ish or kidney-like masses; sometimes they are of a peculiar 
peg or nail-shaped form, the so-called nail-chalk, or they 
lie in layers between other limestones. Marls very rich 
29 
in clay or sand are called clay or sand-marl. They are 
very readily decomposed in the atmosphere, and form a 
rich, more or less sandy soil. 
PLATE VII., Figs. 15 and 16, and PLATE X., Figs. 1 —3 and 5— 6. 
Arragonite. 
The primary form is a right rhombic prism of 116° 11'. 
This form is generally found with truncation of the acute 
lateral edges (Plate VI., Fig. 2), sometimes also with 
truncation of the basal edges, and aggregated as twins 
(Plate X., Fig. 1) ; in this case the crystals are often 
quite transparent or yellowish. The former variety occurs 
at Molina in Arragon, the latter at Bilin in Bohemia. 
Besides these it presents pyramidal and acute forms (Plate 
X., Fig. 3), from Salzburg); fibrous forms are also met 
with, like the sprudelstein of Carlsbad (Plate VII., Fig. 
15), which is partially reddened by oxide of iron, or 
shrub-like forms, sometimes of coralline structure like the 
so-called flos-ferri (Plate VII., Fig. 16), a finely fibrous 
arragonite, with an external pearly or silver lustre. It 
is conjectured that most arragonites have been formed 
at a higher temperature from the carbonaceous waters 
of the limestone mountains, and that the distinction 
between arragonite and calcite is closely connected with 
the relations of temperature; there are arragonites, too, 
which contain strontia, and even oxide of lead, as, for in¬ 
stance, the tarnovicite , a fibrous arragonite from Tarnowitz 
in Silesia, which contains from 2*0 to 3*8 of the carbonate 
of lead. Most of the arragonites are, however, pure, 
simple carbonate of lime, as has already been stated. 
Carbonate of magnesia, and the protoxides of manganese 
and iron, are also found in many, especially in the compact, 
varieties of arragonite ; in this class is the so-called agate- 
marble from Constantine in Algiers (Plate X., Fig. 5), 
which is translucent, and is marked by greenish and occa¬ 
sionally yellowish stripes, and also the veined variety from 
Gibraltar (Plate X., Fig. 6) and Bergamo in Northern 
Italy. These, like the other arragonites, are also distin¬ 
guished from the common limestones by their greater 
weight (= 2*8—3*0), and hardness (3*5—4*0). Their be¬ 
haviour in the fire, and towards acids, has been given above. 
The distribution of arragonite is much more limited than 
that of the calcite, and, except being employed for marble, 
and for small objects of art, as already mentioned, it has 
no further application in the arts. 
Fig. 4.— Bitter Spar and Brown Spar, Dolomite. 
i 
This mineral differs from calcite by its crystalline 
form, which is a rhombohedron of 106° 16 / ; it is essen¬ 
tially distinguished from it, however, by containing car¬ 
bonate of magnesia, almost in equal proportions, some¬ 
times also carbonate of manganese and iron; the hard¬ 
ness (= 3*5—4*01), and the specific gravity (2*81—2*90) 
are also greater. The latter varieties are generally 
called brown-spars, the former bitter-spars, and, if they 
occur in larger crystalline granular masses, dolomite. The 
colour varies from white to yellow, grey, reddish (Plate X., 
Fig. 4), and even to black. The crystals are essentially 
separated from the calcite, inasmuch as they usually present 
intermediate, and more rarely obtuse, rhombohedrons; 
they are drusy, often present curved planes, are but slightly 
transparent, and are, for the most part, of a pearly lustre, 
and are dissolved only slowly in acids; before the blow-pipe 
they are phosphorescent, indeed, and become caustic, im¬ 
parting to the flame a yellow-red colour, but most of them, 
at least all the brown-spars, take a brown or black colour. 
The chemical constituents vary. Dolomite or bitter- 
spar contains, on an average, 55*0 carbonate of lime, and 
45*0 carbonate of magnesia (= Ca G + Mg C); in brown- 
spar the lime, and sometimes the magnesia, is partially re¬ 
placed by carbonate of iron and manganese, the bitter-spar 
from Kolosuruk in Bohemia for three equivalents of Ca C, 
has only two equivalents of Mg C. 
Its occurrence is limited, for the most part, to veins 
containing ores and clefts in the primary and stratified 
rocks : dolomite is found in irregular masses, sometimes, 
however, also in regular beds of remarkable thickness and 
extent, especially in the lime mountains of the Alps, 
Southern Tyrol, of the Zeckstein, shell limestone, and 
white Jura especially. They present sometimes the ap¬ 
pearance of granular limestone, and are employed in 
building. They are also used for the preparation of car¬ 
bonate and sulphate of magnesia. 
The carbonate of magnesia, magnesite , occurs crystal¬ 
lised or crystalline as magnetic spar, similar to bitter-spar, 
and has likewise a rhombohedron of 107° 22' for a primary 
form; it is distinguished from it, however, by greater 
hardness (= 4*0—4*5), and specific gravity (= 2*9—3*1), 
as well as by chemical composition. It is a pure carbo¬ 
nate of magnesia = Mg C, consisting of 48*31 of mag¬ 
nesia and 51*59 of carbonic acid; it is dissolved bv acids 
with effervescence, is phosphorescent before the blow-pipe, 
and, when wetted with a solution of cobalt, gives a rose- 
red colour. There are two kinds of magnesite; the one 
compact, sometimes passing into earthy, is white, and 
contains, for the most part, some silica, as, for instance, 
that from Sassbach at the Kaiserstuhl, and from Hrub- 
schtitz in Moravia. Crystals are found at St. Gothard, 
at Greiner in the Zillerthal, and in the Fassathal, generally 
with talcose or chloritic schist. 
Breunerite is a magnesite mixed with carbonate of 
iron and manganese ; pistomesite consists of three equiva¬ 
lents of carbonate of magnesia and two of carbonate of 
iron, and is found at Flackau in Salzburg. 
The periclase of the Somma at Naples is a magnesia, 
containing oxide of iron, and crystallising in octahedrons 
or cubes. Hardness 6*0, specific gravity 3*75. 
i 
