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long erly considered as so many distinct species, are 
-— mostly indebted for their generally very striking 
external characters to the admixture of matter 
foreign to the species, or to other casual circum¬ 
stances that prevailed at their formation. 
Case 19. Rock crystal: various modifications 
of crystalline forms: small dodecahedral and 
other crystals, vulgarly known by the names of 
Gibraltar diamonds, Bristol diamonds, &c. ; va¬ 
rieties of colour according to which the sub¬ 
stance obtains the familiar denominations of 
smoky topaz or morion, cairngorm, citrine, &c.; 
specimens of rock crystal, enclosing various sub¬ 
stances, such as rutile, brown iron stone, mica¬ 
ceous iron, needle antimony, actinote, asbest, 
chlorite, &c.; groups of rock crystal;— amethist 
quartz of various tints, in grouped crystals, &c. 
Case 20. Common quartz :—among the speci¬ 
mens of this widely diffused substance, which 
offers such great variety in its external aspect, 
the more remarkable are those of hacked, cor¬ 
roded, and cellular quartz from Schemnitz, as 
also the pseudomorphous or supposititious crys¬ 
tals, 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 \—jibrous 
quartzflexible sandstone from the Brazil; 
—ironflint . In this case are also deposited se¬ 
veral 
