96 



LITHOLOGICAL GEOLOGY. 



included are (a) the concretionary, (b) the jointed, and (c) the slaty. 

 They are produced either in the process of consolidation or during 

 subsequent changes. 



103. a. The Concretionary Structure. — T'his kind of structure has 

 been briefly explained in \ 80, and is here further illustrated. 



Fig. G7 is a sphere, — a very common form. The sphericity is 

 frequently as perfect as in a bullet or cannon-ball, though usually 

 more or less ovoidal, and sometimes quite distorted. The size varies 



from a mustard-seed and less to a foot or more ; and generally those 

 that are together in a layer of rock approach a uniformity in size. 

 They often have a shell, or a fragment of a plant, or some other 

 object, at the centre. In other cases they are hollow and filled 

 with crystals. The structure is often in concentric layers. 



Figs. 68 to 75 are views of sections showing the interior. In 68 there is a 

 fossil shell as a nucleus ; in some cases a fossil fish forms the interior of a con- 

 cretion. 



The structure in fig. 68 is represented as solid without concentric layers. In 

 fig. 69 the structure is concentric, the layers either firmly adherent or easily 

 separating. In 70 a variety with a radiated structure is shown, consisting of 

 crystalline fibres diverging from the centre and showing crystalline apices over 

 the exterior surface. In fig. 71 the exterior is concentric but the interior is 

 filled with radiated crystallizations. 



