These are open tracts upon which the blue ground is spread out and left exposed to sun and rain until it crumbles and disintegrates, the process being hastened by harrowing with steam ploughs; this may require a period of three or six months, or even a year.
The coal is all in the form of brown lignite and is not very valuable as a fuel, as it soon crumbles into a fine powder on being exposed to air.
Sometimes it is almost pasty, and crumbles to powder when dried, so as to be susceptible of use as a pigment, forming the colour known as Cologne earth, which resembles umber or sepia.
The material of the bank being loosened by blasting and the cutting action of the water, crumbles into holes, and the superincumbent mass, often with large trees and stones, falls into the lower ground.
A lump of clay, which if dried would become hard and intractable, crumbles into pieces when dried after adding to it 2% of lime.