CLEAVAGE STRUCTURE. 187 



crete granules or crystals, with surfaces of easy fracture between them. 

 When such substances are broken, the fracture takes place between the 

 crystals or granules, producing a rough crystalline or granular surface, 

 entirely different from the smooth surface of vitreous fracture. Marble, 

 cast iron, earthenware, and clay, are good examples of crystalline and 

 granular structure. Now, if a mass thus composed, yield to pressure, 

 every constituent granule is flattened into a scale, and the structure be- 

 comes scaly ; and as the surfaces of easy fracture will still be between 

 the constituent scales, we have cleavage at right angles to the line of 

 pressure. A mass of iron, just taken from the puddling-furnace and 

 cooled, exhibits a granular structure ; but if drawn out into a bar, each 

 granule is extended into a thread, and the structure becomes fibrous ; 

 or if rolled into a sheet, each granule is flattened into a scale, and we 

 have a cleavage structure. 



There can be little doubt that this is the true explanation of slaty 

 cleavage. The change of form which, as we have seen, has taken place 

 in the fossil-shells, encrinal joints, and rounded nodules, has affected 

 every constituent granule of the original earthy mass, so that the struct- 

 ure becomes essentially scaly instead of granular ; the cleavage being 

 between the constituent scales. Sorby, it is true, in his observations 

 on cleaved limestones, recognized the true cause of cleavage, viz., the 

 change of form of discrete particles ; but he regarded this as subordi- 

 nate to change of position. Besides, the particles of Sorby were/or- 

 eign, which Tyndall has shown to be unnecessary ; while the particles 

 of Tyndall are constituent. 



Geological Application. — It may be considered, therefore, as certain 

 that cleaved slates have assumed their peculiar structure under the in- 

 fluence of powerful pressure at right angles to the cleavage-planes, by 

 which the whole squeezed mass is mashed together in one direction and 

 extended in another. Taking any ideal sphere in the original unsqueezed 

 mass : after mashing the diameter in the line of pressure has been short- 

 ened, the diameter in the line of cleavage-*^ has been correspondingly 

 extended, and the diameter in the line of cleavage-strike unaffected (since 

 extension of this diameter in any place must be compensated by short- 

 ening in a contiguous place right or left) ; so that the original sphere has 

 been converted into a greatly-flattened ellipsoid of three unequal diame- 

 ters. The amount of compression and extension may be estimated in 

 the case a by the amount of distortion of shells of known form (Figs. 

 160 and 161) ; in the case c by a comparison of the transverse diame- 

 ter of the block with the length of the folded line f d (Fig. 163) ; in 

 the case d by the relation between the diameters of the elliptic spots. 

 By these means, but principally by the first, Haughton * has estimated 



* Philosophical Magazine, fourth series, vol. xii, p. 409. 



