
CONCRETIONS AND SECRETIONS 121 
in chalk, and is probably, like the latter, in many cases of organic origin. 
Now and again, however, it may have been a deposition from thermal 
water. In such cases it occurs as thin lamine, interleaved with similar 
laminz of limestone—the layers being often highly puckered, crumpled, or 
confusediy contorted and involved, as if the deposits had been disturbed by 
the bubbling up of spring-water before they had become quite solidified. 
These appearances, however, may be otherwise accounted for. The 
siliceous solution may have been originally in a colloid or jelly-like 
condition, containing some percentage of water. Thus, when the mineral 
began to lose its water and solidify, the contraction of its bulk would give 
rise to much distortion and confusion—later accretions of silica filling up 
any fissures or cavities thus produced. Siliceous secretions are not 
uncommon in some argillaceous rocks: nodules and seams of chert 
(often radiolarian), for example, occasionally occur in Paleozoic shales, 
and reniform menzlite appears now and again in marls of later age. 
Even sharp crystals of guartz, generally of small size, have occasionally 
‘been developed in marly clay. 
(4) CALCAREOUS and FERRUGINOUS.—Spherical and nodular cal- 
careous and ferruginous concretions are characteristic of many argillaceous 
rocks and of some sandstones. In laminated clay the mineral solutions 
have made their way most readily along the planes of sedimentation, so 
that the resulting concretions are usually somewhat lenticular, and often 
assume the shape of flattened spheroids. When numerous, they not 
infrequently coalesce so as to form irregular concretionary bands or 
layers. Very often, however, they are scattered sporadically through the 
beds in which they occur. In homogeneous clay-rocks, without apparent 
lamination, the concretions are usually either spherical or variously 
shaped, and often irregularly dispersed. Frequently they have formed 
round a nucleus, which may consist of mineral matter, but is more 
commonly of organic nature, such as a shell, a coprolite, a fish, a 
fragment of plant, etc. A concretion may be compact and homogeneous 
throughout, or may consist of concentric shells, or while externally 
compact it may be much cracked and fissured internally. The cracks 
are widest towards the centre of a concretion, and die out towards its 
circumference, as if the interior had contracted after the outside had dried 
and become consolidated. They are often partially or completely filled 
with subsequently introduced mineral matter, usually calcite. Concretions 
of this kind are known as sef/aria or seftarian nodules, in allusion to the 
septation or partitioning of the interior (see Plate XXVII. 1). Septaria 
are commonly either calcareous or ferruginous. Occasionally, concretions 
consist of concentric shells of different chemical composition. In a 
nodule, composed for the most part of ferruginous matter, one or more 
of the shells may be calcareous; or the core or kernel may be calca- 
reous, and the external shells ferruginous.* Owing to the subsequent 
action of percolating water, the calcareous portions may be completely 
* Occasionally one or more of the concentric shells may consist of 
oxide of manganese (psilomelane). 


