
122 STRUCTURAL AND FIELD GEOLOGY 
removed in solution. In this way, by the removal of a calcareous layer 
from the interior of a nodule, a central ferruginous kernel becomes 
detached, and rattles when the concretion is shaken (K/lapferstein, or 
Rattle-stone). When a calcareous core is entirely dissolved, a nodule, of 
course, becomes hollow. Many nodules, however, are rendered hollow, 
simply owing to the contraction of the interior after the outer shell has 
dried and hardened. 
Concretions of the several kinds referred to in the preceding paragraph 
are all obviously of secondary origin—they are superinduced structures. 
This is shown by the fact that the planes of sedimentation can often be 
seen passing through them, and never curving over them, as would have 
been the case had they been loose stones and boulders covered up 
while lying on lake-floor or sea-bottom. Among familiar examples of 
concretionary calcareous nodules are the so-called /azvy-stones so fre- 
quently met with in alluvial clays. In Germany they are common in 
loess, and are known to the country-folk as ‘“‘Loss-piippchen, Lo6ss- 
mannchen,” etc.,and as “‘ Marlekor” (Kobolds’ playthings) or “ Nakkebréd” 
(Nixies’ bread) in Sweden. Similar calcareous concretionary nodules 
termed “kankar” are abundantly developed in many of the alluvial 
deposits of India. Reference may also be made to the curious calcareous 
concretionary structures which occur in the Tertiary sand of Fontainebleau, 
near Paris. These frequently take the form of single crystals of calcite, 
or of groups and aggregates of such rhombohedral crystals. Ferruginous 
concretions are well represented in this country by the balls and 
nodules of clay-zronstone (spherosiderite) that occur so abundantly in the 
black shales of the Carboniferous System. They vary in size from a hazel- 
nut to flattened spheroids measuring two or three feet across, but these last 
are not common. Many contain a fossil at the centre, while others seem 
to consist wholly of inorganic materials. A large proportion, it may be 
added, are septarian. As calcareous and ferruginous concretions alike 
tend to be developed in the direction of the bedding-planes of the rock 
in which they occur, it frequently happens that contiguous nodules become 
fused together, so as to form more or less continuous seams of limestone 
or of ironstone, as the case may be. Sometimes such seams maintain an 
uniform thickness, but more usually they are lumpy, thickening and 
thinning irregularly. Concretions of disulphide of iron (Pyvite and 
Marcasite) are of frequent occurrence in sandstone, clay, chalk, and coal. 
They vary in size from minute grains up to nodules two or three inches in 
diameter. In the form of nodular concretions marcasite is much more 
common than pyrite, the concretions having usually an internal, fibrous, 
radiating structure. Now and again marcasite, however, assumes a 
crystalline form, as in the flat, spear-headed “twins” which are seen in 
the chalk deposits at Dover and Folkestone. Pyrite does not occur so 
commonly in nodules as marcasite, but has a much wider distribution in 
the crystalline form, crystals and crystalline aggregates appearing in 
many kinds of derivative rocks, either dispersed through a rock-mass 
or lining its minute cracks and fissures. As sporadic crystals or groups 
of crystals, it often appears in clay-slate, but such occurrences fall to be 
