gm cm−3. The basal lavas of the volcanic succession are of significantly higher density but appear to be of very small volume. All the volcanic rocks are thoroughly indurated. Many of the ignimbrites are strongly welded and all of them have undergone devitrification and some recrystallization, so it is not surprising that their densities are similar to those of the associated granitic intrusions.

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