FIRST FLOOR. 
55 
Then come the Agates (16b), chiefly formed of thin layers of General 
porous chalcedony of different colours, though the material of ^^^eralT 
many of the white layers is a compact semi-opal. Most of 
the specimens are now brought from Uruguay, in South 
America, and are cut and polished at Oberstein, in Germany, 
where in former times agates were got in quantity from the 
mountains of the district. Sometimes the layers are plane and 
parallel, and the stone is then an Onyx, useful as a material for 
cameos : or the bands of a section are arranged in sets of straight 
but zigzag lines, and the stone is then called a Fortification- 
agate: but in the ordinary agate the layers are variously 
curved. 
The Moss-agates, or Mocha-stones (16e), are varieties of 
chalcedony, enclosing moss-like forms of oxides of manganese 
iind iron and green earthy chlorite. 
The Carnelian (16e) is a beautiful stone much valued by the 
■engraver : its fracture has a peculiar waxy lustre, and is distinct 
from that of the Sard, which is dull and horn-like. 
Next follows the Opal, including the Precious or Noble 
Opal (16f), among the specimens of which is a fine suite from 
Queensland, presented by Professor Maskelyne, M.P. 
The carbonate of barium, Witherite (18a), is much used in the 
manufacture of plate-glass, and in France in that of beet-sugar. 
Strontianite (18b), the carbonate of strontium, is the mineral 
from which most of the strontium nitrate is made for use in the 
manufacture of fire-works, owing to the fine crimson colour 
which it gives to the flame : it is also much employed in the 
process of sugar refining. 
Cerussite (18b) is the corresponding carbonate of lead, and 
when abundant is a valuable ore of the metal. 
Calcite (18e), carbonate of calcium, is represented by an 
extraordinarily fine suite of specimens, illustrating an almost 
endless variety of crystalline form. The clear variety from 
Iceland is largely used in optical instruments for the polarisa- 
tion of light. 
Chalybite, or Spathic iron ore (20h), is the carbonate of iron, 
and is a most valuable ore of the metal. The most important 
English iron ore. Clay iron-stone, is a mixture of chalybite and 
clay. 
