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

This paper continues the study of the constituent minerals of the Dartmoor granite. The district to which it mainly refers comprises the for-area around Widecombe and a part of the aureole adjoining. No detailed reference will be made at this stage to aureole phenomena. The results to be recorded necessitate a brief description of the two main granite types and of their field relationships. Abundant varieties and modifications of both types occur, but for the Purposes of the present paper they are of subordinate importance.Most of the for-masses and high-level exposures of granite in the area consist in the main of the type known locally as the ‘giant granite‘ — a very coarse-grained, strongly porphyritic roelc rich in biotite, and consistently garnetiferous; it has an index-figure 1 ranging from 7.5 to 12.

This paper describes occurrences of cookeite (not previously recorded as a British species) and hydromuacovite, associated with auriferous pyrite and arsenopyrite, blende, galena, carbonates, and free gold at Ogofau, on the left bank of the river Cothi, half a mile ESE. of Pumpsaint, Carmarthenshire.The hillside around Ogofau is scarred with opencast workings and trenches reputed to be an 'ancient gold mine' dating back to Roman times. Though the mine is described as 'disused' on the Ordnance Survey map (six-inchcs/mile, Sheet XVII, NW.), these surface workings have been exploited for free gold intermittently within the last fifty years, and recent development work (by the Roman Deep Holdings,Ltd.) from a shaft sunk in 1910 is approaching the producing stage.

On the basis of field relationships, the main types of the Dartmoor granite can be referred to four stages of intrusion, the sequence being from relatively basic to thoroughly acid.To this sequence is closely related an increase in the content of tourmaline and in the extent to which autopneumatolysis (with tourmalinization) was effective before solidification of the rock was complete.Autopneumatolysis is well displayed by many minor intrusions referred to the latest stage. Such an intrusive rock is reddened more or less uniformly throughout its entire mass, and volatile constituents exhaled from its margin have reddened a contact-zone of the grey granite into which it is intrusive.

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