MINERALS. 
341 
material had happened under ground. It is pro- 
bable that the shock of the earthquake had disturbed 
some of the slickenside mineral far under ground, 
and occasioned it to part and explode.” 
The presence of minerals in the earth is said to 
be indicated by several circumstances, though we 
believe none of them are to be much depended on. 
Among others may be mentioned the rolling stones 
found in the torrents; veins without metal, but 
composed of stones coloured by metallic oxides ; 
waters holding in solution metallic salts ; but above 
all, a just knowledge of the general constitution of 
the earth. This knowledge is certainly of con- 
siderable importance to the finding of metal in a 
district where it is supposed to exist, as it runs more 
readily through some strata than others. 
In Derbyshire, which abounds in lead mines, the 
strata of earths and stones where the mineral is found 
are arranged in the following order : 
1. A layer of sandstone of a considerble thickness, 
but subject to vary. 
2. A bed of friable slate of 4 or 500 feet in thick- 
ness. 
3. A bed of calcareous earth filled with shells : 
it is of a black colour, and from 100 to 150 feet 
thick. 
4. A bed of toadstone, which vanes from 40 to 
100 feet in thickness. 
5. A second calcareous layer full of marine pro- 
ductions, and about 100 feet thick. 
