The Permian Lavas of Devon

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Abstract
I. I ntroduction T he rocks to be described comprise the relics of a suite of lavas forming two main groups of exposures : (i) the Exeter lavas, constituting a zone of outcrops north of Dartmoor, and (ii) lavas south of Dartmoor occurring near Kingsbridge and in the Maker peninsula south-west of Plymouth. These rocks present features of special interest as being the latest surface manifestations of the Dartmoor cycle of igneous activity, and are products of a later stage of magmatic differentiation than any other known rocks of the region. All the occurrences have been previously recorded and, in part, described, but the probable modes of origin have not hitherto been discussed. The present investigation was, therefore, directed to the study of the suite as a whole in the light of recent petrological progress, the object being to determine the precise character and sequence in time of the Devon lavas and to discuss the genetic relationship with the igneous rocks in the Devon sub-province. (a) Distribution of the Lavas Geographically, the lavas occur in five groups : (i) immediately to the south-west of Exeter, (ii) in the valleys of the Exe, Culm, and Clyst, (iii) in the Crediton valley, (iv) west of Tiverton, and (v) to the south of Dartmoor. With the exception of the rocks in groups (ii) and (v), the lavas occur generally at or near the boundary between the Culm Measures and the Permian. Lavas of group (ii) appear to lie higher in the succession and, with minor

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