DEFORMATION OF ROCKS 451 
dall,t Heim,? Daubrée,3 Harker,* Becker,5 and others, it is the 
purpose to inquire into the attitudes of cleavage and fissility, 
the causal difference between the two, and the relations which 
these structures have to others. The investigators mentioned 
have shown that cleavage and fissility are usually closely con- 
nected with folding, being one of the results of compression. 
It has been held (pp. 195-213) that there are great differences 
in the manner in which masses of rock respond to compression, 
depending upon depth and upon whether they are homogeneous 
or heterogeneous. 
Sorby explains rock-cleavage as mainly caused by the rota- 
tion of mineral particles, and especially mica, so that their longer 
diameters and the cleavage of the mica particles are normal to 
the greatest pressure. The rock readily parts along the greater 
dimensions and cleavage of the mineral particles. The minute 
mica plates were supposed to be fragmental particles deposited 
in the plane of bedding, and to have been rotated by the move- 
ment of the rock to a position normal to the pressure. Sorby 
also showed that the laminar hydrous silicates, such as chlorite, 
develop in situ parallel to the cleavage. He did not think that 
this was true of mica in slates, but believed that the parallel 
mica flakes of mica-schist form in situ during the recrystalliza- 
tion of the rock. | 
Sharpe and Tyndall explain cleavage as due to the flatten- 
ing of the mineral particles by pressure, so that they have a 
parallel arrangement with their shortest axes in the direction of 
greatest pressure. This cause and the causes given by Sorby 
*The development of Slaty Cleavage, JOHN TYNDALL, Philosophical Magazine, 
4th Ser., Vol. XII, pp. 35-48, 1856. ‘ 
2 Mechanismus der Gebirgsbildung, ALBERT HEIM, Band II, 1878, pp. 51-74, mit 
einem Atlas. 
3 Géologie Expérimentale, by A. DAUBREE, Vol. I, pp. 391-432, Paris, 1879. 
4On Slaty Cleavage and Allied Rock Structures, with Special Reference to the 
Mechanical Theories of their Origin, ALFRED HARKER, British Association for the 
Advancement of Science, 55th meeting, 1885, Proceedings, pp. 813-852. 
5 Finite Homogeneous Strain, Flow, and Rupture of Rocks, by G. F. BECKER, 
Bull. Geol. Soc. Am., Vol. 1V, 1891, pp. 13-90. 
