142 Elements of Mineralogy. 



pounded, form a fand rather than a pow- 

 der. 



I, VARIETY. 



Grit 'with a Calcareous Cement > Quadrum. 



A ftone of this fort hath been already de- 

 fcribed under the calcareous genus. When 

 it contains about 50 per cent, or more of filex 

 it belongs to this fpeciesj it commonly contains 

 alfo a fmall proportion of argill and iron ; it 

 hardens by expofure to the air, and then ftrikes 

 fire with fteel, and effervefces flightly with 

 acids : it is fometimes cryftalized in rhom- 

 boids, as at Fontainb'eau. 100 parts of this 

 contain, by the experiments of Mr. LaJJone, 

 62,5 of filex and 37,5 of mild calcareous 

 Earth. Mem. Par. 1777, p. 43. 



The Saxum margdriticum of Linneus be- 

 longs alfo to this fpecies : it confifts of quartz 

 in a calcareous cement. 



II. VARIETY. 



With an Argillaceous Cement. 



Thefe do not effervefce with acids, but they 

 ftrike fire with fteel, though fome forts are too 

 foft to do io until expofed for fome time to the 

 air. The free-ftone from Keredge^ near Mac- 

 clesfield) which is of a clofe grain, is of this 

 fpecies. Its fpecific gravity is 2,544. 



So 



