186 



STRATIFIED OR SEDIMENTARY ROCKS. 



but if the cube, while still plastic, be pressed into a flattened disk, then 

 the scales are turned with their long diameters in the direction of ex- 

 tension and at right angles to the line of pressure, as in B, Fig. 165, 

 and the planes of easy fracture, being still determined by these sur- 

 faces, will be in that direction. 



In proof of this view, Mr. Sorby mixed clay with mica-scales or 

 with oxide-of-iron scales, and, upon subjecting the mass to powerful 

 compression and drying, he always found a perfect cleavage at right 

 angles to the line of pressure. Furthermore, by microscopic examina- 

 tion he found that both in the pressed clay and in the cleaved slates 

 the mica-scales lay in the direction of the cleavage-planes. 



Although cleavage is most perfect in slates, yet other rocks are 

 sometimes affected with this structure. In a specimen of cleaved lime- 

 stone, Sorby found under the microscope 

 unequiaxed fragments of broken shells, 

 corals, crinoid stems, etc. (organic parti- 

 cles), in a homogeneous limestone-paste, 

 lying with their long diameters in the di- 

 rection of cleavage. Originally the lime- 

 stone was a lime-mud with (he supposes) 

 unequiaxed organic particles disseminated. 

 In some cases, however, Sorby recognized 

 the very important fact that the organic 

 fragments which were encrinal joints, had 

 been flattened by pressure — had changed 

 their form instead of their position. A, 

 Fig. 166, gives a section of the mass in the 

 supposed original condition, and B the 

 This observation contained the germ of the 



Fig. 166.— Illustrating- Sorby's Theory 

 of Slaty Cleavage (after Sorby). 



condition after pressure, 

 theory proposed by Tyndall. 



Tyndall's Theory.* — Tyndall was led to reject Sorby's theory by the 

 observation that cleavage structure was not confined to masses contain- 

 ing unequiaxed particles of any kind, but, on the contrary, the cleavage 

 is more perfect in proportion as the mass is free from all such particles. 

 Clay, deprived of the last trace of foreign particles by the sorting power 

 of water, when pressed, cleaved in the most perfect manner. Common 

 beeswax, flattened by powerful pressure between two plates of glass 

 and then hardened by cold, exhibits a most beautiful cleavage structure. 

 Almost any substance — curds, white-lead powder, plumbago — subjected 

 to powerful pressure, exhibits to some extent a similar structure. Tyn- 

 dall explains these facts thus : Xearly all substances, except vitreous, 

 have a granular or a crystalline structure, i. e., consist entirely of dis- 



* Philosophical Magazine, second series, vol. xii, p. 35. 



