Vol. 65.] OVERTHRTJSTS AT TINTAGEL. 269 



outside the map. Types rich in epidote are widely distributed, and 

 are common at Gullastem and Trevena. Some of the lava is so rich 

 in calcite that it was burnt for lime, but the tendency of the rock to 

 run to a slag rendered the practice risky, and it was discontinued. 



At Tintagel Haven the lavas are rich in secondary silica, and the 

 felspars (although showing strain-shadows) are large and well 

 preserved. The original ferromagnesian constituents have in all 

 cases been converted into chlorite and actinolite, and this conversion 

 occurred after the shearing. 



Magnetite is the chief iron-ore ; but pyrite is not uncommon, and 

 sagenitic rutile is often visible. 



(v) The Trambley Cove Beds. 



These resemble the Barras Nose Beds in many respects, and do 

 not require description. It may be noted that, in inland localities, 

 poorly preserved fossils occasionally are found in these slates. 



(vi) The Tredorn Phyllites. 



These phyllites vary slightly in colour, but are usually grey-green. 

 They consist mainly of chlorite and sericite, and often contain small 

 white felspars scattered through the meshes of the rock-substance. 

 This type is common in the west, and is replaced inland by grey- 

 green softer phyllites. Cleavage is not usually well developed, though 

 the rock was quarried at several places for slates. Fossils of Upper 

 Devonian age are abundant in some bands, but are seldom speci- 

 fically determinable : Spirifer vemenili, Fenestella, and crinoids are 

 the common forms. The phyllites are best seen at Trevalga cliffs, 

 in the north-eastern corner of the map (PI. XIII). 



Besides these rocks, bosses of epidiorite occur intruding into all 

 the series described. 



III. Structure of the District. 



Having described the rocks, we may now return to the map. 

 The general strike over the greater part of the area represented is 

 north-north-east and south-south-west ; but Bossiney Bay marks 

 the position where an abrupt change of strike occurs. Prom the 

 coast at Trevalga, for over 12 miles inland, the rocks strike south- 

 south-eastwards. 



The structure of the district will be best understood by studying 

 the map and sections. Beginning at the south-eastern corner of 

 the map, and following the line of section (fig. 1, p. 268), we note 

 that the sequence is perfectly normal. Along the line of section 

 illustrated in fig. 2 (p. 268) the succession is regular to the top of 

 the Tredorn phyllites. Overlying these, we find (i) the Delabole 

 Slates ; (ii) the Woolgarden Phyllites ; and (iii) the Barras Nose 

 Beds. The section shows that the Barras Nose Beds form a mere 



a. J. G-. S. No. 259. t* 



