261 PHYSIOGEAPIIY. CI,ASS n. 



the genus Feld-spar are used in the arts and manufactures. 

 The purest opalescent varieties of Adularia are cut round 

 and polished, and worn as ring-stones, &c. The finest of 

 them are from Ceylon, and are called Moonstones. The 

 Sunstone is likewise prismatic Feld-spar, having exceedingly 

 minute scales of mica dispersed throughout its substance, 

 which are disposed parallel to the face of Pr + co,and reflect a 

 pinchbeck-brown tint. Labradorite, particularly the Ameri- 

 can variety, and Amazone-stone, are cut for various orna- 

 mental purposes, into snuff-boxes, &c. ; this is also the 

 case with Graphic Granite, which consists of a simple varie- 

 ty of common felspar, regularly mixed with long parallel 

 cr} r stals of rhombohedral Quartz, whose transverse angular 

 sections bear some resemblance to certain letters. The 

 pure varieties of prismatic Feld-spar are used in the com- 

 position of the paste of porcelain, also for the enamel with 

 which it is covered ; and the decomposed variet}^ or Porce- 

 lain earth itself is the most important material in that de- 

 partment of manufactures. 



3. PYEAMIDAL FELD-SPAR. 



Pyramidal Felspar or Scapolite. Prismato-pyramidal Fel- 

 spar or Meionite. JAM. Syst. Vol. II. p. 35. 43. Py- 

 ramidal Felspar. Man. p. 162. Dipyre. Scapolite. 

 Meionite. PHILL. p. 45. 137. 143. Schmelzstein. 

 Skapolith. Mejonit. WERN. Hoffm. H. B. II. 1- S. 

 270. 345. 3d. Tetraklasit. Meionit. HAUSM. II. S. 

 511.550. Wernerit. Mejonit. LEONH. S. 479. 481. 

 Meionite. Wernerite. Dipyre. Scapolite. HA " Y. Traite, 

 T. II. p. 580. T. III. p. 119. 242. T. IV. p. 393. 

 Wernerite. Paranthine. Dipyre. Meionite. Tabl. comp. 

 p. 34. 45. 55. Traite', 2de Ed. T. II. p. 582. 58G. 596. 

 T. III. p. 75. 



Fundamental form. Isosceles four-sided pyramid. 

 P = 136 r, 63 48'. Vol. I. Fig. 8. R. G. 



a = V 0-3874. 

 Simple forms. P 1 (*) ; P (/) ; P + o> () ; 



[P + oo 



