64 



THE GEOLOGIST. 



with safety. The hoops effectually prevent the coal from falling off; 

 but it has an odd appearance, like a black crinoline petticoat. 



Suppose then the miner at the bottom of his shaft, it is not all 

 straight forward digging then. There is a structure in coal, and he 

 must take advantage of it. It is full of joints which cut it up into 

 squarish pieces ; one set being backs or cutters, and the other joints ; 

 and. the art is to drive the pickaxe and lever along these two sets 

 so as to work the coal in the easiest directions. 



This structure can be seen even in the little specimens in our own coal 

 scuttles, and is due to the pressure the coal has received since it was 

 hardened. There is nothing crystalline about it, as some have fancied. 

 It is a sort of cleavage. 



As most coal lies on a slope, the first gallery is driven horizontally 

 along it at the lowest level, a, a. This horizontal gallery, which must 



Fig. 5.— Plan of Mine-workings. 



follow the curves of the strata, if there be any, is called the "dip- 

 head level ;" from this they drive galleries upwards, (the coal is brought 

 much easier down than up) from a, a, to A and B in our plan, and cut 

 cross ones at right angles — keeping all at quite regular distances ; and 

 so proceeding forwards and sideways, in squares, to the extent of 

 their working, or of such portions as they choose to work out first. 

 The galleries made by the hewers are called " stalls" ; the pillars of 

 coal left between are called "posts" ; and the usual mode of working- 

 is to go over the whole space in this way, leaving posts large enough 



