CLEAVAGE (from cleure, AS. elf'ofan, Ger. Hieben, to cleave, glubere, to peel, Gk. 7X6ISetp, glyphrin, to hollow out). In geology, a property induced, under certain conditions, dur ing deformation in a rock by virtue of which the rock may be readily split into parallel layers or rods, i.e. parallel to a plane or line. It is a property possessed also by certain original .meisses that have not undergone deformation since their first solidification. Cleavage, in rare cases, may he parallel to planes of bedding that may he present in the rock-mass. The essential condition of rock-cleavage is a parallel dimen sional arrangement of the constituent mineral particles of the rock. In certain minerals, such as mica, parallel dimensional arrangement car ries with it a parallelism of the mineral cleav age. The cleavage of a rock may be observed to occur parallel to the greater diameters of the mineral particles, or to the parallel mineral cleavages. When the two coineide. as in the case of mica, the rock-cleavage produced is parallel to one plane. Where they do not coincide, two rock-cleavages may he produced at angles to each other, as in the case of feldspar, although one may be conspicuous and the other obscure. The property of rock-cleavage is observed in rocks that have yielded to pressure by deformation without conspicuous fracture. This deformation can be induced only where the rock is under such great pressure from all sides that it flows rather than fractures. The planes or lines of rock cleavage are further observed to be normal to the directions in which the rock-masses have been most shortened.
A number of processes probably coiiperate to induce the parallel arrangement of mineral par ticles during the shortening of the rock-mass.
Chief among these is the reerystallization of old mineral particles and the crystallization of new particles through the agency of contained water. This process results in the elongation of the mineral particles of the rock in the plane or line of greatest elongation of the rock-mass as a whole, and in shortening normal to this direc tion: in other words. it results in the flattening of the mineral particles through solution and deposition of mineral material. Other processes which produce rock-cleavage are the rotation into parallel position of previously existing par ticles whose axes have unequal length, and the flattening in situ of original mineral particles through the process known as gliding—i.e. dif ferential movement along certain definite planes and crystals without fracture. Cleavage is found in almost all varieties of rocks which, under pressure, have been made to flow, a] though as a rule it is shown to best advantage in the finer-grained rocks. Rocks possessing the property of cleavage are called 'slates' or 'schists.' BIBLIOGRAPHY. Phillips, "Cleavage and Fo Bibliography. Phillips, "Cleavage and Fo tion in Rocks," in Report of British Association for the Advancement of Science (London, 1856) ; Heim, Mechanismus der Gebirgsbildung. vol. ii. (Basel, 1878) : Tyndall. "Comparative View of the Cleavage of Crystals and Slate-Rocks," in Philosophical Maga;:ine. 4th series, vol. xii. (London. 1S56) Dauhree. Gfiologir experimen tale, vol. i. (Paris, 1879) Van Hise, "Prin ciples of North American Pre-Cambrian Geol ogy," in Sixteenth Annual Report of the United States Geological Survey (Washington, 1896).