all Mineral Substances. 163 
act of being separated into small parts by 
wedges and other tools. Now the inference 
which may be drawn from these premises, is, 
that all kinds of mineral matter, however 
hard and brittle they appear to be in hand spe- 
cimens, will bend, less or more, when formed 
into large flat pieces. 
Very intimately connected with this subject, 
is the sinking of the strata above old coal 
works. Inthe common way of working coals, 
care is taken to get as much of the coal as is 
possible, and none is intended to be left, but 
undertowns, villages, houses, canals, harbours, 
orrivers. If about two thirds of the coal be 
taken away, the remainder cannot support the 
incumbent matter, of course it sinks, squeezes 
the pillars into small coals, and forces them 
out sideways, till all the hollows are filled, 
and the coal ground becomes so firm as to 
support its burthen. When as much coal is 
left as will nearly bear up the roof-strata, the 
sinking takes place slowly, and the coaliers 
then say, the creep has come upon the mine. 
The weight forces pieces of coal from the 
pillar corners, and other weak places, and the 
pillars themselves appear to be forced into the 
pavement ; they are then separated into smaller 
parts, which are soon crushed into small pie- 
