340 
MINERALS. 
the sides of the fissure, and these slabs are joined 
by two polished faces, which seem to be in perfect 
contact without any cohesion. The surfaces are 
coloured with lead ore, but as thin as a covering 
from a black lead pencil. If a sharp-pointed tool, 
which the workmen call a pick, is drawn over the 
vein with some force, the minerals begin to crackle, 
like sulphur excited to electricity; in a few minutes 
after which they explode with violence, and fly out 
as if they had been blasted with gunpowder, inso- 
much that the weight of forty tons have been blown 
out together. These dangerous effects deterred the 
workmen from proceeding for several years ; but at 
length it occurred to them, that this power might 
be used for the carrying on of their work with bet- 
ter advantage than by the common method of 
blasting with gunpowder. Accordingly a workman 
makes a scratch with his pick upon the joint of the 
slickensides, and runs away as fast as he can, to 
escape the explosion, which perhaps loosens as 
much of the rock as ten men would have brought 
away in three months by the ordinary methods. 
In the mines where this phienomenon occurs, the 
workmen were much alarmed on the first of No- 
vember, 1755, about ten o’clock in the morning, 
the time of the earthquake so fatal at Lisbon. The 
rocks which surrounded them were so much dis- 
turbed, that soil, &c. fell from their joints or fissures, 
and they heard violent explosions as of cannon, for 
fear of which they fled to the surface ; and, when 
all was quiet, were surprised to find that nothing 
