Argillaceous Genus. 77 



Brick Clay. 



Its colour is various, reddim, Wuifh, or 

 yellowifh : it always contains iron, and melts 

 into a flag. The beft fort contains little or 

 none of calcareous Earth, but a good deal of 

 a coarfe filiceous fand. 



Coloured Clays. 



Yellow^ red) and brown clays contain moft 

 iron, fometimes difperfed through them, and 

 fometimes united to the iiiiceous part : in this 

 cafe they are more difficultly fufible. The 

 yellow calx of iron is more dephlogifticated 

 than the red, and the red more fo than the 

 brown. When thefe clays contain about 14 

 or 15 per cent, of iron, they become mag- 

 netic after calcination. 



Red chalk, Rubricafabrilis, is,~according to 

 Mr. Rinman, either a clay intermixed with 

 the red calx of iron, which hardens in fire, 

 and then becomes magnetic and browner, and 

 in a ftronger heat melts into a black glafs, and 

 contains from 1 6 to 1 8 per cent, of iron, (but 

 fometimes it contains but 7 or 8 per cent, of 

 iron, and then does not become magnetic by 

 roafting) or it is an impure fteatites, mixed 

 with clay and calx of iron. Hijloria Ferri, 

 189. 



Blue 



