1846.] 



SHARPE ON SLATY CLEAVAGE. 



95 



cleavage is southerly ; to the S. of the Stokenham line it is northerly*. 

 A line of vertical cleavage which must be considered identical with 

 thatof Start Point appears to run to the N.of the Lizard Point ; at least 

 we are told that on the N. of the serpentine of that point, the laminae 

 of the hornblende slate dip to the N, and N.W., which being the 

 reverse direction to the rest of the district, shows that we have here 

 passed the vertical line. The bearing of these two places is about W. 

 by S. to E. by N., which is nearly the line of strike of the cleavage 

 for the district. 



In the N. of Devonshire the cleavage is vertical in the middle of 

 the quarry of black limestone containing Goniatites at Bickington, a 

 mile and a half S. of Barnstaple. The cleavage here strikes nearly 

 E. and W. ; at the N. end of the quarry it dips 70° to the S., and at 

 the S. end 80° towards the N. Between six and seven miles N. of 

 this spot, the cleavage is vertical at Swinham Down with a strike to 

 the E.N.E., and between these two lines the planes usually dip to 

 the N.N.W., but with occasional reversal of dip and irregularity. 

 Between Swinham Down and the Bristol Channel the cleavage 

 always dips S., S.S.W. or S.SE., the angle diminishing as we recede 

 from the vertical line, so that at Hillsborough near Ilfracombe it has 

 fallen to 60° and at Linton to 35°. The Bickington line of vertical 

 cleavage appears the boundary of the Devonshire area of elevation, and 

 the Swinham line that of another parallel area, of which the northern 

 limit must be sought on the opposite coast of Pembrokeshire and 

 Glamorganshire ; the band between Swinham Down and Bickington 

 being neutral ground lying between the two areas. 



We thus establish two lines of vertical cleavage, 60 miles apart, 

 the one at Stoke Fleming, the other at Bickington, to serve as bound- 

 aries to the area to be examined. In the centre of this area there 

 is a broad band of country over which the cleavage planes undulate 

 in low flat waves, so that considered as a whole they are nearly hori- 

 zontal. The axes of these flat curves bear between E. and E.N.E., 

 and the cleavage dips alternately to about N.N.W. or S.S.E. at angles 

 which rarely reach 10°. The curves of the cleavage frequently cor- 

 respond with an anticlinal arch of the bedding, both having the same 

 axis ; but the inclination of the beds is always greater than that of 

 the cleavage. The section fig. 19 would represent any one of these 



Fig. 19. 



The continuous lines represent the bedding, and the dotted lines the cleavage. 



waves as they are seen near Launceston, or on the coast near Tin- 

 tagel. In other cases the two axes have a slightly difl'erent direc- 



* Report on Cornwall, &c., p. 77. 



