ENDOGENETIC DEFORMATION 779 



other, tlicy form the apices of a series of equilateral triangles and 

 the points around which the greatest tension is focused will be the 

 centers of these triangles. The smallest number of cracks about 

 these central points which will relieve the tension in all directions is 

 three. If these cracks are symmetrically arranged about the point, 

 the angle between any two of them is 120°, which is the angle 

 between the two sides of a hexagonal prism. As shown in the 

 diagram (Fig. 162), this uniform contraction about equidistant 

 points will result in the formation of a set of uniform hexagonal 

 prisms. Unequal development or failure of the cracks to develop 

 about one of the points may lead to five-sided or other irregular 

 prisms. 



Drying mud will in like manner cause a development of a series 

 of prisms which typically are hexagonal, but from lack of homo- 

 geneity of the material through unequal drying, or from some other 

 cause, are frequently irregular polygons. The prisms are seldom 

 very high, though in exceptional cases the cracks may penetrate to 

 a depth of ten feet. In general a fraction of an inch is the usual 

 height of the dried mud prisms, the sides of which are slightly 

 raised, giving a concave surface. (See ante, Chapter XVII.) 



Prismatic jointing is also sometimes found in undisturbed hydro- 

 genic rocks. Examples of these occur in some gypsum beds of the 

 Paris Basin. It is quite probable that this structure is due to 

 pressure exerted by hydration of anhydrite and so belongs to the 

 diagenetic processes. 



DEFORMATIONS DUE TO EXTRANEOUS CAUSES— 

 EXOGENETIC DEFORMATIONS. 



This type may be divided into (A) Gravitational. (B) Orogenic, 

 and (C) Contactic. 



A. Gravitational Deformations. 



a.' Structures Due to Movements. 



4. Intraformational Brecciation. This has already been dis- 

 cussed in the chapter on Autoclastic rocks, but is again mentioned 

 here as a structural feature. It is probably in all cases an extreme 

 of the next type, and so may be considered in connection with that 

 subject. Here belong the edgewise conglomerates of many lime- 

 stone formations. 



