278 MB. H. DEWEY ON [Aug. I909, 



V. Tectonics oe the District. 



We are now in a position to discuss the causes which gave rise to 

 the structures shown in the sections and map (PL XIII). We have 

 seen that the disposition of the beds is due to an anticline which 

 pitches north-westwards, and that this structure continues towards 

 the east for a distance of at least 12 miles. It has been further 

 seen that buckling occurred to the south of the nose of this anti- 

 cline, and produced minor folds which increase in amplitude north- 

 westwards, until they are replaced by overthrusts. 



But it may be asked whether there is evidence in the rocks 

 themselves of pressure sufficiently great to produce these folds. 

 There is plenty of evidence that great pressure affected the rock- 

 masses ; and, without going into details, it may be pointed out 

 that the Barras Nose and Trambley Cove Beds are intensely 

 brecciated ; that the lava often includes shear-lenticles ; and that 

 the phyllites are frequently in a structureless state. Under the 

 microscope the rocks seldom contain unbroken minerals, but 

 exhibit long, curving, parallel bands of chlorite and actinolite, 

 enclosing ' eyes ' of harder minerals. 



All of the rocks are contact-altered, and contain secondary 

 minerals : the secondary minerals of commonest occurrence being 

 ottrelite and chloritoid, white and brown micas, and actinolite. 

 Sericitization occurred before the development of the ottrelite and 

 white micas. Later than these are minerals of pneumatolytic 

 origin, which occur in veins cutting across the strike of the beds at 

 several localities, as at Trewethet Gut, where schorl and quartz- 

 veins are exposed. 



The question arises as to how this contact-alteration was effected. 

 There is no exposure of granite nearer than the Bodmin Moor mass, 

 which is 5 miles away ; but it is possible that an underground 

 spur of this granite-mass runs in a general north-westerly direction 

 beneath the metamorphosed area, at a depth of about 2000 feet, 1 

 and produced the metamorphism of the overlying rocks. 



Briefly to recapitulate this part of this paper, it may be said that 

 sediments and contemporaneous igneous rocks of Upper Devonian 

 age were folded into an anticline pitching north-westwards ; that 

 the beds were crumpled on the south side of the nose of this fold ; 

 that the crumpling increased in amplitude, until overthrusts re- 

 placed the minor folds towards the north-west of the crumpled 

 area ; and that denudation and erosion have since revealed these 

 structures in the Tintagel district. 



I should here like to acknowledge the kind assistance which I 

 have received from Dr. Teall and Mr. Clement Beid. They have 



1 Clement Eeid, ' Geology of the Land's End District ' Mem. Geol. Surv. 

 1907, p. 8. 



