1848.] 



SHARPE ON SLATY CLEAVAGE. 



119 



organic remains. The lines y y show the direction of the cleavage- 

 planes. The plane of bedding a a has been broken into a very irre- 

 gular series of steps by some force which has either moved each 



sheet of slate forward in ad- 

 vance of its neighbour, or has 

 caused an uneven expansion 

 of the parts of the rock. The 

 fossils have suffered from this 

 movement of the surface. 

 The whole bed is covered by 

 small irregular ridges parallel 

 to the cleavage, which make 

 it impossible to measure the 

 angles accurately. 



In figure 7, « « represents 

 the section of the surface of 

 a bed of black calcareous 

 slate in the Bickington lime- 

 stone quarry, North Devon, 

 which is bent over in a steep 

 arch, and dips rapidly to the 

 north ; it is intersected by 

 cleavage y y, dipping nearly 

 70° to the south. In this 

 instance the surface of the 

 bed « a is bent into parallel 

 waves without its continuity 

 being broken : but the out- 

 lines of the beds below h h 

 and c c may be traced in the 

 solid rock unchanged; so that 

 the disturbance of the surface seems only to have taken place where 

 the looser packing of the rocks allowed room for some play of their 

 parts. 



Figure 8 represents another case of the same kind, observed in the 

 Brathay flag-stone quarry near Ambleside ; a a being the surfr.ee of 

 a bed, and yy the planes of cleavage. 



In cases like the last two, the whole surface of the beds a a is 

 covered with irregular flutings running in the direction of the strike 



