CAUSE OF CLEAVAGE 26 1 



causing the rock to part into j) lates w ith a rough or undulating 

 surface, due to the presence of parallel flakes of some mineral. 



Many unmodified igneous rocks have a marked cleavage, which 

 is occasioned by the arrangement of the mineral grains with their 

 long axes parallel, or by a parallelism in the cleavage planes of 

 these minerals, or by both factors combined. ■ In cleaved sedi- 

 mentary rocks the cleavage planes may coincide with the planes 

 of stratification. Much more commonly, however, they intersect 

 the latter at all possible angles, keeping a constant direction for 

 long distances (parallel to the axes of the folds in which they 

 occur), while the bedding planes change with the dip from point 

 to point. Ordinary roofing slate is one of the best possible ex- 

 amples of a cleaved rock, and the structure is often called slaty 

 cleavage, to distinguish it from mineral cleavage (see p. 13). 



Cause of Cleavage and Fissility. — It is very generally agreed 

 among geologists that slaty cleavage is a result of compression ; 

 for, disregarding certain igneous masses, it occurs only in rocks 

 which show other 

 evidences of having 

 been subjected to 

 compression. On 

 the other hand, the 

 mechanics of the 

 problem are some- 

 what obscure and 

 have given rise to 



,..-- r • FlG. 114. — Diagram showing relation of cleavage and 



differences of opin- + .. c .. , 



1 stratification planes. 



ion. The most prob- 

 able view seems to be that the cleavage planes are developed 

 at right angles to the compressing force, and are due to the 

 arrangement of the constituent mineral particles of the rock 

 with their longest diameters, their cleavage planes, or both, in 

 parallel directions. Further, that " this arrangement is caused, 

 first and most important, by parallel development of new minerals ; 

 second, by the flattening and parallel rotation of old and new min- 

 eral particles ; and third, and of least importance, by the rotation 



