194 STRUCTURAL GEOLOGY. 



stratification that it seems best to take it up here. It is 

 found in many kinds of rocks, but most perfectly in 

 slates, and is therefore often called slaty cleavage. 



Definition. Cleavage is easy splitting in. certain di- 

 rections. There are many kinds of cleavage due to dif- 

 erent causes. For example, many crystals split perfectly 

 in certain directions. This is called crystalline cleavage, 

 and is due to molecular arrangement. Certain stratified 

 sands split easily into broad flag-stones in the direction 

 of the laminae. This is lamination cleavage, and is due to 

 the arrangement of the grains by the sorting power of 

 water. Again, wood splits easily in the direction of the 

 silver grain. This wood-cleavage is due to the arrange- 

 ment of the wood-cells. 



Slaty Cleavage. Now, there is also an easy splitting 

 of rocks in definite directions, which occurs on an im- 

 mense scale, and in certain slates is a very marked struc- 

 ture. The direction of cleavage is usually vertical or 

 highly inclined. Whole mountains are thus cleavable 

 from top to bottom, and rocks over thousands of square 

 miles are often made up of such thin sheets. It is by 

 splitting along these lines of easy fracture that roofing- 

 slates, ciphering-slates, and blackboard-slates are made. 



On casual examination of strata the cleavage-planes are 

 liable to be mistaken for fine laminm, and we are apt to 



FIG. 110. Cleavage-planes cutting through strata. 



think that we are examining a beautiful example of 

 highly inclined strata. But a closer examination will 

 usually show the lines of stratification running in an 

 entirely different direction. In Fig. 110, the strong lines 



