Vol. 65.] OVERTHROWS AT T1NTAGEL. 267 



The ottrelite-phyllites of Tintagel were described by Mr. W. M. 

 Hutchings, 1 and this was the first record of the occurrence of 

 ottrelite in British rocks. Mr. J. Parkinson ' carefully described a 

 somewhat similar rock from Davidstow, but identified the secondary 

 mineral as clinochlore. These two minerals, however, do not alone 

 'spot' the phyllites ; for another mineral, which agrees neither 

 physically nor optically with any described chlorite or chloritoid, 

 is very abundant in rocks from widely separated localities. 



The three kinds of mineral, nevertheless, resemble one another 

 very closely, and their presence on the surface of the phyllites 

 renders the identification of the rock in which they occur easy in 

 the field. Although the cleavage is poor, the rock constitutes the 

 best building-stone in the district, and it is largely quarried. 



At "Woolgarden Farm, St. Clether (off the map), this phyllite is 

 especially well exposed ; but the rocks are also well exposed at 

 Bossiney, near Gillow Quarry, and north of Lanterdan Quarry, 

 where their characters may be studied. 



(iii) The Barras Nose Beds. 



These consist, for the most part, of blue-black gritty slates. 

 Under the microscope they are seen to be almost wholly siliceous ; 

 but they also contain strands of chlorite and much carbonaceous 

 matter. On cross-fracture well-marked black and white banding 

 occurs. The rock is hard and brittle. In many coast-localities 

 pressure has produced intense brecciation, and in such cases the 

 breccia is cemented and veined with infiltrated quartz. 



Tourmaline-needles are characteristic of many of the specimens, 

 especially of those from the south side of Gullastem. 



Barras Nose is practically built up of these thin gritty slates, 

 which lie bare on the slopes of the headland so as to be easily 

 studied. 



(iv) The Volcanic Series. 



Above the Barras Nose Beds a great thickness of sheared lavas 

 dip northwards. Their distribution is seen on the map (PI. XIII). 

 From their present condition it is difficult to say what was their 

 original constitution, but some indication is obtained from less- 

 sheared examples ; those occurring at Trebarwith Strand in the 

 south of the map may be taken as examples. At this locality, they 

 are vesicular pillow-lavas very rich in calcite. 3 



Usually, the rock is in the condition of a green or purple schist 

 composed of chlorite or actinolite. Porphyritic felspars are preserved 

 in some cases, particularly well seen near Treglasta in Davidstow, 



1 Geol. Mag. dec. iii, vol. vi (1889) p. 214. 



a Quart. Journ. Geol. Soc. vol. lix (1903) pp. 408-28. 



3 It is probable that these lavas are part of the same sheet as that occurring 

 at Portquin Bay and neighbourhood, described by Mr. Eeid and myself in this 

 Journal, vol. lxiv (1908) pp. 264-69. 



