Some Forms of Silica. 217 
There are several varieties of the mineral known as opal. 
The most valued kind exhibits a rich play of colours, due to 
iridescence. They all contain from five to twelve per cent. of 
water of hydration. Opal is probably the early stage of such 
minerals as chalcedony, onyx, agate, and carnelian, which were 
formed by the infiltration of silica into cavities or fissures in rocks. 
Chalcedony occurs in several varieties. The structure exhibits 
radiating crystals resembling fibres. Chrysoprase is an apple- 
green variety; carnelian, a bright red; sard, a brownish red ; 
agate is variegated; mocha stone and moss agate are chalcedony 
with dendritic or moss-like markings disseminated through them ; 
onyx is an agate with colours arranged in horizontal layers ; 
cat’s-eye is a chalcedony with a peculiar lustre, due to the 
inclusion of filaments of asbestos; jasper is another variety. 
Quartz is silica in the crystalline condition. The crystals 
belong to the rhombohedral system, and occur in six-sided prisms 
with pyramidal ends. Rock-crystal is the purest variety, and it 
is to this form that the term crystal was first applied. It is also 
known as pebble, and is used for making spectacles and other 
optical instruments. Some of the varieties rank among the 
precious stones. Amethyst is quartz coloured purple by man- 
ganese, and is not uncommon, although crystals suitable for 
cutting are scarcer. Cairngorm is a brown or yellow variety 
found in Scotland, and is much used in Scotch jewellery. 
Quartz approaches the precious gems in its amount of hardness.* 
When fused by the intense heat of the oxy-hydrogen blowpipe, 
it may be drawn out into fine flexible elastic threads. Heated 
in a current of steam it undergoes partial sublimation. Fine 
crystals are often found accompanying mineral veins, and also 
lining cavities of rocks, or replacing other minerals. It is by no 
means uncommon to find cavities in flints lined with small 
glistening crystals of quartz. These must have been formed 
subsequently to the upheaval from the sea-bottom. The growth 
of large crystals most probably took place with extreme slowness 
from aqueous solution. Crystals frequently contain cavities 
with various fluids, such as water, liquefied carbon dioxide, and ~ 
hydrocarbons such as naphtha. They often attain a large size. 
There are some in the Natural History Museum about a yard 
high and a foot in diameter. I was shown a very perfect small 
crystal the other day from the Pitch Lake, in Trinidad. It was 
perfectly clear, and each end was terminated by six-sided pyra- 
mids. It had been formed in contact with pitch, instead of hard 
rock, and consequently had grown uniformly in every direction. 
Vein quartz is of a milky-white colour, and occurs in large 
masses not showing definite crystals. It is found filling veins, 
—— ae 
* It scratches glass easily. 
