78 MINERALOGY. 
muschelkalk ; it forms a principal portion of wood ashes, and the bones of 
animals, into which it is introduced by water holding it in solution. When 
crystallized it is known as calcareous spar. Calcareous spar has the most 
diversified crystallization to be met with in the mineral kingdom. It is 
likewise one of the dimorphous substances, or those whose forms do not all 
belong to the same system. Calcareous spar is consequently distinguished 
as arragonite, and as calcareous spar proper. In the former the crystals 
belong to the trimetric system (jigs. 34 and 35). The crystals are right 
rhombic prisms, and are often grouped together, as in fig. 18. In the latter, 
or calc spar proper, the crystals belong to the hexagonal system. The 
forms are derived from the holohedral primary form of the double six-sided 
pyramid, or from a rhombohedron in which the angle is 105° 5’, some of 
which are represented in pl. 35, figs. 27, 29, 32, and 36. The hexagonal 
prisms are frequently pure, and such crystals often form very fine specimens, 
as in pl. 35, figs. 8, 16%, 16°; they are often terminated by the faces of the 
double six-sided pyramid, as shown by figs. 36, 45, and 47. Rhombohedral 
crystals, particularly secondary rhombohedrons, are also very common, 
together with various combinations of surfaces of rhombohedrons with those 
of six-sided prisms and pyramids. fig. 20 represents a druse of 
rhombohedral segments ( fig. 17) of dogtooth spar, with the very interesting 
crystal form of fig. 43, or the scalenohedron (scalene dodecahedron). 
The principal forms of crystallization of calc spar are shown in 
pl. 35, figs. 25, 28, 33, 37, 40, 41, 44, 45, 46, 47, 49, and pl. 36, fig. 37. In 
pl. 35, figs. 10, 11, 14, 21, 22, 24, and pl. 36, fig. 29, may be seen druses 
with many of the different modifications. Pl. 35, fig. 19, represents the 
hexagonal prism, with a finely crystalline coating of a second crystallization 
on the first, as a bed. The variety, pl. 36, fig. 3, 1s called double spar. 
2. Carbonate of fron. 
This mineral is often found associated with carbonate of lime, carbonate 
of magnesia, carbonate of manganese, and receives various names in its 
different modifications of constitution and form. The primary form of its 
crystallization is an obtuse rhombohedron: it also occurs in various 
secondary derivative forms. Pl. 35, jig. 3, represents a variety from 
Steinheim, called Sph@rosiderite, a carbonate of iron of spheroidal form 
and radiated texture. Spathic iron is the usual name of the pure carbonate 
of iron; it often contains some carbonate of magnesia and lime. It occurs 
in vast beds, and yields an iron excellent for conversion into steel. Its 
crystals are shown in pl. 36, fig. 36, having the obtuse rhombohedron as 
their primary form. Spathic iron also occurs in crystal druses of the 
character seen in figs. 6 and 14. By increasing the proportion of lime and 
magnesia, to a degree sufficient to form dolomite, we have brown spar. 
The crystals of this have a very great similarity to those of simple 
calcareous spar; they are, however, readily recognisable, by becoming 
brown when exposed to the weather, owing to the manganese and iron 
they contain. 
3. Carbonate of Strontia. 
This is met with in the form of strontianite, in no great abundance, and 
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