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
The system of intrusive sheets in the Lower Carboniferous formations of the north of England, familiar to geologists throughout the world as the Great Whin Sill, has a voluminous early literature which it is unnecessary to summarize here. It will be sufficient to refer to the classical papers by Topley and Lebour on the field relationships (1) and by Teall on the detailed petrography (4), each of which contains an ample bibliography of the earlier literature. The most recent general account is by Prof. E. J. Garwood (9), who contributed a careful discussion of the evidence bearing on the period of intrusion.Teall's great paper closely followed on one devoted to some of the dikes of the north of England (3), and in it he drew attention to the remarkable petrographical Similarity between the rocks of the Heft and High Green dikes and that of the Whin Sill (4, p. 656).
I. G eneral D escription of the V olcanic A reas U ntil 1891 the possibility was entertained that Ruwenzori, like the other high mountains of Central Africa, might be of volcanic origin. As a result of his explorations during that year, Dr. F. Stuhlmann (1894) showed that this preliminary suggestion was without foundation, adding that no signs of volcanic activity had been detected in the regions traversed. In 1894, however, G. F. Scott Elliot recognized three well-marked volcanic belts in the adjoining country east and southeast of Ruwenzori ( Elliot & Gregory, 1895, p. 674 ; Elliot, 1896, p. 168 ). He described the bedded tuffs, craters, and crater-lakes of Toro (fig. 1) as occurring in the Vijonga area (Lake Kijongo), north-west of Fort Portal; the Butanuka or Kyatwa area, south of Fort Portal, locally forming the boundary of the low Toro plains to the east; and the Katwe and Kaihura Straits area (north-west of the Kazinga Channel between Lakes George and Edward). It is remarkable that Stanley failed to recognize the volcanic origin of the famous salt-lake of the Katwe crater. He visited the locality in 1889 and commented on the deep depression and the salinity of the lake, and even recorded the occurrence of a spring of sulphurous water. Scott Elliot also indicated that Karimi Hill, south-west of Ruwenzori (now in Belgian territory), might be of volcanic origin, and suggested that the different belts of cones and craters were alined on fissures radiating outwards from the uplifted massif of Ruwenzori. The nature of Karimi Hill
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.
In the following pages an account is given of the rock assemblages produced at the contact of a dolcrite intrusion with chalk. Owing to the intense localized assimilation of calcium carbonate by the dolerite magma, unusual rock types are produced. The products of the endogenous contact-zone are dealt with in detail. Those of the exogenous contact-zone have in part been described already, but some additional data are incorporated in this account.
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