126 OKIGIN AND DEVELOPMENT OF OEE DEPOSITS. 



Hope's Nose, in Devon, where the same cleavage runs uninter- 

 ruptedly through, two sets of beds which are distinctly of different 

 ages, the lower set being much contorted. Here it is plain that the 

 lower beds had become consolidated, contorted, upheaved and 

 denuded before the upper beds could be deposited, while these 

 latter had also been consolidated before cleavage was set up. 

 Prof. Sedgwick suggested that the cleavage might be due to 

 some hind of crystallization on a large scale, but this is negatived 

 by the microscopic observations of Sorby and others already 

 quoted. 



But the Rev. 0. Fisher has called attention to the fact* that 

 to produce cleavage the pressure must be in all cases accompanied 

 by a sideways strain or "Shear." He concludes therefore that 

 the internal movements of rock particles which result in slaty 

 cleavage are due to those rocks having been elevated, and left 

 after elevation in a position too lofty for equilibrium. The 

 "shear" which follows, owing to the action of gravity when for 

 any reason faulting cannot take place, produces slaty cleavage. 



Mr. Fisher shews that the direction of shear need not be 

 the same as that of the resultant cleavage ; on the contrary it 

 may differ from it as much as 45°, and diminishing from that as 

 a maximum, the two directions may become nearly though 

 never quite identical as the shear is increased. f He also shews 

 that shear alone would be competent to produce cleavage but 

 for the faulting that would then take place, pressure being 

 necessary to prevent faulting. 



This pressure also produces condensation, hence the general 

 rule that, other things being equal, the more dense a slate the 

 better its quality. 



M. Ed. Janettaz has also investigated this subject experi- 

 mentally, and the structure he calls longrain, long, andjll appears 

 to be a kind of secondary potential cleavage, similar to and 

 parallel with the shear just mentioned. In experimenting on 

 the plastic clay of Issy near Paris, a quantity was placed in a 

 box having an outlet at the side. When this clay was forced 

 out by application of a pressure from above equal to 40 k. per 



*Geol. Mag. 1885, p. 397, et seq. 

 flbid, p. 400. 



