CONCRETIONARY STRUCTURE. 



87 



of concretionary solidification distinct from the others. This concretionary structure, 

 when wholly unobservable in the solid unaltered rock, is frequently developed by 

 the action of atmospheric agencies, and sometimes so perfectly that the mass separates 

 into thin concentric plates. This takes place in some kinds of both granite and sand- 



Fig. 82. 



Fie. 83. 



:« 



stone. The rock, after partial alteration, peels off in concentric layers; and a bluff of 

 granite which has undergone the change sometimes appears as if made up of huge 

 rounded bowlders piled together. A sandstone often looks like an excellent building- 

 stone, which, after an exposure of a few months, will fall to pieces in concentric shells. 



Ffc. 84. 



Fig. 84 is a case of concretion in a sandstone alongside of a small fissure, observed 

 in Australia. The two concretions measured twenty feet across. They consisted of 

 layers from half an inch to two inches thick, which separated rather easily. The rock 

 elsewhere was without concretions. 



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 a 

 separate concretion. The action of the sea had worn the surface and brought the 

 Structure out to view. 



