BELLE ISLE TORSION-BALANCE SURVEY 13.49 
quantitative problem of the determination of the dimensions of the 
upper part of a shallow salt dome. The quantitative predictions were 
in error by 27-40 per cent, but, qualitatively, the work undertaken was 
accomplished completely by the balance. In other surveys with the 
torsion balance, equally good and better results have been obtained. Ina 
survey to determine whether or not at Bryan Heights cap rock might 
be present on the flanks, the predicted depths proved to be in error by 
+200-300 feet at depths of 1,500 feet; however, in our report, we had 
estimated the probable accuracy of our predictions at approximately 40 
per cent, but, rather to our surprise, the subsequent drilling proved the 
accuracy of the predictions to be nearly 60 per cent and cap-rock masses 
which we somewhat hesitatingly predicted proved actually to be present. 
In the Hoskins Mound survey, the errors in the predicted depth at four 
points were proved to be respectively 7, 5, 2, and 0.7 per cent (9). 
The accuracy of quantitative calculations of this type decreases 
with the depth to the top of the anomalies, provided that the density 
situation remains constant. Geologtcally, the density may change with 
depth, and through the complication of the density situation, the accu- 
racy of quantitative calculations through a certain vertical zone may 
increase with depth. The cap rock at all depths in the Gulf Coast is 
heavier than the surrounding sediments, but, compared with the salt, 
the sediments are slightly lighter or of almost the same density at the 
surface and become progressively denser with depth and at great depth 
are heavier than the salt. At certain depths, therefore, there is a neutral 
zone in which the salt can not be detected by gravitational measurements. 
Below those depths the presence of the salt can be detected by gravita- 
tional measurements and the accuracy of quantitative calculations in 
regard to dimensions of the salt through a certain vertical zone increases 
with depth, although on account of the law of decrease of accuracy with 
depth, the absolute accuracy may be very low. Practically, the dimen- 
sions of the cap rock can be calculated with fair accuracy for depths less 
than 3,000 feet and for any domes in the Gulf Coast area; but, on account 
of the enormously greater thickness of light sediments near the coast 
and their low specific gravity, predictions in regard to the dimensions of 
the salt of the domes near the coast are nearly valueless. Surveys across 
the inland domes of the coastal group suggest that calculated predictions 
in regard to the salt may be of some use. 
REFERENCES 
1. A.F. Lucas, “Louisiana Salt Resources,” Amer. Manuf., Vol. 63 (1898), 
Pp. QI0-11. 
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