CONCEETIOXAEY STRUCTURE. 



85 



(6.) There are also markings which are attributed to the flowing of 

 thick mud. There are others, produced apparently by small eddyings 



of water in clay or mud which work out concavities that afterward 

 become filled with clay and look as if made by the valves of shells. 



(2.) Kinds of structure not properly a result of deposition, and 

 mostly of subsequent origin. — The kinds of structure here included 

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

 are produced either in the process of consolidation or during subse- 

 quent changes. 



a. The Concretionary Structure. — This kind of structure has been 

 briefly explained on p. 63, and is here further illustrated. Concretions 



Figs. 67-79. 



in massive sandstones are usually spherical, but in laminated sand- 

 stones or shales more or less flattened. 



