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Sometimes the separation is perfectly easy and clean, owing to the 
smoothness of surface possessed by the head of the mass, which 
appears with a brilliant polished surface, as if it had been covered 
with a varnish, or some matter which had been in a liquid state. 
Sometimes, as is observed by Mr. Williams, to whose excellent 
work, on the subject of coal, I acknowledge myself much indebted, 
the roof, as it is called by the English, and the crop, as it is called 
by the Scotch colliers, falls down for a certain space below its 
ordinary level, and presses the coal, or squeezes it much thinner, 
especially in the middle of that space. These accidents of the roof 
pressing down nearer than it should be to the pavement, as the 
bottom is called, and squeezing the coal in that part to less' than its 
ordinary thickness, exist in very different dimensions. Sometimes it 
thus presses down, and cuts off the thickness of the coal to the extent 
of several fathoms. Sometimes these depressions are not above two 
or three feet in diameter, when they are called, by the Scotch colliers, 
a bonnet-case, or pot-arse. But all these accidents are not of such 
small dimensions as the bonnet-case ; some of them are not only two 
or three feet, but two or three yards in diameter, and sometimes even 
thirty or forty yards and more. 
The stratum of coal is seldom so regular and good near the sur- 
face of the ground; whereas the seams of coal which are lower 
down, as well as the strata which accompany them, have much 
more firmness and compactness, and also manifest more regularity 
of stratification. 
Frequently the strata, instead of being continued in a regular 
horizontal direction, have their lines broken and disordered bv 
fissures and breaks. These are generally termed slips, and very 
frequently will be found to have thrown the coal a considerable 
way out of its regular course. Sometimes the strata will be found 
to have slipped down, on one side of the breach, a considerable 
number of feet ; perhaps many fathoms : and sometimes it will be 
