CLEAVAGE STRUCTUEE. 



185 



tical spots of finer material. In clay-deposits of the present day it is 

 also common to find imbedded little round nodnles of finer material. 

 It is probable that the greenish 

 nodules in slate were also 

 rounded nodules of finer clay in 

 the original clay-deposit from 

 which the slate was formed by 

 consolidation. But in cleaved 

 slates these nodules are always 

 very much flattened in the di- 

 rection at right angles to the 

 cleavage-planes, and spread out 

 in the direction of these planes. 

 (e.) Experimental Proof. — 

 Finally, experiments by Sorby 

 and by Tyndall show that clay 

 (the basis of slates), when sub- 

 jected to powerful pressure, ex- 

 hibits always a cleavage, often 

 a very perfect cleavage, at right 

 angles to the line of pressure. FlG - 164 ~ A Block of cleaved slate < after Jukes >- 



Physical Theory. — Cleavage is certainly produced by pressure, but 

 the question still remains : How does pressure produce planes of easy 

 splitting at right angles to its own direction ? "What is the physical 

 explanation of cleavage ? 



Sorby's Theory.* — Mr. Sorby's view is that all cleaved rocks con- 

 sisted, at the time when this structure was impressed upon it, of a plastic 



mass, with unequiaxed foreign particles dis- 

 seminated through it ; and that ~by pressure 

 the unequiaxed particles were turned so as 

 to bring their long diameters in a direction 

 more or less nearly at right angles to the 

 line of pressure, and thus determined planes 

 of easy fracture in that direction. Usually, 

 as in slates, the plastic material is clay, and 

 the unequiaxed particles are mica-scales. 

 Let A, Fig. 165, represent a cube of clay 

 with mica disseminated. If such a cube be 

 dried and broken, the fracture will take 

 place principally along the surfaces of the 

 mica, which may therefore be seen glisten- 



Fig. 165.— Illustrating Sorby's The- • _ „ n » j? xi j* j. 



ory of siaty cleavage (after Sorby). mg on the uneven surface of the fracture ; 







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A 





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B 



* Philosophical Magazine, second series, vol. xi, p. 20. 



