HOOFING SLATE, 77 
CLAY SLATE FAMILY. 
120. CLAY SLATE, or ROOFING SLATE, is a kind 
(if stone of foliated texture, and greyish, black, brown, green, 
or bluish colour. 
. -It breaks into splinters, does not adhere to the tongue, yields 
generally a clear sound when struck, and is nearly thrice an 
-heavy as water. 
Vast and extensive beds of slate occur in different 
parts of the world ; and this mineral sometimes consti- 
tutes even a principal portion of mountains. In our 
.own country there are many important quarries of it, 
.particularly in Westmoreland, Yorkshire, Wales, and 
.Derbyshire. 
The uses of slate are numerous and important ; but 
its principal use is for the roofing of houses. For this 
purpose it is split into thin plates or laminee. These 
are fastened to the rafters by pegs driven through 
.them; acd are made to lap over each other at the edges, 
in such manner as to exclude the rain and other 
.moisture. The kinds which are preferred for this pur- 
pose are such as have the smoothest surface, and split 
into the thinnest plates. It is requisite that slates should 
be damp when they are split, otherwise this cannot be 
done without difficulty. Hence it is .generally cus- 
tomary to split the masses as soon as possible after they 
have been separated from the rock. 
Slate should not be porous. If it be so, rain and 
snow water will pass through it, and destroy the wood- 
.work of the house on which it is placed. Porous slate 
is also liable to have moss and lichens grow upon and 
cover it. These plants retain moisture long-, and keep 
the surface, and even the interior of the slate, moist; so 
that, during the winter season, by the freezing of the 
moisture, the slate is apt to split and fall into pieces. 
To ascertain whether the slate be of requisite compact- 
ness, it should be completely dried, then weighed, and 
afterwards soaked for some time in water. When taken 
