Vol. 59.] THE TINTAGEL AND DAVIDSTOW DISTRICT. 413 



Slates, and may be traced down to the shore, with the exception 

 of the middle third- of the cliff, where their place is taken by the 

 dark slaty variety. The typical rocks, therefore, are found with 

 considerable irregularity. In the same way, the road from the 

 Prince of Wales's Quarry to Treknow Mill 1 passes through a series 

 of rocks mapped as Hall well-Cottage Beds, and these, locally (as, for 

 example, east of the MillEoad), are excellently banded. No doubt 

 the whole is approximately on one horizon. Nevertheless, not 

 only are the beds themselves variable, but here, as in other places, 

 the true order may be confused by small faults, which are inappre- 

 ciable because exact horizons are difficult to determine. Over 

 Waterpit Downs and north-west of Condolden surface-indications 

 are all that are available, and the position of the underlying beds 

 may be most readily determined by the presence of a dark slabby 

 rock, with lustrous fracture-faces and a fibrous structure produced 

 by the orientation of its crystals of white mica (p. 424). This 

 rock is found in situ in the railway-cutting which lies to the west 

 of Hendraburnick Farm, and is considered as part of the Hallwell- 

 Cottage Series. 



The underlying rocks (Pen pe thy Beds and Slaughterbridge 

 B e d s), as well as the epidiorites, are described in Part II. Over- 

 lying the Tredorn Beds are carbonaceous blue slates, well developed 

 in the neighbourhood of Lesnewth. They have not been studied h\ 

 me, and are not described in this paper. 



In the petrographical description which follows, the beds are 

 taken in descending sequence, with the exception of the Upper and 

 Lower Blue-Black Slates, which are described together near the 

 end of the paper. 



Part II. — Description of Microscopical Detail. 



The Tredorn Beds. 



The rocks from the following localities form a well-marked type : — 

 Abbott's Hendra Farm, north-west of St.Clether, the small quarry to 

 the south of Hallworthy.Treegreen Farm, the road south of Otterham 

 Station, parts of the South-Western Railway-cutting near Lesnewth, 

 and Lanterdan Quarry on the coast. Thin sections show these rocks 

 to be fine-grained and rather opaque phyllites, composed of minute 

 flakes of a pale-green chlorite and sericite, set in a translucent base 

 which, in many instances, appears to be isotropic. Specks of iron- 

 oxide are common, and for the most part are haematite ; but ilmenite 

 occurs, frequently showing excellent sagenitic r utile. Tourmaline, 

 ratile-grains, and, locally, minerals resembling ottrelite and colour- 

 less garnet are accessories. 



The rock found north of the farm of Tregrylls, on the road 

 to Lesnewth, affords an instance of a second type of the Tredorn 

 phyllites, and one which is characteristic of these beds westward 

 as far as the coast. The Tregrylls rock is harder than the Abbott's- 

 Hendra type, of a slabby fracture, dark slate-grey in colour, and 



1 The road from Camelford Station to Trebarwith Strand. 



