182 



STRATIFIED OR SEDIMENTARY ROCKS. 



Fig. 158.— Cleavage-Planes cutting through Strata 



nation. Many rocks may be thus split into large coarse slabs called 

 flag-stones, and are used for paving streets, or even sometimes as roof- 

 ing-slates. This may be called flagstone cleavage, or lamination cleav- 

 age. Again (p), the arrangement of the ultimate molecules of a mineral 

 under the influence of molecular or crystalline forces gives rise to an 

 exquisite splitting along the planes parallel to the fundamental faces 

 of the crystal. This is called crystalline cleavage. Again (c), the ar- 

 rangement of the wood-cells under the influence of vital forces gives 

 rise to easy splitting of wood in the direction of the silver-grain. This 

 may be called organic cleavage. 



Now, in certain slates and some other rocks is found a very perfect 

 cleavage on a stupendous scale. Whole mountains of strata may be 

 cleft from top to bottom in thin slabs, along planes parallel to each 

 other. The planes of cleavage seem to have no relation to the strata, 

 but cut through them, maintaining their parallelism, however the strata 



may vary in dip 

 (Fig. 158). Usually 

 the cleavage-planes 

 are highly inclined, 

 and often nearly 

 perpendicular. It 

 is from the cleaving of such slates that roofing-slates, ciphering-slates, 

 and blackboard-slates are made. This remarkable structure has long 

 excited the interest of geologists, and many theories have been proposed 

 to explain it. 



On cursory examination of such rocks, the first impression is, that 

 the cleavage is but a very perfect example of flag-stone or lamination 

 cleavage — that the cleavage-planes are in fact stratification-planes, and 

 that we have here an admirable example of finely laminated rocks 

 which have been highly tilted and then the edges exposed by erosion. 

 Closer examina- 

 tion, however, will 

 generally show the 

 falseness of this 

 view. Fig. 159 

 represents a mass 

 of slate in which 

 three kinds of 

 structure are dis- 

 tinctly seen, viz., 

 joint faces, A, B, 

 C, J, /; stratification-planes, S S S, gently dipping to the right ; and 

 cleavage-planes, highly inclined, D D, cutting through both. Cleavage- 

 planes are therefore not stratification-planes. 



Fig. 159. 



I) J 



-Strata, Cleavage-Planes, and Joints. 



