The Dartmoor Granites: their Genetic Relationships

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Abstract
I. I ntroduction S ince Sedgwiek and Murchison initiated the structural study of the West of England nearly a century ago a large amount of data has been applied to the interpretation of the granites of Cornwall and Devon—their age and origin, their space-form and attitude relative to the country-rocks, and their probable continuity at shallow depth. Concerning the space-form of the Dartmoor granite, the ideas of De la Beche (1839) agree essentially with the modern conception of a batholith. R. N. Worth (1888-9) considered the exposed granite to be the lower portion of a volcanic focus denuded of its superstructure. On the other hand, A. R. Hunt (1894) considered the granites to be wholly plutonic. A laccolithic emplacement was suggested by W. A. E. Ussher (1888) , but he withdrew this theory four years later. The laccolith theory was advanced as a probability by the authors of the Geological Survey Memoir (1912, No. 338) ; laccomorphic internal features were described, and the composite character of the intrusion was recognized. Similar features, correlated according to type and emplacement-sequence, were described by the present authors (1923, 1926) and by Mr. C. W. Osman (1928) . Mr. R. H. Worth considers the West of England granites to be protrusions from a granite wedge forming the axis of the peninsula (1930, p. 94), and Dr. R. H. Rastall describes the Cornish granites as cupolas on a batholith with underground connexions at small depth (1931). None will question the reality of the cupola-feature, but the unqualified interpretation of the

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