MATERIALS OF THE EARTH 49 



In size, concretions may vary from microscopic dimensions 

 to huge masses, 8, 10, or even more feet in diameter. The varia- 

 tions in shape are also great. The conditions of growth have 

 much to do with the form. A concretion which starts as a sphere 

 may find growth easier in one plane than another, when it becomes 

 discoid. Two or more concretions sometimes grow together, 

 giving rise to complicated forms. 



Fig. 28. Section of a septarian nodule (clay ironstone). About 3, natural 



size. (Geikie.) 



Near the surface, the action of water commonly tends to the 

 decomposition of the rock; but below a few hundred, or at most 

 a few thousand feet, its general effect is to solidify the rock, for 

 at these depths deposition exceeds solution, and oxidation, car- 

 bonation, etc., go on much more slowly than near the surface, 

 or not at all. Thus oxidized and hydrated sediments may be 

 buried to great depths, and under the pressure and perhaps the 

 high temperature of these depths, deoxidation and dehydration 

 may take place, with resulting diminution of volume. 



