NATURE OF CLEAVAGE. 



have undergone enormous compression. Often they occupy only a 



third or even a tenth of their 

 original horizontal space, as is shown 

 by the distortion of the fossils they 

 \ contain, and sometimes by the 

 w \ folding of hard sandstone layers 

 which could not yield in the same 

 way to the pressure. This com- 

 pression alters the rock so that 

 it no longer absorbs water freely or 

 decomposes into mud. And while 



Fig. 6. -Verticil Cleavage in Folded Strata. ^ Q or igi na l planes of deposition 



are still well marked by changes in the colour of the layers, the 

 rock no longer splits into the strata or laminae of which it was 

 composed, but has acquired the property of dividing into much 

 more regular and sometimes exceedingly thin films by division 

 planes which cross the rocks in different quarries at every possible 

 angle to the original stratification. This property, to which we 

 owe the slates of commerce, was named by Professor Sedgwick 

 cleavage. This rock cleavage is quite distinct from the cleavage 

 of minerals, for that always depends upon the crystalline form 

 of the mineral, and is parallel to one or more of the faces of 

 the particular crystal system to which the mineral belongs. Thus, 

 what is named the cubic system includes besides the cube many 

 other figures, such as an eight-sided double pyramid called an 

 octahedron, a twelve-sided figure named a dodecahedron, &c. ; and 

 though the mineral called fluor-spar usually crystallises in cubes, its 

 eight corners cleave off and convert the cube into the octahedron. But 

 rock cleavage is a mechanical property which is due to rearrangement 

 of the rock particles and crystals by pressure. This has been proved 

 experimentally by Mr. Sorby and Professor Tyndall, who have pro- 

 duced cleavage experimentally by pressure in clay, wax, and other 

 substances. Professor Ramsay has observed that cleavage is only 

 produced in contorted rocks when they are subject to some weight 

 of rock from above as well as lateral pressure ; since when near the 

 surface the rocks yield and fracture, and the particles are not forced 

 to rearrange themselves at right angles to the direction of the com- 

 pressing force. 



Fig. 7. Clay Splitting Obliquely. 



Fig. 8. Same Rock converted into Slate 

 Splitting Vertically. 



The production of cleavage may perhaps be better understood 

 from this diagram. First, we suppose the clay to be unaltered, 



