face of a kidney. When
split open they prove to be traversed by a network of cracks, which are
usually filled with calcite and other minerals. These white infillings
of the fissures resemble partitions; hence the name from the Latin
_septum_, a partition. Sometimes the cracks are partly empty. They vary
up to half an inch in breadth, and are best seen when the nodule is cut
through with a saw. These concretions may be calcareous or may consist
of carbonate of iron. The former are common in some beds of the London
Clay, and were formerly used for making cement. The clay-ironstone
nodules or sphaerosiderites are very abundant in some Carboniferous
shales, and have served in some places as iron ores. Some of the largest
specimens are 3 ft. in diameter. In the centre of these nodules fossils
are often found, e.g. coprolites, pieces of plants, fish teeth and
scales. Phosphatic concretions are often present in certain limestones,
clays, shelly sands and marls. They occur, for example, in the Cambridge
Greensand, and at the base of certain of the Pliocene beds in the east
of England. In many places they have been worked, under the name of
"coprolite-beds," as sources of artificial manures. Bones of animals
more or less completely mineralized are frequent in these phosphatic
concretions, the commonest being fragments of extinct reptilia. Their
presence points to a source for the phosphate of lime.

Another very important series of concretionary structures are the flint
nodules which occur in chalk, and the patches and bands of chert which
are found in limestones. Flints consist of dark-coloured
cryptocrystalline silica. They weather grey or white by the removal of
their more soluble portions by percolating water. Their shapes are
exceedingly varied, and often they are studded with tubercules and
nodosities. Sometimes they have internal cavities, and very frequently
they contain shells of echinoderms, molluscs, &c., partly or entirely
replaced by silica, but preserving their original forms. Chert occurs in
bands and tabular masses rather than in nodules; it often replaces
considerable portions of a bed of limestone (as in the Carboniferous
Limestones of Ireland). Corals and other fossils frequently occur in
chert, and when sliced and microscopically examined both flint and chert
often show silicified foraminifera, polyzoa &c., and sponge spicules.
Flints in chalk frequently lie along joints which may be vertical or may
be nearly