THE 



AMERICAN JOURNAL OF SCIENCE 



[FOURTH SERIES.] 



Art. I. — Current Theories of Slaty Cleavage; by Geobge 



F. Becker. 



The theory that slaty cleavage is due to pressure normal to 

 the cleavage is old and very generally esteemed satisfactory. 

 Sedgwick and others held cleavage to be mainly a phenomenon 

 due to the crystallization or recrystallization of minerals in an 

 appropriate orientation, and this idea with modifications has 

 been advocated of late years by Messrs. Yan Hise and Leith. 

 Mr. Leith's recent paper * is the most authoritative exposition 

 of it. 



This geologist acknowledges that my theory of slaty cleavagef 

 applies in certain cases to which he gives the name of fracture 

 cleavage. According to him, such cleavage is mainly character- 

 ized by the presence of actual partings within the mass, but 

 sometimes shows flow structure as well. He distinguishes flow 

 cleavage from fracture cleavage, however, attributing to the 

 former a greatly preponderating importance in nature and 

 ascribing it to causes distinct from those which produce fracture 

 cleavage. To this latter he attributes the fissility of those rocks 

 in which a parallel arrangement of mineral constituents is ab. 



*Rock Cleavage, U. S. Geol. Sixty:, Bull. No. 239, 1905. 

 ''f Like Tyndall and Daubree, I consider a parallel arrangement of flattened 

 grains unessential to cleavage. Rupture takes place (as Messrs. Van Hise 

 and Leith concede) on planes of maximum slide or maximum tangential 

 strain. Rupture is a gradual process and cohesion is impaired through flow 

 before it is destroyed. Impaired cohesion in my theory is cleavage. Cleavage 

 developes most perfectly when the stress tending to produce it is persistent 

 in direction, because viscous resistance is then small. In a rotational strain 

 there are two sets of mathematical planes on which maximum slide takes 



Am. Jour. Sci. — Fourth Series, Vol. XXIV, No. 139.— July, 1907. 

 1 



