336 PHYSIOGRAPHY. CLASS n. 



beds. Uncleavable Quartz often accompanies Calcedony, 

 in the vesicular cavities of amygdaloidal rocks, and even in 

 agate balls. Thus also Menilite is found in adhesive slate. 

 Some varieties are met with in metalliferous veins, along 

 with hexahedral Lead-glance, dodecahedral Garnet-blende, 

 &c. It occurs in petrifactions in sandstone. 



5. Uncleavable Quartz is, in its different varieties, more 

 plentiful in Hungary than in any other country in the 

 world. It has for a long time been the only locality of pre- 

 cious Opal, which is found at Czerwenitza near Caschau, 

 along with common and semi-opal in a kind of porphyry. 

 Traces of it have been met with at Huberisburgin Saxony. 

 Fine varieties have been lately discovered in the Faroe 

 islands, and most beautiful ones, sometimes quite transpa- 

 rent, near Gracias a Dios in the province of Honduras, 

 America. Common opal occurs in great quantities at Tel- 

 kobanya near Eperies, and in other parts of Hungary, in 

 the Faroe isles, in Saxony, c. An apple-green variety 

 is found at Kosemutz in Silesia with Chrysoprase ; and 

 the red and yellow, bright coloured varieties of Fire-ojnil, 

 near Zimapan in Mexico. Semi-opal occurs in several of 

 the countries mentioned above ; also near Frankfort on the 

 Maine, in Austria, Moravia, Poland, Siberia, &c. In Sax- 

 ony, Bohemia, and Cornwall, it is met with in metalli- 

 ferous veins. Hyalite is found in amygdaloidal rocks, near 

 Frankfort, in irregular veins ; near Schemnitz in Hun- 

 gary, in porphyry ; also in Bohemia, and various other 

 countries. Brown Menilite occurs at Menil Montant near 

 Paris ; the grey variety has also been discovered in that 

 neighbourhood. Opal Jasper is formed wherever unclea- 

 vable Quartz happens to be mixed with oxide of iron, as at 

 Telkobanya in Hungary, near Almas and Tckoro in Tran- 

 sylvania, &c. Wood opal is frequently found at Kremnitz 

 and Telkobanya in Hungary, and in many districts of Tran- 

 sylvania, sometimes in very large trees. 



C. Precious opal is considered as a gem, and generally cut 

 with a convex surface. It is of considerable value, if large, 

 pure, and posse sing vivid colours. 



