244 LUPTON : GOLD, SLATE, AND SALT MINES IN GREAT BRITAIN. 
The gold of North Wales is found in veins traversing the slate 
rocks of the Lower Silurian system, and parallel to intrusive beds or 
veins of trap or greenstone. 
These same beds of Silurian slate are found ten miles to the 
northward, and are traversed by similar veins of greenstone, and here 
at Ffestiniog it is not the intrusive vein that is valuable but the slate 
rock itself, which is capable of being spht up into most valuable and 
perfect slates. The most valuable bed of slate, called the " Old 
Vein," lies at an angle of 23° and dips under a mountain, where it is 
mined on the Pillar and Stall system. In one of these slate 
mines there are twenty miles of underground roads. The roof is 
standing on the pillars all over, and no attempt has been made to 
remove the pillars, nor will be made for generations yet to come, if 
ever. In this respect a slate mine differs from an ordinary coal mine, 
where the pillars are worked out soon after they have been formed. 
The slate vein (it is called a vein, though it is really a stratified 
rock) is reached by levels driven into the mountain side and by an 
inclined road, up which the slate trucks are drawn from the lowest 
part of the mine by a steam-engine. Right and left of the incline 
levels are driven in the slate : the levels are about twenty yards 
apart in a vertical line. 
The vein is 46 yards thick ; the levels are driven about seven 
feet high, in the top of the vein ; the roof is marked by a parting of 
white clay, called a clay slant. The levels are connected by cross- 
heads every twenty-thi^ee yards ; these cross-heads are all in the 
same straight line from rise to dip, and make a series of parallel in- 
clines. The cross-heads are widened out from the level to a width of 
45 feet, leaving pillars 24 feet wide ; the stalls or breasts are worked 
from the level towards the rise, the clay slant forming the roof ; the 
floor of the working place is carried level till it reaches the base of 
the vein (fig. 2). As a breast is carried up from a lower level it cuts 
away the rock forming the level above, and therefore if the upper 
level is to be maintained as a road it is necessary to put a timber 
bridge over the working, or else move the level more to the " rise," 
which can easily be done by driving through the pillars. 
When beginning a breast it is easy to examine the roof, but as 
