FIRST PERIOD OF THE ANCIENT WORLD. 9 
a, Roof of Coal; Argil, with sand. 
6, Pot, or Caldron-bottom. 
c, Bituminous Coal. 
d, Pavement of Coal; Vire-clay. 
In working the bed of coal, the miner generally knows that he is ap- 
proaching one of these, by the coal becoming twisted, and more difficult to 
work ; and this continues till the trouble in the roof is passed. The general 
form is that represented in the figure, when, of course, the mouth of the pot is 
inverted. Its sides are generally lined with coal, from one-eighth of an inch 
to an inch in thickness, and the pot or cavity is filled up with stone of the ar- 
gillaceous kind, or fire-clay, having generally less mixture of sand than the 
roof-stone around. The under surface of the stone which fills the pot, is ir- 
regular and waving, not smooth, like the roof adjoming. Although the 
coal which lines the pot is connected with the main bed of coal, it is of a 
texture altogether different, having a bright appearance like jet, and breaks 
into very minute cubical pieces. Sometimes is has no bitumen in it, and is 
of the nature of glance-coal. The sides of the pot are generally as smooth 
as glass, with small furrows or grooves in a vertical direction, so that there is 
very little tenacity between the sides of the pot and the stone which fills it 
up. This circumstance renders these troubles very dangerous, particularly 
when they are of a large size, as they fall without giving any warning. The 
peculiar singularity attending this trouble, is the twisted texture and altera- 
tion which are found in the bed of coal immediately under and adjoining it, 
without any mixture of the stone in it which fills up the pot. There is 
sometimes no lining of coal; and it generally happens that a piece of the 
B 
