GEOLOGY 



on Babbacombe beach, and near the granite at South Brent a (quartz- 

 mica-diorite) kersantite. 



Lamprophyres are met with in the Culm at Rose Ash in north 

 Devon. In the Upper or Middle Devonian on the east of Ingsdon House. 



Some dykes of decomposed rocks allied to the Exeter traps occur 

 in the Lower Culm north of Kitton Barton (south of Ashbrittle). 



Felsites. — Felsitic dykes occur between Tavistock and Plymouth in 

 Cann Wood near Bickleigh, on Roborough Down near Walkhampton, 

 at Horrabridge, Lopwell and Morwell Down, in Upper Devonian slates. 

 Between Modbury and Kingston at Whympston Wood there is a spheru- 

 litic granophyre and several exposures of spherulitic banded and nodular 

 felsites near Tor, Wastor and Shearlangston. Small exposures of felsite 

 are met with in Putshill Copse and near Mounts east-north-east of Wood- 

 leigh (? of date of Permian eruptions). The Bittadon felsite in the 

 Morte slates has been alluded to. Decomposed elvans occur in Middle 

 Culm rocks on Itton Moor, at Gribbleford Bridge, etc., on the north of 

 Dartmoor. At Meldon and in Cann quarry white granite dykes, or 

 aplites, are visible. Near Christow there is a small mass of quartz- 

 porphyry. Near WooUey west of Bovey Tracey a rhyolitic rock borders 

 the granite. 



GRANITE 



The Dartmoor granite is about 225 square miles in extent. It is, 

 generally speaking, a coarsely porphyritic granite with large orthoclase 

 felspars, quartz, biotite and muscovite, and is more or less schorlaceous 

 especially towards the contact margin. At Trowlesworthy Tor a red 

 granite, consisting of red felspar schorl and fluorspar, has been noted by 

 Worth.' Finer grained granite is met with, here and there, in intrusive 

 elvans very difficult to trace, sometimes in nearly horizontal bands, some- 

 times in small patches, and sometimes at the margin from which it passes 

 into the coarser variety. MacMahon * mentions the Tavy valley between 

 Hill Town and White Tor as a good locality for studying the relations 

 of the coarse and fine granites, which here suggest ' the imperfect mixing 

 of two portions of the granite magma in different conditions of fluidity.' 

 Rutley ' described a junction at Brazen Tor where amphibolite is in 

 contact with schorl-spotted fine grained granite passing within 1 00 yards 

 into porphyritic granite. 



Dr. Busz noted three varieties of granite in contact with Culm 

 hornfels in a quarry near Bovey Tracey — coarse granite with large ortho- 

 clase crystals, fine grained granite, and rock with large crystals of plagio- 

 clase yellowish orthoclase and large quartz grains in a fine grained grey 

 matrix. 



The presence of schorl, and of fluid inclusions in the quartzes, has 

 been shown by Hunt* to be characteristic of the Dartmoor granite, and 



' Rowe, A perambulation of Dartmoor (third edition revised, etc., 1896), p. 244. 

 ^ ^art. Joum. Geol. Soc. vol. xlix. p. 389. 

 ' Geol. Survey 'Memoir On Brent Tor,' p. 38. 

 * Geol. Mag. June, 1899, p. 256. 



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