therly direction along the
known lines of the glacial movement. It was found that these lines
converged at a point near Hudson's Bay. The data were too meager and the
base line too short for this long projection, and the indicated source
of the diamonds can be regarded as the merest speculation. However, with
the finding of additional diamonds in the drift, as seems very likely,
the refinement of this method might conceivably bring results in time.

THE USE OF MAGNETIC SURVEYS IN TRACING MINERAL LEDGES

Magnetic surveys are often useful in tracing iron-bearing rocks beneath
the surface, in the discovery of outcrops of such rocks, and in working
out their lines of connection. This method is in general use for the
crystalline iron ores in the Lake Superior region, Canada, the
Adirondacks, and elsewhere in the glaciated portions of the United
States. It is not so useful for the brown ores and the Clinton ores of
the southeastern United States, which are only slightly magnetic and can
be commonly located by other methods.

Where the ore is strongly magnetic, and is associated with other rocks
which are non-magnetic, the nature of the magnetic field determined by a
surface survey with vertical and horizontal needles may tell something
about the shape and size of the ore body. Commonly, however, magnetic
ores are associated with leaner magnetic rocks,--with the result that
the magnetic survey, unless it happens to lead to an outcrop of ore,
indicates only the general area through which underground exploration
might be warranted. In the hematitic iron ores of Lake Superior,
magnetism is less pronounced than in the magnetites; and in the soft
hydrous hematites, like those of the Mesabi district, it may cause only
slight disturbance of the magnetic needle. This disturbance is usually
sufficient to locate the position of the iron-bearing formation, though
not the position of the ore.

Where the iron formation has been highly metamorphosed, and rendered
resistant to weathering and erosion so that it will not concentrate into
ore, it is likely to have higher magnetic attraction than the richer
ores. For this reason an area of strong magnetic attraction is
ordinarily regarded as not particularly favorable to the finding of
important hematite deposits. However, this attraction may be very useful
in tracing out the formation to a place where it is less metamorphic,
less resistant to erosion, less likely to outcrop, and yet more