The miners who went from the Pennines to the mining areas around Dubuque in
the late 1820s, 30s and 40s would be well acquainted with the idea of a
'bearing bed' or horizon within a bed of rock where ore was most likely to
be found. An impermiable bed, like shale, overlying a limestone or
sandstone and acting as a cap rock was a common concept. I doubt that they
would have thought of this knowledge as "a familiarity with geological
principles", rather as practical mining.
You do not say, but presumably the transition between the Maquoketa Shale
and the Galena Dolomite is a horizontal (roughly) feature rather than a
vertical one. Are the ore deposits in veins which outcrop? In which case
one would expect the pits to be more or less along the expected line
(random) of the vein. Or, do they spread horizontally along the transition?
In which case the pits would be rather more randomly scattered.
Mike Gill
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