266 MR. H. DEWEY ON [Aug. 1909, 



a still higher plateau rising to 600 feet occurs in the south-eastern 

 corner of the map. Two small streams dissect the lower plateau, 

 the one on the north forming the "Rocky Valley ; and the other, on 

 the west, the Trevena Valley. The whole area forms a land-mass 

 which protrudes from the general coast-trend of North Cornwall, 

 and from this mass several smaller headlands jut out westwards. 

 The order of succession of the rocks is as follows : — 



(vi) The Tredorn Phyllites. 



(v) The Trambley Cove Beds. 

 (iv) The Volcanic Series, 

 (iii) The Barras Nose Beds, 

 (ii) The Woolgarden Phyllites. 



(i) The Delabole Slates. 



Epidiorites (intrusive into all the above). 



(i) The Delabole Slates. 



These slates are mainly composed of matted flakes of sericite with 

 some chlorite, pyrite, and ilmenite, and garnets are plentiful. Small 

 grains of zircon are apparently the only clastic minerals in the rock. 

 jDa t The beds can best be studied in the great quarry at Delabole, 

 about 5 miles to the south-east of Tintagel. Here two varieties are 

 seen : one is grey-blue in colour, and furnishes the well-known 

 roofing-slate ; the other is grey-green, and is rejected in the process 

 of working. The two kinds of rock are local variations of, and pass 

 by insensible gradations into, one another. The grey-blue slate was 

 described and analysed by J. A. Phillips l and other workers. 



Certain bands in the quarry contain fossils in abundance, the 

 best known fossil being the so-called ' Delabole butterfly ' (Spirifer 

 verneuili). 



The grey- green beds are less well cleaved than the grey -blue : they 

 are often spotted with oval white patches, and at times by rusty 

 iron-ore spots. Flakes of micaceous pyrite, weathering into brown 

 circular zones, spot the typical grey-blue slates, and the spotting is 

 a valuable aid to their recognition in the field. Both varieties occur 

 in the Lanterdan and other quarries marked on the map. 



(ii) The Woolgarden Phyllites. 



These beds vary slightly from place to place, but by far the most 

 usual form is a banded phyllite. The banding is seen on cross- 

 fracture to be due to alternating layers of gritty and chloritic or 

 sericitic material. The sericitic variety is usually seen, and has a 

 crystalline, saccharoidal appearance ; but the chloritic variety is in 

 places developed to the exclusion of the sericitic one. Another 

 feature which distinguishes this phyllite is the constant presence 

 of a secondary green mineral, which at times can be identified as 

 ottrelite and at others as clinochlore. 



1 Quart. Journ. Geol. Soc. vol. xxxi (1875) p. 319. 



