EXPERIMENT BY DAUBREE. 75 



Others have made experiments with similar results. It is well known 

 that east iron, building stone and similar substances, crushed in testing 

 machines, do not yield on planes parallel to the support, but at angles 

 approximating to 45°, when the slabs are broader than they are thick. 

 Not all of the cracks pass through the masses experimented upon. The 

 slabs are somewhat deformed, and, in short, the phenomena are strictly 

 comparable with those of M Daubree's experiment, though less bril- 

 liantly illustrated. 



Jointing and Cleavage. — -Many geologists have been struck by the inti- 

 mate manner in which jointing and cleavage (whether schistose or slaty) 

 are associated, and there seems to have been a growing tendency to assert 

 or imply a relationship between them, even in spite of an assumed theo- 

 retical difference in origin. Professor William King advanced the 

 hypothesis in 1857 that slaty cleavage was derived from jointing, the 

 jointed surfaces having been welded under pressure. This conclusion, 

 indeed, has not, to my knowledge, been adopted by any other observer; 

 but rejection of the conclusion does not imply rejection of the facts upon 

 which it was based — viz., that dislocated jointing occurs " developed to a 

 degree of fineness bordering on that of mineral cleavage," as at Carragrian, 

 near Galway, and the occasional alternation of joints with parallel slaty 

 cleavage* Professor A. Heim distinguishes cleavage due to microscopic 

 dislocated joints from cleavages unattended by joints, or true slaty cleav- 

 ages. Of these he makes two classes : " micro-cleavage," consisting in a 

 flattening of the component grains of the rock, and cleavage due to the 

 re-arrangement by pressure of previously existing scales in positions more 

 and more nearly perpendicular to the line of pressure. All three varie- 

 ties are associated so intimately, according to Heim, as to be found in 

 one and the same thin section. Even in cases of pure micro-cleavage 

 relative movement without fracture in adjoining cleavage planes may be 

 detected.f M Daubree speaks of " the surfaces of slipping which pro- 

 duce schistosity ; "J Dr H. C. Sorby has described cases of discontinuity 

 on a microscopic scale which lead to cleavage; Messrs (leikie, Peach and 

 Home describe fluxion structure and shearing as productive of schistosity 

 and highly cleaved rocks, the planes of cleavage being parallel to the 

 thrust plane, ^ and other similar observations could be cited to show that 

 relative tangential motion and slaty cleavage are at least most intimately 

 associated in nature. 



Phenomena of slaty Cleavage. — Workers in slate distinguish not only the 

 cleavage laces, but also " side " and " end." Most slates can be split only 



* Trans. R. Irish Acad., vol. 25, 1875, p. 612, et passim. 

 fMech. der Gebirgsbildung, vol. 2, 1.S7S, pp. 54-56. 



X Etudes SynthiHiqiii's, 1879, p. 321. 

 I Nature, vol. 31, 1S84, pp. 29-35. 



