86 Elements of Mineralogy. 



calx of iron, from whence its various colours 

 are derived, and a proportionably imaller 

 quantity of the other ingredients. 



Argillaceous FiJ/ile Stones. 



Thefe and many other of different genera, 

 have been comprehended under the denomi- 

 nation of Schijli, but to avoid ambiguity, I 

 think it expedient to confine this name to 

 flones of the argillaceous genus. 



SPECIES VI. 

 Roof Slate, Shijlus Tegularis. 



Of this fpecies there are many varieties, 

 none of which have been hitherto analyfed, 

 except the bluifh purple flate chiefly ufed here, 

 which I have lately examined, and to which, 

 as to a ftandard, I {hall refer the other forts. 



I, VARIETY. 



Argill Intimately mixed 'with 1,77 of its 

 weight of Siliceous Earth, 0,3 of its 'weight 

 of Magnefia-) 0,15 of Calcareous Earth, both 

 Jlightly aerated, and nearly 0,54 of its 

 weight of Iron, befides ajlight admixture of 

 Mineral Oil, 



Bluiflo Purple Slate. 



It does not ftrike fire with fteel, and may 

 be {lightly fcraped with the nail ; it is very 



brittle, 



