246 LUPTON : GOLD, SLATE, AND SALT MINES IN GREAT BRITAIN. 
depth of the hole. The drill-hole is about one inch diameter, and is 
nearly filled with black gunpowder, in appearance something like 
sporting gunpowder ; a very little tamping is lightly rammed on the 
top. The effect of filling the hole with x^owder is to produce a 
simultaneous pressure from top to bottom, and the effect of gentle 
tamping is to diminish the violence of the explosion. Hard tamping 
would cause a violent explosion that would destroy the slate. 
In some mines a lower bed of slate is worked under the " Old 
vein." This is separated by a layer of silicious rock, called chert, 
about two yards thick ; this chert is the fioor of the " old" vein and 
the roof of the " new" vein ; the vein is about 26 yards thick, and 
the pillars are left in it exactly below the pillars in the old vein. 
It has happened in at least one large mine that the pillars have 
been left too small, with the result that they have suddenly fallen 
and all the ground above, letting down millions of tons of rock, and 
doing great injury to the mine. This incident of course has many 
parallels in the annals of coal-mining. Up to the present time slate 
mining has been of far more importance to Wales than gold-mining ; 
the annual value of the slates produced at the mines and quarries 
of Wales exceeds one million sterling. 
When the slate is sent to the surface it is cut and split into 
lumps of convenient sise, and then it is put on a saw-bench and cut 
by a circular saw so as to give it a smooth end, when a chisel can be 
introduced to split it ; the splitting is all done by hand with a chisel 
and hammer ; the slates when split are cut by a revolving knife to 
the correct size and shape. The slates must be split as soon as they 
reach the bank, because if allowed time to dry they are not easily 
split. One of the most remarkable conditions of slate-mining is the 
immense proportion of waste material ; it is common , to find that out 
100 tons of slate rock only five to ten tons are actually made into 
slates, the rest is all tipped into the waste heap. This is the reason 
of the costly nature of slate-mining, because the cost of getting one 
ton has to be multiplied by twelve in order to find the cost of the 
slates sent to market ; the mine or quarry has to be twelve times as 
large as the space required for the finished slates; and enormous waste 
heaps must be formed, and locomotives provided, to carry the waste 
