170 



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 ordi- 

 nary level, and presses the coal, or squeezes it much thinner, espe- 

 cially 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. Some- 

 times 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 ac- 

 cidents 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 th 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 03^ 

 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 



