72 A SYSTEM OF 



SECT. LXVI. 



H. Rhombic Quartz, Spatum fcintillans Felt- 

 fpatum *. 



This has its name from its figure f , but 

 feems to be of the fame fubftance as the 

 jafper. I have not however ranked them 

 together, for want of true marks to diftin- 

 guifh the different forts of the flinty tribe 

 from one another. 



This kind is found, 



1. Sparry. 



a. White. 



b. Reddifh brown, occurs in the Swediih 

 and alfo in the foreign granites. 



c . Pale yellow. 



d. Green ifh. 



This lafl mentioned refembles very 

 much the fchorl or cockle-fpar, (Seel:. 

 Ixxiii.) but is neither fo eafy to melt in 

 the fire, nor of fo exact a figure. 



2. Criftallifed. 



with them in form of an earth, which perhaps we do not 

 know in that (late. It may be afked, Whether this earth will 

 be du&ile as clay, or rough to the touch as powder of bricks ? 

 Perhaps Tripoly is produced in this manner. 



* The Germans and other nations call this fubftance 

 Feid-fpat, that is feild or vague fpar ; a name very inadequate 

 to its nature, as it touches no wife on the fparry clais. I 

 have therefore, in my Lectures, given it the Engl ifh name of 

 Khombic Quartz, a name very fignificant, as the name quartz 

 exprefTes its clafs ; and the quality of it, of always breaking 

 into angular (rhombic) fragments, is alfo expreffed by the 

 adjective of rhombic. D. C. 



.. f What Cronftcdt means, that the feid-fpat has its name 

 from its figure, is unintelligible* D. C. 



A. In 



