28 
Figs. 9—14.— Rhombohedral Carbonate of Lime, 
Calc-Spar, Double Spar, Calcite. 
The primary form is an intermediate rhombohedron of 
105° 5 ! —105° 8 ; (Fig. 9), in which directions all crystals 
may be alike easily split. There is still, however, a fourth 
crossing of laminae, which falls perpendicularly on the prin¬ 
cipal axis, and therefore corresponds to the planes of trun¬ 
cation of the vertex. This rhombohedron is seldom found 
simple, but, for the most part, with single or several trun¬ 
cations of the vertical edges, as Fig. 10 shows ; sometimes 
as a secondary, and sometimes as an acute rhombohedron 
(Fig. 12), and from three to six different rhombohedrons, 
combined with one another, not unfrequently occur; they 
may be recognised by their pyramidal planes, of which 
every alternate three are grouped round the principal axis. 
A separate series is formed by the unequal edged hexagonal 
double pyramids, scalenohedrons (Fig. 11), which occur 
sometimes by themselves, sometimes united as twins (Fig. 
14), sometimes combined with dissimilar rhombohedrons, 
especially frequently in clefts and veins of the different 
chalk formations. A third series is formed by the hexa¬ 
gonal prisms, which originate in truncation of the basal 
angles of the rhombohedron, and occur sometimes in com¬ 
bination with the acute, sometimes with the obtuse rhom¬ 
bohedron, sometimes without these, with right terminal 
planes. Fig. 13 represents the first secondary rhombo¬ 
hedron, with the hexagonal prism, as it so frequently occurs 
in the metallic veins at the Harz, the Schwarzwald, the 
Erzgebirge, etc. All prisms have shorter tabular, and 
longer, frequently much striated, forms; the rhombohedrons 
have frequently long spear-like and aeicular crystals, so 
that the calc-spar represents a variety of forms, equalled 
by no other mineral, about 800 having been described. 
Stem-like, foliated, fibrous, crystalline-granular, dense, 
perfectly compact, earthy-friable, stalactitic (Fig. 17), with 
radiated fracture, pisiform or globular (Fig. 18), roe-stone¬ 
like and oolitic forms are the most usual. Under the 
name limestone, are comprised all those varieties of car¬ 
bonate of lime which occur in large masses, but when these 
are white or finely coloured, and capable of polish, they are 
called generally, marbles. Thus the granular white marble 
of the primary formations is called saline or statue-marble, 
also absolutely white marble, and, according to the locality 
where it is found, Carrara, Tyrolese, Parian, and Hymetan 
marble. The other kinds are divided according to their 
colour; where it is single, into yellow, red, grey, black; 
where double, into black and white (Plate VIII., Fig. 5), 
grey and black (Fig. 7), variegated (Figs. 8 and 9), etc. 
Fire-marble, or lumachelli, is the name given to a limestone 
capable of polish, and coloured by fragments of a mussel or 
snail (Plate VIII., Figs. 2 and 6) ; such marbles as con¬ 
tain corals are called coralline marbles (Fig. 9); those which 
present figures of ruined walls, castles, buildings, etc., are 
called ruin marbles (Plate VIII., Fig. 1) ; brecciated 
marbles are such as are composed of fragments of differ¬ 
ently coloured limestones (Plate VIII., Figs. 3 and 5; 
Plate IX., Fig. 4). Those calcareous stalactites which are 
marked with layers, and which take on a polish, are called 
striped marbles (Plate IX., Fig. 2) ; all those where two 
or more colours are especially conspicuous are called varie¬ 
gated marbles (Plate IX., Figs. 1 and 5—8), according to 
the formation we distinguish between salt and fresh-water 
limestone. Limestone appears very frequently, not only 
as the petrifying medium of the remains of conchiferous 
animals, but also as the mother-stone of the petrifactions 
which are often embedded in it by thousands. As in all 
the limestone of the stratified formations, these remains 
consist of the coverings of marine animals, this is called ma¬ 
rine limestone. An example of this is seen in Fig. 6, 
Plate VIII., which represents a tertiary marine limestone 
from the tertiary formation, with sections of turbinated shells 
(■Pyramidella Turbinella ?), cut through in different direc¬ 
tions ; also in Plate IX., Fig. 3, which is a fragment of 
gryphitic chalk, with bivalve shells (gryphcea arcuata), from 
the lias near Stuggart. Such limestones are called fresh¬ 
water limestones, as, from their containing the remains of 
land and freshwater animals {Helix, Pupa, Lymnceus, Palu- 
dina, Planorbis, Valvata, etc.), can be recognised as the 
deposits of fresh water. To these belong, for example, 
Figs. 1 and 2, Plate IX., which are from the ter¬ 
tiary formation of the Swabian Alps at Bottingen, near 
Miinsingen, and present a similar appearance to the spru- 
delstein of Carlsbad (Plate VII., Fig. 15). Mineral water 
limestone is the name applied to such varieties of calcite as 
are known to have their origin in mineral springs which 
are rich in carbonic acid, as, for instance, the tufaceous 
limestones of Canstatt, -which are produced under the eyes 
of the observer. Tufaceous limestone usually includes all 
the more recent kinds of calc-spar that have been produced 
either in mineral springs or in rivers. They are generally 
extremely porous, of an earthy fracture, sometimes crystal¬ 
line-cellular, sometimes they take reticulated, interlaced, 
shrub-like forms by encrustation of chara and similar 
plants; they occasionally assume coralline or peg-like 
forms. This is especially the case near waterfalls, where 
the spray sometimes produces even very fine clustered or 
globular forms. 
According to the formations we distinguish between 
the plutonic or primary limestone, stratified limestone of 
the different formations, Silurian, Devonshire, lias, Jurassic, 
chalk, tertiary diluvial, and alluvial limestone. There are 
also limestones of volcanic origin, as, for example, at the 
Kaiserstuhl there is a layer of crystalline lime mixed with 
mica in the dolerite, and vulcanised or stratified limestone 
changed by volcanic minerals, which is usually remarkable 
for its marble-like mixture of colours, as, for example, 
Plate IX., Figs. 5—8, and limestone of the Jura formation 
from the neighbourhood of volcanic rocks in the Swabian 
Alps. 
The distribution of limestones is extraordinarily great, 
and they play an important part in each formation. The 
application of this mineral to quick-lime and other chemi¬ 
cal purposes, to architecture, for the construction of build¬ 
ings and roads, for monuments, sculpture, and also to 
lithography, etc., are generally known. 
Chalk-marl is the name applied to the limestones 
which are more or less rich in clay, aud which are soluble 
in acids, with separation of the muddy clay. The amount 
of clay contained in marl varies from 25 to 30 per cent, 
and when burned it forms a hydraulic mortar, that is, a 
cement which hardens under water, and is extremely 
valuable for building in water or by the shore. These 
rocks are found in most of the chalk-formations, especially 
in the lias and Jura. The marls frequently form round- 
