162 On the Flewibility of 
coal-miners. When they make greater hol- 
lows than usual, the stratum immediately 
above the coal, gives way in a little time, in 
consequence of its being either pressed on 
by the incumbent matter, or, not being able, 
if only a thin stratum, to support its own 
weight. In both cases it bends downwards 
into the hollow, till it either separates at in- 
tervals into small layers, or falls all at once 
to the pavement. A stratum of sand-stone 
several yards thick, but divided into separate 
layers, will bend six feet in a hollow space, 
whose sides are ten by nine yards. This rate 
of bending, or degree of flexibility, may be 
reckoned more than that of the sand-stone 
strata in general, and more than that of the 
coaly strata; but it is less than that of the 
slate clay strata. The ratio of the flexibility 
of all strata however, is modified very much 
by the existence or want of the seams of up- 
right distinct concretions : for if these seams 
are numerous, as soon as a stratum is bent a 
little, it falls to pieces. We have not an op- 
portunity of determining if any strata or rocks 
but what belong ‘to the coal formation, are 
flexible on the great scale, but I have seen 
some varieties of limestone, and primitive 
slate, which bend considerably, when in the 
SS 
