
CHAPTER’ Vili 
CONCRETIONARY AND SECRETIONARY STRUCTURES 
Siliceous Concretions—Flint, Chert, Menilite. Calcareous and Ferrugin- 
ous Concretions—Septaria, Composite Nodules, Rattle-stones, Fairy- 
stones, Kankar, etc. Clay-ironstone Nodules, Pyrite, Marcasite, 
Gypsum, Dendrites. Concretionary Sandstones, Argillaceous Rocks, 
and Limestones. Concretionary Tuffs. Concretions in Crystalline 
Igneous Rocks. Secretionary Structures — Amygdules, Geodes, 
Drusy Cavities. 
Concretionary Structures may occur in almost any kind 
of derivative rock. Sometimes they affect the mass of a rock; 
at other times they take the form of various sized spherical or 
lenticular bodies or zodules, scattered regularly or irregularly 
through a rock, or they may appear as more or less interrupted 
layers or vertical and ramifying veins, or as well-formed 
crystals. In most cases they owe their origin to the gradual 
aggregation of mineral matter originally diffused through the 
mass of the rock in which they occur. Occasionally, however, 
the mineral matter has been introduced from the outside by 
percolating water. The commonest concretions are siliceous, 
calcareous, and ferruginous, and there is a strong tendency 
in spherical concretions to assume internal radiating and 
concentric structures, 
(2) SILICEOUS.—Among the most familiar examples of siliceous con- 
cretions are the /zu¢s, which occur so frequently in chalk. Flint nodules 
are usually irregular in form, and vary in size up to a foot or more in 
diameter. They are white externally, and brown to black internally. 
They often enclose or partially enclose fossils, more particularly sponges. 
Usually they are arranged in lines that coincide with the bedding-planes 
of the chalk. They may coalesce to form more or less interrupted sheets 
or seams of flint (some three or four inches thick), which follow the bed- 
ding of the chalk, or may traverse it, as irregular vertical or ramifying 
veins. In the older limestones chert plays much the same part as flint 
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