Hev. 0- Fisher — On Cleavage and Distortion. 403 



specific gravity at 15°-5 C. is 2-64:4; and the loss on ignition is 

 5"08 per cent. The slate examined had the same specific gravity 

 as the Devonian, viz. 2'64; and the loss by ignition was 2*44 per 

 cent. If we divide the water in the Devonian, so that one portion 

 combined with the rocky matter will give the composition of slate 

 rock, we shall have, 



Devonian rock= 2-77 water 



2-31 water ^ — 1 f V 



94-92 rocky matter ) 



100-00 



That is, Devonian rock=: 2-77 water 



97-23 slate rock 



100-00 



If then 2-77 per cent of water was to be removed from the Devo- 

 nian rock, the remainder might be taken to have the composition of 

 a slate. Probably the loss of this amount of water would cause the 

 material to shrink somewhat ; but we will neglect that cause of 

 shrinking, and credit the whole of it to compression. 



The specific gravity of the remaining mass of the Devonian (its 

 volume being unaltered) will bear to its former specific gravity the 

 same ratio, that its present mass bears to its former mass, so that 

 it will now be 97-23-^ 100 x 2-644=2-57. We must next suppose 

 this substance compressed, until its specific gravity becomes that 

 of slate. The volume after compression will be to that before 

 compression inversely as the specific gravities : and 2-64 being the 

 specific gravity of slate, the volume of the compressed rock will 

 be, 2-56-f-2-644, or 0'97 of its original volume. Thus we see that 

 a compression of less than three per cent, would suffice to reduce 

 the Devonian rock to slate. But being confined on all sides (as 

 would be the case deep in the earth), the rock could not expand 

 laterally, and the change of dimension could take place only in 

 the direction of the pressure ; so that we may say that 100 feet 

 thickness of the Devonian rock of Turnford would not quite be 

 compressed into 97 feet on being converted into slate. This is a 

 compression of only three per cent. 



It is probable that, if a substance was homogeneous, a shearing 

 action, accompanied by compression, would not induce cleavage. But 

 the rocks which have been cleaved were not homogeneous. Every 

 particle, therefore, being liable to distortion, would be thinned in the 

 direction of the shortest axis, and elongated in that of the longest 

 axis of the ellipsoid. The breadth only of the particle, measured 

 in the direction of the strike, would remain unaltered. These 

 changes would convert the rock into an assemblage of flattened ovoid 

 particles of unequal strength, and render it liable to cleave, the 

 fracture running among the particles parallel, on the average, to the 

 major diametral plane of the ellipsoid. This agrees with the structure 



