
124 ' STRUCTURAL AND FIELD GEOLOGY 
Other kinds of concretionary structures are frequently met with in sand- 
stones. Small quantities of carbonate of lime or carbonate of iron, 
diffused through the rock, tend to aggregate so as to form irregular 
concretionary masses of sandstone, which are much harder than the 
surrounding rock. Cracks and crevices in concretionary masses of this 
kind are often filled or lined with crystalline siderite or calcite, as the case 
may be. The hardened rock (known as “kingle” in Scotland) breaks 
with a splintery fracture, and is rejected by the quarrymen as unsuitable 
for building purposes. 
Argillaceous rocks hardly less frequently assume concretionary forms. 
Now and again a whole bed of shale may exhibit the structure—the 
rock appearing to be composed of an aggregate of various sized 
spheroids. The spheroids usually show a concentric arrangement—the 
concentric shells being in some cases separated from each other by thin 
films of ferruginous matter. 
Calcareous rocks often enough acquire a concretionary structure— 
the most pronounced examples of the kind being furnished by dolomitic 
or magnesian limestone, as already described (p. 65). Reference may 
also be made to the odlitic structure of certain limestones, calcareous 
tufas, and ironstones, which, however, in most cases is original (see p. 70). 
Concretionary structures, comparable to those that characterise so 
many derivative rocks, can hardly be said to occur in igneous rocks. 
Exception, however, must be made of the Zuffs, in many of which concre- 
tionary ferruginous and calcareous nodules occur, while now and again 
tuff itself may exhibit concretionary structure, such as that seen occasion- 
ally in argillaceous shales. But the concretionary structures that affect 
many crystalline igneous rocks differ from those which occur in derivative 
rocks in being original and not superinduced. For example, the dark, 
irregular shaped aggregates of ferromagnesian minerals which appear in 
many granites, gabbros, and other plutonic masses, and the sporadic 
nodular masses of olivine so frequently met with in basalt, are early 
segregations from the original molten magma. They are not, like the 
septarian nodules described above, younger than the rocks in which they 
occur. So, again, the aggregates of spherical bodies which constitute 
orbicular diorite or napoleonite are not superinduced but original 
structures—they consist of radially and concentrically arranged felspar 
and hornblende—the two primary and essential ingredients of the rock 
(see Plate XIV.). Similar original concretionary structures are met with 
in other crystalline igneous rocks, as, for example, the ball-granite 
(“ Kugel-granit ”) of Finland. 
Secretionary Structures.—These are especially character- 
istic of certain types of igneous rocks, but may occur in 
almost any kind of rock having a cellular or cavernous 
structure. They consist of mineral matter which has been 
deposited on the walls of cavities, usually in successive 
layers, and thus they may be said to increase from without 
