162 NATURAL HISTORY. [NORTH 
The five following Cases contain the acid or oxide of 
silicium (silica,, quartz), the numerous' varieties of which, 
formerly considered as so many distinct species, are mostly 
indebted for their generally very striking external charac¬ 
ters to the admixture of matter foreign to the species, or 
to other casual circumstances that prevailed at their form¬ 
ation. 
Case 20. Amethyst quartz of various tints, in grouped 
crystals, &c.-— Rock crystal : various modifications of crys¬ 
talline forms: small dodecahedral and other crystals, 
known by the names of Gibraltar diamonds, Bristol dia¬ 
monds, &c.; varieties of colour, according to which this 
substance obtains the familiar denominations of smoky 
topaz or morion, cairngorm, citrine, &c.; specimens of rock 
crystal enclosing various substances, such as rutile, brown 
iron-stone, micaceous iron, acicular antimony, actinote, 
asbest, chlorite, &c.; groups of rock crystal; some speci¬ 
mens of rock crystal in a wrought state are added, among 
which is Dr. Dee's show-stone (see Walter Scott's Demon¬ 
ology), &c. 
Case 21. Common quartz : among the specimens of this 
widely diffused substance, which offers such great variety 
in its external aspect, the more remarkable are the hacked, 
corroded, and cellular quartz from Schemnitz, as also the 
pseudomorphous or supposititious crystals, principally 
derived from modifications of calcareous and fluor spars; 
and, with regard to colour, the blue quartz, called siderite , 
from Salzburg, and the rose or milk quartz , which are both 
used as ornamental stones ;— -fibrous quartz ;—flexible 
sandstone (itacolumite ) from Brazil and China;— flelid 
quartz , from Nantes; — iron-flint. In this Case are also 
deposited several varieties of stalagmitic quartz or quartz - 
sinter , the most remarkable among which are the siliceous 
concretions deposited by the celebrated hot spring in Ice¬ 
land, the Geyser; another variety of it is the pearl- 
sinter from Santa-Fiora in Tuscany (whence it has ob¬ 
tained the name of fiorite), and from the island of Ischia. 
With these are placed specimens of the ceraunian sinter 
or those enigmatical siliceous tubes which were discovered 
in the sands of the Senner Heath in the county of Bippe 
(where, on account of their probable origin, they are 
