DISINTEGRATION, &c. OF ROCKS. 139 
air, and is readily broken at its surface nto small 
fragments by implements of husbandry coming into 
collision with it. ‘These fragments are peculiar in 
being of a somewhat rounded figure, and on striking 
them, they are found to consist of continued coatings 
like an onion ; these concave pieces gradually 
moulder down into soil, and the remaining nuclei 
or centres not broken down into the same hollow 
fragments remain in the ground, and by atmosphe- 
ric influences as well as by the blows received in 
the operations of husbandry soon acquire a very 
rounded figure, and may excite surprise and specu- 
lations of various kinds in the minds of persons 
who have not proved the great susceptibility of this 
rock to decomposition and rounding. ‘The soil it 
constitutes must be of very indifferent quality. 
The schistose rocks are especially liable to disin- 
tegration, decomposition, and fracture, or loosening 
of parts at their surfaces. Compact slate or grey 
dunstone, loosens most readily at its surface on 
every fresh exposure, or disturbance by the plough 
or pick, and falls into dust, or small pieces in pro- 
portion as it is moved about and exposed to the 
action of the elements, forming eventually a good soil 
for general purposes, called “ Dun-soil” or “ Dun- 
land.” Several varieties of slate also are prone to 
dissolution and softening; some of it, where exposed 
to continued damp, gets as soft as cheese, and if 
decayed wood be at hand, the rock moreover acquires 
a black colour by infiltration of carbonaceous par- 
ticles. A great deal at the surface becomes light 
brown and hoary, through removal of original colour 
by exposure to the sun and air, and there is one 
coincidence with this worthy of note, that as fossils 
are ordinarily found towards the exterior of our rocks, 
or the surfaces of their beds, so they are not uncom- 
monly seen in the substance of decomposing or 
decomposed slate. The generality of slaterock suffers 
S$ 2 
