540 
LEAD. 
men, and produce to their owners a clear income of 
more than sixteen thousand pounds a year. In 
working some of these mines, we are told the 
miners often find large breaks in the rock, like 
grottos, entirely encrusted with very beautiful spar, 
which by torch-light appear as if covered with pre- 
cious stones. These openings are generally closed 
up as soon as they are perceived, that the workmen 
may not be tempted to leave their business in order 
to collect and sell the spar, by which they are en- 
abled to make a great deal of money. 
The productive veins of lead in Derbyshire run 
in layers of calcareous earth, more or less mixed 
with shells ; and sometimes penetrated to the depth 
of more than twelve hundred feet. Their situation 
is almost vertical, and their direction generally from 
the north-west to south-east. The calcareous layers 
are frequently interrupted by beds of toadstone, so 
called from being formed of white grains upon a 
black ground. The thickness of the calcareous 
layers is from one to two hundred feet, and the 
toadstone is sometimes of an equal volume ; but 
this substance is found to vary exceedingly in this 
respect. We have been thus particular in descri- 
bing the layers, as it leads to a circumstance worthy 
of remark, and which we have formerly noticed in 
our Introduction to the minerals. — It is, that the 
veins of metal, though very rich in the calcareous 
earth, immediately cease upon meeting with toad- 
stone, so that not a vestige of metal is to be found 
in that substance. 
