240 DESCRIPTIONS OF MINERALS. 
cinth or fire-red reflections. It comes from Mexico and the 
Faroe Islands. 
Common Opal, Semiopal. Common opal has the hardness 
of opal and is easily scratched by quartz, a character which 
distinguishes it from some siliceous stones often called semi- 
opal. It has sometimes a milky opalescence, but does not 
reflect a play of colors. ‘The lustre is shghtly resinous, and 
the colors are white, gray, red, yellow, bluish, greenish to 
dark grayish-green. Translucent to nearly opaque. Phillips 
found nearly 8 per cent. of water in one specimen. 
Hydrophane. his variety is opaque white or yellowish 
when dry, but becomes translucent and opalescent when 
immersed in water. 
Cacholong. Opaque white, or bluish white, and usually 
associated with chalcedony. Much of what is so called is 
nothing but chalcedony; but other specimens contain water, 
and are allied to hydrophane. It contains also a little alu- 
mina and adheres to the tongue. It was first brought from 
the river Ca¢h in Bucharia. 
Hyalite, Muller’s Glass. A glassy transparent variety. oc- 
curring in small concretions and occasionally stalactitie. 
It resembles somewhat a transparent gum arabic. Composi- 
tion, Silica 92:00, water 6°33 (Bucholz). | 
Menilite. A brown opaque variety. in compact reniform 
masses, occasionally slaty. Composition, Silica 85:5, water 
11:0 (Klaproth). It is found in slate at Menil Montant, 
near Paris. : 
Wood Opal. An impure opal, of a gray, brown or black 
color, having the structure of wood, and looking much like 
common silicified wood. It is wood petrified with a hy- 
drated silica (or opal), instead of pure silica, and is distin- 
guished by its lightness and inferior hardness. Specific 
gravity, 2. 
Opal Jasper. Resembles jasper in appearance, and con- 
tains a few per cent. of iron ; but it is not so hard, owing to 
the water 1t contains. 
Siliceous Sinter has often the composition of opal, though 
sometimes simply quartz. The name is given to a loose, 
porous siliceous rock usually of a grayish color. It is de- 
posited around the Geysers of Iceland and the Yellowstone 
Park, in cellular or compact masses. sometimes in fibrous, 
stalactitic, or cauliflower-like shapes. It is often called gey- 
serite. Pearl sinter, or fiorite, occurs in yoleanic tufa in 
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