BELLE ISLE TORSION-BALANCE SURVEY 1343 
The assumptions (A and B) of Table II gave a very much better 
agreement of the calculated with the observed gradient profiles than did 
the geologically more probable assumptions of Table I. The assumptions 
of Table II evolved more or less mathematically in the process of the 
calculations. Geological analyses of these assumptions showed that 
geologically they were possible but somewhat improbable. Assumption 
A makes the specific gravity of the sediments slightly lower than seems 
probable and necessitates a content of approximately 15 per cent of an- 
hydrite in the salt. Assumption B uses the geologically most probable 
densities for the sediments but uses a density for the salt which connotes 
an average anhydrite content of approximately 24 per cent. But the 
average anhydrite content of the salt in the near-by Weeks and Avery 
Island salt mines is approximately 1 per cent, and in the two analyses 
of Belle Isle salt which are available, the percentage of NaC/ in the one 
analysis is 92.8 and in the other, 96.4. The density used for the salt in 
Table I connotes an anhydrite content of 7 per cent. An average anhy- 
drite content of considerably more than 7 per cent in the salt theoretically 
is not impossible, but has not been encountered in the shaft or the wells 
into the shale. The assumptions of Table II, therefore, were discarded on 
account of their geologic improbability and the geologically most prob- 
able assumptions of Table I were used, although mathematically they 
did not give as good results as did those discarded assumptions. 
The known data in regard to the depth of the top of the cap and of 
the salt were utilized and before the calculations were made for profiles 
in the unexplored areas, sections were calculated through the areas in 
which the conformation of the salt and cap were best known. 
The calculated structural sections for the profiles in the sotthwest 
quadrant are shown in Figure 3. Each structural section is the final 
choice out of a long series of sections calculated for that particular pro- 
file. If the structural sections which were calculated with the discarded 
density assumptions of Table II had been retained, the predictions re- 
garding the position of the edge of the cap and depth to the top of the 
cap would have been the same, but the thickness of the cap would have 
been predicted as approximately 100 feet. 
The calculated structure contours on the top of the cap, or, in the 
absence of the cap, on the salt, are shown in Figure 4. The structure 
contours were made to conform to the known data from the drilling in 
the northeast quadrant. 
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