*4 Slate. [Book VI. 



a red colour, more or lefs deep, in the fire, and melts 

 pretty eafily into a greenifa glafs. It ccmfifls of a 

 mixture of pure clay, filiceous and martial (or iron) 

 earths, containing alfo a fmall quantity of vitriolic acid. 

 The clay is alfo found mixed with calcareous earth, 

 when it conftitutes marie. It is alfo found in an in- 

 durated ftate, either pure or mixed with inflammable 

 fubftances, and vitriolic acid, in which cafe it forms the 

 ores of alum. 



VIII. Argillaceous foffile ftones, to which the de- 

 nomination of SCHISTI properly belongs. The mod 

 remarkable ftones comprehended under this divifion 

 are the blueifh purple fchiftus, or common roof flate ; 

 the dark blue fchiftus, or writing flate j the pyritaceous 

 fchiftus i the bituminous fchiftus ; the argillaceous grit, 

 which is alfo called fand-ftone, and free-done; the 

 killas ; the Bag-ftone, and the toad-ftone. 



The method of fplitting the flate, ufed for the 

 roofing of houfes, is the following : they expofe it in 

 blocks to be well wetted and foaked by the rain ; 

 when the froft comes upon it afterwards, it rarities the 

 water, and cracks and opens all the joints of the ftone, 

 'fo that when the froft has completed its work it lies 

 in loofe flakes or Olivers. Thus the elements perform 

 with eafe what no manual art could have eafily acconv* 

 plifhed* 



