118 



PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [NoV. 1, 



<?; 



obviously mechanical ; the resistance offered by the oblique sheet of 

 quartz to the pressure forward of the mass, has caused the bending 

 of the sheets of slate at the ends which abut against the quartz. 



A slight change in the direction of the clea- 

 Fig. 5. vage may frequently be observed in its passage 



^. from one bed of rock to another, arising pro- 

 / bably from beds of different hardness offering 

 different degrees of resistance to pressure. 



Figure 5 represents the section of a fragment 

 of slate from Langdale, in which the thin beds 

 a a and h h are rather softer than the other beds 

 c c ; the lines y y show the direction of the 

 planes of cleavage, which are perpendicular in 

 passing through the beds c c, but dip 80° to the 

 southward in the beds a a and b b, the beds 

 dipping to the north at an angle of 60°. 



Here the change in the direction of the 

 cleavage in passing from bed to bed is fully 

 10°, which is an unusual amount of deviation ; 

 but a variation of two or three degrees under 

 such circumstances is often met with. The 



K 



cause of these irregularities must also be me- 

 chanical. 



The surface of particular beds of slate is 

 sometimes found broken up into a series of 

 steps parallel to the strike of the cleavage : 

 the front of each step is formed by the plane 

 of cleavage, and the top by a portion of the 

 bed which has been displaced from its original 

 position and appears to lie at an angle towards 

 (f the original plane of bedding. 



6 represents the section of a fragment of slate from the 



** Pilton beds " two miles north of Barnstaple : the upper broken 

 line a a shows the present form of the surface of a bed covered with 



