CONCRETIONARY STRUCTURE. 



97 



In fig. 72 the interior is irregularly cavernous, as if it had cracked thus in 

 drying. In fig. 73 there is a similar result, hut with more numerous and smaller 

 cracks, making a reticulation of them; and when these cracks are subse- 

 quently filled by carbonate of lime, heavy spar, or other material, by a process 

 of infiltration, it becomes a kind of septarium, and forms frequently a beautiful 

 object when polished. Some flattened concretions of this kind are a yard in 

 diameter. In 74 the interior is irregularly hollow, and filled around with a layer 

 of crystals (quartz crystals are the most common in such a condition), forming 

 what is called a geode, — a little crystal grotto. In fig. 75 the concretion is 

 hollow and contains another small concretion. This variety is not uncommon. 

 They rattle in the hand when shaken. 



Fig. 76 o, b are different views of flattened or disk-shaped concretions; 

 77 is another, approaching a ring-shape ; 78, a combination of three flattened 

 concretions ; 79, another, which is remarkable for the symmetry of its compound 

 form while so irregular. 



Fig. 80. 



Fig. 81. 



Fig. 80 is part of a clay layer made up of flattened concretions. A concre- 

 tionary layer often graduates insensibly into one in which no concretions are 

 apparent, through the coalescence of the whole. Fig. 81 represents a rock 

 made up of concretions of the size of peas, — a calcareous rock called pisolite 

 (from pisum, a pea). Each concretion has a concentric structure, the layers 

 easily peeling off. The oolite (named from wov, egg) is similar, except that 

 the concretions are as small as the roe of fish, or even as fine as grains 

 of sand. 



Fig. 82. 





7/'// ; 



Fig. 83. 



104. Fig. 82 exhibits a crystalline rock with spherical concretions imbedded 

 in its mass and not separable from it, — each layer (of the three represented 



8 



