
CONCRETIONS AND SECRETIONS 123 
considered under the head of metamorphism. No hard-and-fast line can 
be drawn between the changes which produce concretions and concre- 
tionary structures in “unaltered” rocks, and those which have induced 
the aggregation and crystallisation of mineral matter in certain “altered ” 
or “metamorphic” rocks. 
Sulphate of lime is not so often met with in concretions as carbonate 
of lime and ferruginous compounds. In some clay-rocks, however, 
gypsum concretions are common enough. Sometimes these appear as 
large perfect crystals and twins of the mineral, but more frequently as 
lenticular nodules, or layers, an inch or more in thickness. 
The oxides of manganese and iron occur not only in nodular forms, 
but frequently appear as thin films coating the surfaces of the natural 
division-planes of rocks, such as joints and bedding-planes. They usually 
assume delicate plumose or plant-like forms resembling sprigs of moss, 
etc., and hence are termed dendrites or dendritic markings (see Plate 
XXVIII.). Although usually appearing only on division-planes, now 
and again they ramify through the substance of fine-grained rocks, such 
as certain limestones, on sections of which the markings often simulate 
belts of trees, hedgerows, etc. (andscape-maréle), 
CONCRETIONARY RocKs.—Not only do mineral solutions tend to 
form concretions of various kinds in rocks, but the rocks themselves have 
not infrequently acquired a concretionary structure. Some sazdstones, for 
example, seem to be largely composed of aggregates of ball-like or larger 
spheroidal masses. Few sandstones, indeed, do not in places show some 
indications of this concretionary structure. The spheroids are now and 
again enclosed in dark brown ferruginous crusts, the rock within being 
often bleached, and it may even be reduced to the condition of loose 
sand. When sandstone of this character is exposed by quarrying, the 
freshly cut rock may show concentric bands of a dark brown or red 
colour, some of which may be an inch or less in width, while others may 
- exceed several feet. The origin of the structure is obscure. The 
ferruginous matter may have been introduced by percolating water, but 
some of it at least has been abstracted from the sandstone itself. The 
concentric shells of ferruginous matter shown in Plate XXVII. 2 are not 
hard to explain, and their mode of formation may throw some light on 
that of the larger concretionary masses to which reference has just been 
made. They owe their origin undoubtedly to the presence of disseminated 
granules or crystals of some ferruginous mineral, almost certainly pyrite 
or marcasite. By the action of water soaking into the stone the mineral 
is broken up chemically, and a ferruginous solution formed, which spreads 
outwards as a drop of ink does on blotting-paper. Evaporation taking 
place around the outer margin of the solution, iron-oxide is precipitated, and 
the first ring or shell is formed. The process is repeated by the formation 
of a second shell inside the first, and thereafter the production of successive 
concentric shells is continued, each forming inside of its predecessor, 
until the ferruginous solution is exhausted. In some cases, a portion of 
the ferruginous mineral at the centre may remain, but it is usually so 
small and so much altered that its original character is hardly recognisable. 
eee ee 
