THE GEOLOGIST. 
with safety. The hoops effectually prevent the coal from falling off ; 
but it has an odd appearance, like a black crinohue 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 hes on a slope, the first gallery is driven horizontally 
along it at the lowest level, a, a. This horizontal gallery, w^hich 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 upicards, (the coal is brought 
much easier down than up) from a, a, to A and B in om^ 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 
theii' working, or of such poi'tions as they choxDse 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 
