CONCRETIONARY STRUCTURE. 



87 



A concentric structure or condition often occurs which is not of concretionary origin. 

 In Fig. 84, the two concentric areas in a sandstone (observed by the author in Australia) 

 have together a diameter of twenty feet, and is made of concentric layers one half to two 

 inches thick. They adjoin a fissure which admitted water; and the rusty bandings may 



Fig. 82 



Fig. 83. 





-A, Mi 





L 





have been due to the oxidation of pyrite at the centre, and the spreading outward of the 

 iron-bearing solution. 



In other common cases, massive rocks that are much fissured or jointed, as some 

 granite, doleryte, dioryte. etc., undergo easy decomposition as far as water can pene- 

 trate along the fissures. 



Fig. 84. 



As the process of oxidation goes on, the blocks into which the rock is divided by the. 

 fissures first have a colored border, and afterward become reduced to great rounded 

 bowlder-like masses, with often a concentric banding in shades of brown (Fig. 85 A.). 



Fig. 85 A. 



Fig. 85 is from an argillaceous sandstone which before consolidation had been inter- 

 sected by slender mud-cracks, and subsequently, on hardening, each areolet became 

 separately concentric. The wear of the sea had brought the structure out to view. 



