Bulletin 54. Geophysical Methods of Exploration and their ...

GEOPHYSICAD INVESTIGATIONS 49
arc in the aggregate very large, thus indicating a considerable
reserve of ore which has not yet been touched."
Although mining companies have removed more than 27,000,000
long tons of ore from New Jersey's developed iron ore deposits,
and in 1940 alone produced 1,292,794 long to_s, the prospective
ore-deposits outlined by Edison's sm'veys have remained untouched
and arc still awaiti_g development. Perhaps this will
be accomplished by iron-mining companies on their own initiative,
but if the State of New Jersey wishes to insure the continuation
of its mining industries, it would seem to be a wiser policy
not to leave this development to chance, but to pob_t the way to
assured reserves of ore by a rcsurvey of the entire iron ore belt
with modern geophysical equipment to be followed by enough
core-drilling of the best prospects to show indisputably that these
are large etmugh and the ore of good enough quality to interest
any company in need o£ ore.
:[_ this same area lie the Franklin Furnace zinc deposits, but
as far as the writer is aware no geophysical measurements have
been made there. However, the amouut of franklinite present in
the ore would indicate that ore-bodies could be found with the
magnetic method providing there were no other complicating
factors, since frankli_fite has a very high magnetic susceptibility.
In com_cctiol_ with absolute magnetic measurements over the
United States the U. S. Coast and Geodetic Survey has established
several magnetic stations in the Hightands_ and although these
serve to show the regional variatio_s in the earth's magnetic
field, they show little relation to the local geology.
Gravitational measuremcnts.--A few gravitational statiorls have
also been estahtished in the Highlands by the U. S. Coast and
Geodetic Sm'vey as part of their general network covering the
country, but they have not carried out a detailed gravitational
survey. The region lies on the flank of a marked gravit_ltional
trough which extends into Pennsylwmia and New York, and any
geologic interpretation of gravitational work in this ares would
have to depend upon the residuals left after removing the effect
of this regional variation. (Nor the areal gravitational relations
i_ New Jersey see Fig. 17).
Electrical measuremc_ts.--Prohably the fi_'st attempt at geephysical
prospecting by electrical methods i_ New Jersey was
an investigation of five dam sites ill the Highlands area conducted
by the Schlumberger Electrical Prospecting Methods
in the winter of 1929-1930. 0£ the sites examined three were on
NEW JERSEY GEOLOGICAL SURVEY