332 PHYSIOGRAPHY. CLASS II. 



are cut and polished into ring-stones and seals, and for vari- 

 ous other ornamental purposes. Agate is also used for the 

 same thing. The most important applications, however, 

 made of rhombohedral Quartz, are those depending upon 

 its consisting of silica. It enters into the composition of 

 glass, both white and coloured, as, for instance, smalt. It is 

 added to the mass of porcelain in the state of an impalpable 

 powder, and forms part of the paste also in other kinds of 

 pottery. It is used as a flux in the melting of several kinds 

 of ores, particularly copper ores, and in other metallurgical 

 processes. The use of flint in gun-locks is well known. Ly- 

 dian-Stone is employed for trying the contents of mixtures 

 of gold and silver. Sand-stone yields various applications 

 for architectural and other purposes, as the construction of 

 melting furnaces, mill-stones, &c. As a peculiar variety of 

 it, the flexible sandstone from Villa Hicca in Brazil de- 

 serves to be noticed, which probably owes its flexibility to 

 scales of mica dispersed throughout its mass. Sand with 

 slaked lime forms mortar. It is also used for the improve- 

 ment of roads, as in some countries also flint. 



3. CNCLEAVABLE QUARTZ. 



Indivisible Quartz (excepting subsp. 1. 6. 7 8. and 9.). 

 JAM. Syst. Vol. I. p. 283. Uncleavable Quartz. Man. 

 p. 203. * Hyalite. Muller's Glass. Opal. Hydrophane. 

 Menilite. Cacholong. Siliceous Sinter. PHILL. p. 8. 10. 

 12. 13. 16. 22. Opal. Hialith. Menilit. Opaljaspis. 

 WERK. Hoffm. H. B. II. 1. S. 131. 134. 156. 177- 

 Opal. Eisenopal. HADSM. II. S. 421. 428. Opal. 

 LEONH. S. 131. Quarz hyalin concretionne. Quarz 

 re'sinite. HAUY. Traite', T. II. p. 416. 433. Tabl. comp. 

 p. 25. 27. Traite', 2de Ed. T. II. p. 270. 



Regular forms and cleavage unknown. 



Fracture conchoidal, of various degrees of perfec- 

 tion, sometimes highly perfect. 



Lustre vitreous, in some varieties inclining to resi- 

 nous. Colour white, yellow, red, brown, green, 



