86 



LITHOLOGICAL GEOLOGY. 



Fig. 67 is a sphere, — a very common form. The sphericity is 

 frequently as perfect as in a bullet, though the form is usually more 

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

 a mustard-seed and less to a yard 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 sheH 

 as a nucleus; in some cases a fossil fish forms the interior of a concretion. 



The structure in Fig. 68 is solid without concentric layers. In Fig. 69, it is concentric. 

 Ik 70, it is radiated or consists of crystalline fibres diverging from the centre and show- 

 ing crystalline apices over the exterior surface. In Fig. 71, the exterior is concentric, 

 but the interior radiated. 



In Figs. 72, 73, the interior was cracked in drying; 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 is frequently beautiful when pol- 

 ished. In Fig. 74, the interior is 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; but most geodes are not of concretionary origin. In 

 Fig. 75 the concretion contains another small concretion; a variety not uncommon. 



Figs. 76 a, b are different views of flattened or disk-shaped concretions ; 77 is another, 

 approaching a ring in shape ; 78, 79, combinations of flattened concretions. Fig. 80 is 



Fig 80. 





Fig. 81. 



v 





Pi 



f^J* 



i ■ \. 



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part of a clay layer made up of flattened concretions. A concretionary layer often grad- 

 uates 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 pealing off. Oolyte (named from w6p, egg) is similar, ex- 

 cept that the concretions are in size like the roe of fish or even grains of sand. 



Fig. 82 exhibits a crystalline rock with spherical concretions imbedded in its mass 

 and not separable from it, — each layer (of the three represented in each concretion) 

 consisting of different minerals: for example, garnets the centre, feldspar the middle 

 layer, and mica the outer; and all making a solid mass. The constitution of such con- 

 cretions is very various. In rocks containing feldspar, they usually consist largely of 

 feldspar, and sometimes of feldspar alone, or of feldspar with some quartz. The con- 

 cretions in pitchstone and pearlstone (called spherulites) are almost purely feld- 

 spathic, and often separate easily from the rock. 



Fig. 83 represents basaltic columns, like those of the Giants' Causeway, having the 

 tops concave : at each joint in the columns, in such a case, there would be the same 

 concavity. This tendency to break with concave or convex surfaces has been attributed 

 to a concretionarv structure. 



