626 DYNAMICAL GEOLOGY. 



expand. Cavities in rocks are often lined with crystals, while the 

 rock itself is but a compact mass of crystalline grains. 



Long-continued heat, short of fusion, favoring a slow aggrega- 

 tion of the particles, sometimes produces crystals, or a crystalline 

 structure. Heating steel to a certain temperature changes the 

 fineness of the grains, — which is a change of crystalline texture 

 without fusion. 



Cleavage. — Cleavage is usually parallel to one or more planes or 

 diagonals of the fundamental form. 



The minerals mica and gypsum are examples of very easy cleavage. 

 Calcite has easy cleavage in three directions making a fixed angle 

 (105° 5') with one another parallel to the faces of the fundamental 

 rhombohedron. Feldspar has easy cleavage in one direction, and in 

 another a second cleavage, a little less perfect, at right angles, or 

 nearly so, with the first. Quartz has no distinct cleavage. 



Cleavage in rocks. — Eocks may derive a cleavage-structure from 

 one of the constituent minerals. Thus, mica schist cleaves into 

 thin laminae because of the abundance of the very cleavable mine- 

 ral mica. Mica may give cleavage even to a quartz rock. Gra- 

 nite often has a direction of easiest fracture, due to the fact that 

 the feldspar crystals have approximately a uniform position in 

 the rock, bringing the cleavage-planes into parallelism. 



Cleavage-structure must not be confounded with the existence 

 of planes of fracture in rocks, called joints. Mineral coal, trap, 

 sandstone, often break into angular blocks ; but were there true 

 cleavage, the cleavage-structure would be general along some 

 one or more fixed directions in the mass or block, and not be 

 limited to certain planes of fracture. Cleavage follows particular 

 directions, but not particular planes. 



The cleavage-structure of a rock like mica schist, due to a cleav- 

 able mineral, is usually called foliation, to distinguish this character 

 from slaty cleavage (see p. 101). 



Concretionary structure. — Examples of concretionary forms are 

 given on pages 96-99. There is a general tendency in matter 

 to concrete around centres, whether solidifying from fusion, solu- 

 tion, or vapors. These centres may be determined (1) by foreign 

 substances which act as nuclei, or (2) by the circumstances of 

 solidification, which, according to a general law, favor a commence- 

 ment of the process at certain points in the mass assumed at the 

 time. As the solidifying condition is just being reached, instead 

 of the whole simultaneously concreting, the process generally be- 

 gins at points through the mass, and these points are the centres 

 of the concretions into which the mass solidifies. 



