110 
THE PRINCIPAL FLOOR. 
exhibited from Derbyshire, Cornwall, &c. Egremont in Cum- 
berland lias furnished some beautiful illustrations, including the 
interesting “ butterfly twins ; 55 whilst the Furness District in 
Lancashire has yielded crystals richly tinted with hsematite. 
The Derbyshire scalenohedra are known as “ dog-tooth spar.” 
Case K. — Carbonate of Lime, continued. 
Many of the massive forms of carbonate of lime having been, 
already described in connexion with the marbles (p. 22) in the 
Hall, it only remains to notice among the marbles here exhibited — 
the giallo antico, or yellow marble of Sienna, generally known 
as Sienna marble ; the onyx marble , from Algeria and Mexico, 
a stalagmitic form similar to the “ alabaster ” of the ancients ; 
and the fire marble, or lumachella, from the lead mines of 
Bleiberg, in Carinthia, remarkable for the brilliant iridescence of 
its fossils. 
Tke Fontainebleau sandstone or limestone is an aggregate of 
rhombohedrons of carbonate of calcium, containing a largo 
quantity of sand mechanically mingled. 
In connexion with the varieties of carbonate of lime will bo 
found some interesting examples of the production of pearls on 
the shell of the pearl oyster and pearl mussel, by the artificial 
process of introducing some object which will produce irritation 
to the mollusc. 
A very distinct physical condition of carbonate of calcium is 
presented in the mineral called Aragonite , from having been 
first discovered at Aragon, in Spain. In addition to the fine 
rhombic crystals may be noticed the coral-like stalagmitic forms 
occurring chiefly in the iron mines of Styria, and known as flos 
ferri , or “ flower of iron ” 
Case L. — Dolomite; Apatite. 
Carbonate of calcium frequently contains a variable amount 
of carbonate of magnesium, and when the two compounds occur 
united in nearly equivalent proportions, they form the species 
Dolomite , already noticed among the building stones (p. 33). 
As the proportion of magnesium increases, the species passes 
into Magnesite , or carbonate of magnesium. 
The' remainder of this case is occupied with a series of 
specimens illustrating the varieties and mode of occurrence of 
the valuable mineral called Apatite. In different varieties the 
composition of apatite varies, but it consists essentially of 
phosphate of calcium associated either with a chloride or fluoride 
of calcium, or with both. This mineral is not unfrequently 
found in company with tin ores. The extensive use of phosphate 
of lime as a fertiliser gives considerable commercial value to 
the deposits of this mineral occurring in Spain, Nassau, Norway, 
and Canada, The massive varieties are frequently called Phos- 
phorite. With the minerals in this case are grouped a few 
