perience for this assumption than in theory. A study
of the shape of a great many ore-shoots in mines of fissure type
indicates that when the ore-shoots or ore-bodies are approaching
vertical exhaustion they do not end abruptly, but gradually shorten
and decrease in value, their bottom boundaries being more often
wedge-shaped than even lenticular. If this could be taken as the usual
occurrence, it would be possible (eliminating the evident exceptions
mentioned above) to state roughly that the minimum extension of an
ore-body or ore-shoot in depth below any given horizon would be
a distance represented by a radius equal to one-half its length. By
length is not meant necessarily the length of a horizontal section,
but of one at right angles to the downward axis.

On these grounds, which have been reenforced by much experience among
miners, the probabilities of extension are somewhat in proportion
to the length and width of each ore-body. For instance, in the A
mine, with an ore-shoot 1000 feet long and 10 feet wide, on its
bottom level, the minimum extension under this hypothesis would
be a wedge-shaped ore-body with its deepest point 500 feet below
the lowest level, or a minimum of say 200,000 tons. Similarly,
the B mine with five ore-bodies, each 300 hundred feet long and
10 feet wide, exposed on its lowest level, would have a minimum of
five wedges 100 feet deep at their deepest points, or say 50,000
tons. This is not proposed as a formula giving the total amount of
extension in depth, but as a sort of yardstick which has experience
behind it. This experience applies in a much less degree to deposits
originating from impregnation along lines of fissuring and not at
all to replacements.

DEVELOPMENT IN NEIGHBORING MINES.--Mines of a district are usually
found under the same geological conditions, and show somewhat the same
habits as to extension in depth or laterally, and especially similar
conduct of ore-bodies and ore-shoots. As a practical criterion, one
of the most intimate guides is the actual development in adjoining
mines. For instance, in Kalgoorlie, the Great Boulder mine is (March,
1908) working the extension of Ivanhoe lodes at points 500 feet
below the lowest level in the Ivanhoe; likewise, the Block 10 lead
mine at Broken Hill is working the Central ore-body on the Central
boundary some 350 feet below the Central workings. Such facts as
these must have a bearing on assessing the downward extension.

DEP