580 DOMAIN VI. CARBONACEOUS. 



served, that where the coal approaches the slip, 

 it has lost its bitumen, whence it is argued, that 

 the slip rose heated from beneath ; while others 

 only infer, that the bitumen has been absorbed 

 by the humid rock. In confirmation of the for- 

 mer opinion, it is added, that in the north of 

 Ireland the layers of flint become red and light 

 when they approach the whindyke; and speci- 

 mens which I have seen, certainly bore every 

 mark of having been affected by great heat, 

 iridescent. The iridescence of coal often penetrates a large 

 mass, and appears in almost every direction. In 

 the peacock coal of Wales or Somersetshire, this 

 iridescence often assumes a strong resemblance 

 of what are called the eyes in a peacock's tail*. 

 In that of the valley of Llangolen, the iridescence 

 consists of steel-purple, crimson, green, yellow^ 

 and blue, disposed in zones. But by far the 

 most beautiful of this kind, is a coal found in 

 small portions, near Valenciennes, in which 

 crimson, green, blue, and yellow, perfectly opa- 

 lise or interchange; so that the substance has 

 more splendour than even the noble opal. The 

 exquisite vivacity of the tints can only be equal- 

 led by some of the celebrated iron ores of Elba; 



* Mr. Parkinson, Org. Rem. vol. i., informs us that peacock 

 coal is found in Somersetshire at a considerable depth, the surface 

 being mingled with fossil shells, and vestiges of fern. 



