continuously in the form of a
spiral as in the Bordenave system.

To those wishing to pursue the subject further, the following books
among others may be suggested:--Sabin, _Cement and Concrete_ (New
York); Taylor and Thompson, _Concrete, Plain and Reinforced_ (London);
Sutcliffe, _Concrete, Nature and Uses_ (London); Marsh and Dunn,
_Reinforced Concrete_ (London); Twelvetrees, _Concrete Steel_
(London); Paul Christophe, _Le Beton arme_ (Paris); Buel and Hill,
_Reinforced Concrete Construction_ (London). (F. E. W.-S.)

CONCRETION, in petrology, a name applied to nodular or irregularly
shaped masses of various size occurring in a great variety of
sedimentary rocks, differing in composition from the main mass of the
rock, and in most cases obviously formed by some chemical process which
ensued after the rock was deposited. As these bodies present so many
variations in composition and in structure, it will conduce to clearness
if some of the commonest be briefly adverted to. In sandstones there are
often hard rounded lumps, which separate out when the rock is broken or
weathered. They are mostly siliceous, but sometimes calcareous, and may
differ very little in general appearance from the bulk of the sandstone.
Through them the bedding passes uninterrupted, thus showing that they
are not pebbles; often in their centres shells or fragments of plants
are found. Argillaceous sandstones and flagstones very frequently
contain "clay galls" or concretionary lumps richer in clay than the
remainder of the rock. Nodules of pyrites and of marcasite are common in
many clays, sandstones and marls. Their outer surfaces are tuberculate;
internally they commonly have a radiate fibrous structure. Usually they
are covered with a dark brown crust of limonite produced by weathering;
occasionally imperfect crystalline faces may bound them. Not
infrequently (e.g. in the Gault) these pyritous nodules contain altered
fossils. In clays also siliceous and calcareous concretions are often
found. They present an extraordinary variety of shapes, often
grotesquely resembling figures of men or animals, fruits, &c, and have
in many countries excited popular wonder, being regarded as of
supernatural origin ("fairy-stones," &c.), and used as charms.

Another type of concretion, very abundant in many clays and shales, is
the "septarian nodule." These are usually flattened disk-shaped or
ovoid, often lobulate externally like the sur