628 DYNAMICAL GEOLOGY. 



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

 lamina?, because of the abundance of the very cleavable mineral mica. 

 Mica may give cleavage even to a quartz rock. Granyte 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, sand- 

 stone 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. 89.) 



2. Concretionary Structure. — Examples of concretionary 

 forms are given on pages 85-88. There is a general tendency in matter 

 to concrete around centres, whether solidifying from fusion, solution, 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 commencement 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 begins at points through the mass ; 

 and these points are the centres of the concretions into which the 

 mass solidifies. , 



The concretions in the same mass are usually of nearly equal size : 

 hence, (3) the points at which solidification in any special case begins 

 are usually nearly equidistant. The great uniformity of size in the 

 concretions of most beds of rock shows that foreign bodies do not 

 generally determine the positions of the centres, although they often 

 act as nuclei. 



Basaltic columns are a result of concretionary structure formed in 

 cooling (p. 87), in accordance with the principles just explained: each 

 column corresponds to separate concretionary action. The size of the 

 columns is determined by the distance apart of the points which take 

 the lead (these points lying in the centres of the columns) ; and this 

 is determined by the rate of cooling ; and this, mainly, by the thick- 

 ness of the mass to be cooled : the thicker the mass, the slower the 

 cooling, and the larger the columns. The cracks separating the col- 

 umns from one another are due to contraction on cooling. 



Iron-stone, sandstone, and clayey concretions in beds of rock, are 



