778 PRINCIPLES OF STRATIGRAPHY 



2. Eiitcrolitliic Structure. Tliis has already l)een discussed in 

 the preceding chapter. In so far as it can be shown that this struc- 

 ture is a purely diagenetic one, brought about by the swelling of 

 the mass either through crystallization or hydration, it properly 

 belongs to endogenetic deformations. If, on the other hand, it 

 should be proven that enterolithic structure in some rocks is pro- 

 duced by the pressure of the overlying mass and the consequent 

 creep and rearrangement of the particles under pressure, such de- 

 formation must be classed under the gravitational section of the 

 exogenetic class. 



The important distinction between enterolithic and other de- 

 formations, such as folding under lateral pressure, or gliding in a 

 given direction, lies in the fact that the enterolithic structure folds 

 in all directions — is apolar or multipolar instead of unipolar. 



Fig. 162. Diagram illustrating the formation of contraction joints and of 

 desiccation fissures. The normal form is hexagonal, but, as 

 shown in the left-hand member, an irregular pentagonal form is 

 developed when one side is suppressed. 



3. Contraction Joints — Basaltic Jointing. This is seen in the 

 formation of mud cracks in lutaceous sediments such as clay or 

 argillo-calcareous mud and in the development of prismatic jointing 

 in cooling basalt and other igneous rocks. In the latter case the 

 prisms always form at right angles to the enclosing surface, such 

 as the upper and lower surface of a sill or laccolith, when the prisms 

 will be vertical or curved, or the lateral walls of a dike, when the 

 prisms will be horizontal. The prevalence of six-sided forms in 

 these prisms suggests that the crystallizing force centers about 

 equidistant points in the magma, each of which will draw an equal 

 amount of the surrounding matrix toward it from all sides. This 

 results in other equally spaced points between these around which 

 the tension in all directions is greatest. Since the points around 

 which attraction takes place are all equally distant from one an- 



