68 STRUCTURAL AND FIELD GEOLOGY 
as rock-like masses in beds, veins, and cavities (especially in 
limestones). Not infrequently it is found replacing limestone 
(see under ORE-FORMATIONS). As a rock it usually contains 
many impurities, such as clay, quartz, oxide of manganese, 
etc. It passes into ferruginous clay, etc. Spathzc [ron-ore is 
a granular or compact aggregate of siderite, occurring in 
beds and as veins, especially among the older geological 
systems. Clay-zronstone is a variety of spathic iron-ore, 
containing much clay. It is brown to dark grey or black, 
and appears as thin beds and layers, or in the form of balls 
and nodules. It is a very common rock among the 
argillaceous strata of the Carboniferous system. In some 
cases it appears to have been deposited on the floors of 
ancient lakes, lagoons, and estuaries; in other cases it is of a 
concretionary nature—the ferruginous matter originally 
diffused through an argillaceous bed having become aggre- 
gated around fossils or other foreign bodies, so as to form 
nodules of various size. Many of these nodules are septarian 
(Plate XXVII.). Blackband-tronstone is simply a clay-iron- 
stone, containing a large proportion of carbonaceous matter 
(from 10 to 52 per cent.). Magnetic Tron-ore sometimes 
occurs in beds amongst fossiliferous strata, in which case it has 
resulted from the alteration of limonitic ore. Magnetic iron- 
sands are often met with in regions where certain igneous 
rocks abound, from the disintegration of which the magnetite 
has been derived. (For other occurrences of magnetic iron- 
ore, see under ORE-FORMATIONS.) 
III. Organically derived Rocks 
The more important rocks included in this division are 
largely composed of organic remains—plant or animal as the 
case may be. Some, however, are due, or partly due, rather 
to the action of living organisms. Amongst the latter are 
Flint and some kinds of Calcareous Tufa and Ironstone. 
The origin of flint has already been briefly considered (p. 67). 
Bog iron-ore not infrequently owes its origin to the action of 
the minute plants known as diatoms, which are able to 
separate iron from water, and to deposit it about and within 
their substance as a hydrate. Water-loving plants are also 


