A HISTORY OF DEVONSHIRE 



in this and other respects it difFers markedly from the granitoid fragments 

 in the Middle Culm rocks. At Clasiwell Pool south of Princetown the 

 granite is kaolinized, also at Shipley Bridge and Leftlake (north-west of 

 Brent) where there were formerly china clay works. 



West of Cornwood there are two isolated masses of granite ; the 

 smaller is Hemerdon Ball ; the larger, on Headon Down, is worked for 

 china clay, also worked on the adjacent part of the main mass on Lee 

 Moor, and near Cadover Bridge on the Plym. 



The Dartmoor granite is divided laterally by impersistent planes 

 which give it an appearance of stratification with dips toward the 

 surrounding rocks. The tors are due to the intersection of these planes 

 by more or less vertical or inclined joints. 



Contact Alteration. — From its rather regular northern boundary and 

 greatest axial breadth the granite extends southward with most irregular 

 boundaries. Its longest axis is from north to south. The limit of 

 alteration or contact zone seldom exceeds a mile in breadth, but is often 

 less, the breadth being determined by the subterranean slope of the 

 granite. Within this zone are Culm rocks, chiefly Lower Culm, from 

 Tavistock round the north margin to Holne on the east. Upper 

 Devonian slates occur on Wallaford Down and, except perhaps near 

 Dousland, from Cornwood to Tavistock. The cleavage or schistosity of 

 the rocks was affected by dynamic agencies before metamorphism. 



MacMahon' has described felsites and rhyolites at Sourton Tors, a 

 mica diorite at South Down, and tuffs with matrix rendered similar by 

 contact-metamorphism to the base of a rhyolite from Sourton Tors and 

 from Meldon. On the flank of Cox Tor ' beds of fine grained volcanic 

 dust' have been 'converted into a mixture of augite and felspar.' The dol- 

 erites have been converted into epidiorites. Banded dark green and cream- 

 coloured rocks occur on Peak Hill near Dousland. These were examined 

 by Teall, who considered their original character as probably decomposed 

 igneous rocks containing carbonate. Similar banded rocks with a por- 

 cellanized appearance occur near Ivybridge, Wrangaton and South Brent ; 

 and in their vicinity dark rocks with chiastolite are met with above 

 Ivybridge and in the contact zone near South Brent. It is probable 

 therefore that Culm and Upper Devonian rocks with associated volcanic 

 rocks occur on the margin of the granite in these localities in which 

 there are also intrusive greenstone masses. An example of the latter 

 near Dousland was considered by Teall to have been originally an 

 ophitic dolerite. Garnets are found at Shaugh and Peak Hill amongst 

 other localities, axinite near Sourton, Walkhampton and South Brent, etc. 

 Teall detected a foliated scapolite pyroxene rock amongst the altered 

 rocks near Dousland.'' The Upper Devonian slates usually exhibit a 

 spotted alteration on the west of Dartmoor and become mica schists. 



Dr. Busz ' described the contact zone by the Avon above South 

 Brent as composed of andalusite hornfels, andalusite schist, chiastolite 



* ^art. Joum. Geol. Soc. vol. 1. p. 338. 

 * Proc, Somerset Arch, and Nat. Hist. Soc. (1900), vol. xlvi. ' Op. cit. 



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