s probable that it is especially
affected by the angle at which stratification or lamination planes
are inclined to the direction of the hole. A hole has been known
to wander in a depth of 1,500 feet more than 500 feet from the
point intended. Various instruments have been devised for surveying
deep holes, and they should be brought into use before works are
laid out on the basis of diamond-drill results, although none of
the inventions are entirely satisfactory.

CHAPTER X.

Stoping.

METHODS OF ORE-BREAKING; UNDERHAND STOPES; OVERHAND STOPES; COMBINED
STOPE. VALUING ORE IN COURSE OF BREAKING.

There is a great deal of confusion in the application of the word
"stoping." It is used not only specifically to mean the actual
ore-breaking, but also in a general sense to indicate all the operations
of ore-breaking, support of excavations, and transportation between
levels. It is used further as a noun to designate the hole left
when the ore is taken out. Worse still, it is impossible to adhere
to miners' terms without employing it in every sense, trusting
to luck and the context to make the meaning clear.

The conditions which govern the method of stoping are in the main:--

_a_. The dip.
_b_. The width of the deposit.
_c_. The character of the walls.
_d_. The cost of materials.
_e_. The character of the ore.

Every mine, and sometimes every stope in a mine, is a problem special
to itself. Any general consideration must therefore be simply an
inquiry into the broad principles which govern the adaptability of
special methods. A logical arrangement of discussion is difficult,
if not wholly impossible, because the factors are partially
interdependent and of varying importance.

For discussion the subject may be divided into:

1. Methods of ore-breaking.
2. Methods of supporting excavation.
3. Methods of transport in stopes.

METHODS OF ORE-BREAKING.

The manner of actual ore-breaking is to drill and blast off slices
from the block of ground under attack. As rock obviously breaks
easiest when two sides are free, that is, when corners can be broken
off, the detail of management for blasts is therefore to set the holes
so as to preserve a corner for the next cut; and as a consequence
the face of the stope shapes into a series of benches (Fig.
22),--inverted benches in the case of overhand stopes (Figs. 20,
21). The size of these benches will in a large measure depend on
the depth of the hol