"Vol. 65.] OVERTHRUSTS AT TINTAGEL. 265 



17. Oil OVERTHRUSTS at TiNTAGEL (NoETH CORNWALL). 1 By HENRY 



Dewey, F.G.S. (Read April 7th, 1909.) 



[Plate XIII- Map.] 



Contents. 



Page 



I. Introduction 265 



II. Description of the Area covered by the Map 265 



III. Structure of the District 269 



IV. The Coast-Sections 272 



V. Tectonics of the District 278 



I. INTEOD POTION. 



On mapping the country to the north of Bodmin Moor for the 

 Geological Survey, it was found that the slate-rocks, as noted by 

 Mr. John Parkinson, are separable itito several well-marked litho- 

 logical types. These types are of widespread occurrence, and strike 

 across the country in parallel zones in a general west-north-westerly 

 and east-south-easterly direction with a constant dip to the north- 

 north-east. 



But, although this stratigraphical succession occurs over so wide 

 an area, it apparently broke down in the Tintagel district, where 

 a repetition of the strata was found near the coast. Subsequent 

 investigation, however, proved the repetition to be due "to overthrusts 

 from the west-north-west. With the district affected by overthrusts 

 I propose to deal in the present paper. 



Several observers have written about the geology of Tintagel, but 

 as none of them interpreted the structure, it is only necessary to 

 give references to their papers in the following list : — 



H. S. Eoase, Trans. Roy. Geol. Soc. Cornwall, vol. iv (1832) p. 166. 



A. Sedgwick & Sir R. I. Murchison, Trans. Geol. Soc. ser. 2, vol. v, pt. iii 



(1840) pp. 672-713. 

 A. Sedgwick, Quart. Journ. Geol. Soc. vol. viii (1852) p. 1. 

 David Williams, Trans. Roy. Geol. Soc. Cornwall, vol. vi (1846) p. 122. 

 Sir H. T. de la Beciie. ' Report on the Geology of Cornwall, Devon, & 



West Somerset ' 1839, pp. 56-58. 

 D. Shakpe, Quart. Journ. Geol. Soc. vol. iii (1847) p. 74. 

 S. R. Pattison, Trans. Roy. Geol. Soc. Cornwall, vol. vii (1865) p. 3. 

 W. M. Patchings, Geol. Mag. dec. iii, vol. vi (1889) pp. 53, 101 & 214. 

 J. Parkinson, Quart. Journ. Geol. Soc. vol. lix (1903) pp. 408-28. 



II. Description op the Aeea covered by the Map (PI. XIII). 



The area represented on the map covers about 5 square miles, 

 with about 4 miles of coast-line. The district forms a plateau ending 

 off in cliff's which are between 300 and 400 feet high. A portion of 



1 Communicated by permission of the Director of H.M. Geological Survey. 



