The Petrography and Correlation of the Igneous Rocks of the Torquay Promontory

Abstract

The Middle and Upper Devonian rocks of the Torquay Promontory give evidence of a volcanic phase in Middle Devonian times, recurring in the Upper Devonian, and completed by a hypabyssal phase, which was post-Culm and pre-Permian. The tuffs are basic in nature, of glass and lava fragments, cemented by calcite. The only lava recognized is a spilite, probably Upper Devonian.Complementary rocks: a soda-porphyrite and an augite-lamp-rophyre, have been recognized at Babbacombe.An albite-dolerite is described from Black Head, carrying small amounts of olivine and with the characteristic albite-segregation veins. This mass has been shown to include the Ilsham Manor and the Red Rocks outcrop at Babbacombe. Metasomatic change due to silicification is characteristic of its flanks: this is especially developed at Anstey's Cove, where quartz-hæmatite veins are later than the cleavage of the slates.It has been shown that there is strong evidence for the post Culm date of this intrusion and hence for the tectonic movements of the area. The latter have been in the sequence:—tilting, intrusion, folding, thrusting, cleavage, and faulting, Some strike faults may be anterior to all these, but were repeated after the cleavage movements.