promising for the discovery of iron ore. For instance, on the east end
of the Mesabi and on the east and west ends of the Gogebic district,
magnetic surveys trace the iron formation with great ease to points
where the attraction is low and the conditions for exploration more
favorable.

The magnetic needle has also been used in the search for nickel ore in
the Sudbury district of Ontario, but without great success, because of
the variety of rocks other than nickel which are more or less magnetic,
and because of the slight magnetic properties of the nickel ore itself.
In a large-scale exploration of this type, conducted some years ago, a
favorable magnetic belt was discovered, and a pit was sunk to water
level but not to bedrock. Years later, the extension of this pit by only
a few feet disclosed one of the great ore bodies of the district.

Experimental work on the use of the magnetic needle on copper deposits
has yielded some interesting and suggestive results, but this
investigation is still under way and the results have not been
published.

THE USE OF ELECTRICAL CONDUCTIVITY AND OTHER QUALITIES OF ROCKS IN
EXPLORATION

In addition to magnetism, rocks and ores have other properties
susceptible to observations made at a distance, such as electrical
conductivity, transparency to X-rays, specific induction, elasticity,
and density. All these qualities have been of interest to geologists in
some connection or another, but none of them have yet been used
effectively in exploration for mineral resources. The only one of these
properties that has thus far seemed to promise practical results is
electrical conductivity. The results yet obtained are slight, and this
kind of investigation has rested under something of a cloud, due to
extravagant claims of inventors. Nevertheless, there has been a
considerable amount of scientific work by physicists, geologists, and
engineers, supplemented by special war-time investigations of rock and
earth conductivity in connection with ground telephones and the tapping
of enemy conversations, which seems to indicate a distinct possibility
of practical results in the future,--perhaps not so much in locating
specific ore bodies as in locating general types of formation and
structures,--which may serve to supplement other methods of search.[38]

The transmission and reflection of sound waves in rocks have also been
more or less investigated with reference to their possible military use.