
488 GEOTECTONIC (STRUCTURAL) GEOLOGY. [Book IV. _ 
of a stratum or dispersed irregularly through it (Fig. 204). 
They most commonly consist of ferrous or calcic carbonates, or 
of silica. Many clay-ironstone beds assume a nodular form, and 
this mineral occurs abundantly in the shape of separate nodules in 
shales and clay-rocks. The nodules have frequently formed 
round some organic body, such as a fragment of plant, a shell, bone, 
or coprolite. That the carbonate was slowly precipitated during the 
formation of the bed of shale in which its nodules lie may often be satis- 
factorily proved by the lines of deposit passing continuously through 
the nodules (Fig. 205). In many cases the internal first-formed parts 





CS =a eel SS SS 
—— SS Fic. 205.—CoNCRETIONS SURROUND- 
$$ ING ORGANIC CENTRES, AND EX- 
Fie. 204.—Concretions or LIMESTONE HIBITING THE CONTINUATION OF 
IN SHALE. yHE LINes OF STRATIFICATION OF 
THE SURROUNDING SHALES. 
of a nodule have contracted more than the outer and more compact 
crust; and have cracked into open polygonal spaces which are com- 
monly filled with calcite (Fig. 30). Such septarian nodules, whether 
composed of clay-ironstone or limestone, are abundant in many shales, 
as in the Carboniferous and Liassic series of England. 
Alluvial clays sometimes contain fantastically shaped concretions 
due to the consolidation of the clay by a calcareous or ferruginous 
cement round a centre. These are known in Scotland as fairy- 
stones, in the Valley of the Rhine as Léss-puppen, Léss-manchen, 
and in Finland as Imatra-stones (Fig. 206). They not uncommonly 
show the bedding of the clay in which they may have been formed. 
Their quaint imitative forms have naturally given rise to a 
popular belief that they are petrifactions of various kinds of organic 
bodies and even of articles of human manufacture. In Norway they 
enclose remains of fishes and other organisms." 
Concretions of silica occur in limestone of many geological ages 
(p. 117). The flints of the English chalk are a familiar example, 
but similar siliceous concretions occur in Carboniferous and Lower 
Silurian limestones. ‘The silica in these cases has not infrequently 
been deposited round organic bodies, such as sponges, sea-urchins, 
and mollusca, which are completely enveloped in it, and have even 
themselves been silicified. Iron-disulphide often assumes the form 
of concretions, more particularly among clay-rocks, and these, though 
presenting many eccentricities of shape—round like pistol-shot or 
cannon-balls, kidney-shaped, botryoidal, &c.—agree in usually pos- 
sessing an internal fibrous radiated structure. Phosphate of lime is 
found as concretions in formations where the coprolites and bones of 
reptiles and other animals have been collected together. 
Concretions produced subsequently to the formation of the rock 
* Kjerulf, “ Geologie des siid], und mittl. Norwegens” (1880), p. 5. 
