FIRST FLOOK. 
53 
lisation, and generally presents a more or less splintery fracture ; 
but in one kind, Flint (15b), the fracture is conchoidal, some- 
times conical, as is well shown by specimens in the case. 
Chalcedony (15b) has a lustre nearly that of wax, and is 
either transparent or translucent. The specimens from Uruguay 
(15d) are of especial interest as containing imprisoned water. 
The Heliotrope, or Bloodstone (16a), is a green stone with red 
blood-like spots. 
Kext follow the Plasma and Chrysoprase, green stones : and 
the Sard, generally a brownish red ; as also the Sardonyx, its 
banded variety : all of them much prized by the ancients 
because, though hard and tough enough to resist ordinary wear 
and tear, they are more suited to the display of the engraver's 
skill than are the harder and more precious stones. 
Then come the Agates (16b), chiefly formed of thin layers of 
porous chalcedony of different colours, though the material of 
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 
and 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. 
Strontianitc (18b), tlie carbonate of strontium, is tlie mineral 
