130 
POPULAH SCIENCE REVIEW. 
especially when they contain organic remains, such as sponges, 
choanites, ventriculites, &c., presenting often a very beautiful 
arrangement, due to the structure of these fossilised organisms. 
Flint pebbles of this kind are commonly found at Brighton, 
Dover, and the Isle of Wight, where they are derived from the 
wear and tear of the chalk cliffs in which they were originally 
imbedded. 
Opal — a hydrated variety of silica — has also been found in 
various parts of Gfrt.at Britain, where its commoner forms are 
not nnfrequently met with. That beautiful gem, the fire opal, 
is stated to occur at several localities in Cornwall ; but we are 
not aware that any British specimens have as yet been usefully 
emplo}md in this country. 
Disthene, or kyanite, from the Gfreek word signifying blue, 
is a mineral occasionally used in jewellery ; it is a silicate of 
alumina, and occurs in thin bladed crystals generally imbedded 
in metamorphic rocks in some parts of the Highlands of Scot- 
land, in the mica schist of Donegal, Ireland, and finely crystal- 
lized in the Shetlands. 
lolite is also a translucent blue stone, said to occur dissemi- 
nated in granite and gneiss, both in Ireland and Scotland. 
This stone, when viewed in different directions, exhibits a change 
of colour, whence it derives the name dichroite. 
Zircon, a silicate of zirconia, known as the Jargoon or Hya- 
cinth, is said to be found in Scotland and Ireland ; its hardness 
equals that of topaz, and its specific gravity is about 4*5. 
Equally rare in Britain are the Corundum or Sapphire (pure 
alumina) and Spinel (aluminate of magnesia). The former is 
stated to be found in Donegal, Wicklow, and at Carrock Fell in 
Cumberland ; and the latter, as rolled grains, in the auriferous 
sands of Wicklow. With the exception of corundum from Cum- 
berland, there are no British specimens in the national collection. 
Some varieties of felspar are occasionally employed in jewel- 
lery, as, for example, the opalescent Adularia, or Moonstone. 
The stone in the box (fig. 8 e) is a polished specimen of Green 
Felspar or amazon stone. 
The Beryl, Emerald, or Aquamarine, are names given to stones 
of similar composition, but which differ slightly in their colour 
and transparency. They are silicates of alumina, with a little 
glucina, and generally crystallise in hexagonal prisms' with ver- 
tically striated fiices, a character which often serves to distin- 
guish them from quartz. Beryl is harder than topaz, but is 
scratched by sapphire, and has a specific gravity of about 2*6. 
This mineral varies greatly in colour ; the pale green variety is 
called aquamarine, whilst the darker and more brilliant ones 
are termed emeralds. Beryls are found in small blue crystals, 
with topaz and tinstone, at St. Michael’s Mount, and in well- 
