ommon salt. This process has been going on
from Cambrian time down through all the intervening geologic ages, and
can be observed to be actually operative today in various localities.
The beds of salt so formed are found interstratified with shales,
sandstones, and limestones, and are frequently associated with gypsum.
On a broad scale, they are always lens-shaped, though they vary greatly
in extent and thickness.

The necessary conditions for the formation of extensive salt beds
include arid climate and bodies of water which are essentially
enclosed--either as lakes, as lagoons, or as arms of the sea with
restricted outlets,--where evaporation exceeds the contributions of
fresh water from rivers, and where circulation from the sea is
insufficient to dilute the water and keep it at the same composition as
the sea water. Under such conditions the dissolved salts in the enclosed
body become concentrated, and precipitation may occur. A change of
conditions so that mud or sand is washed in or so that calcareous
materials are deposited, followed by a recurrence of salt-precipitation,
results in the interstratification of salt beds with shales, sandstones,
and limestones.

For the formation of very thick beds of salt, and especially of thick
beds of fairly pure composition, however, this simple explanation of
conditions is insufficient. The deposits of Michigan and New York occur
in beds as much as 21 feet in thickness, with a considerable number of
separate beds in a section a few hundred feet thick. Beneath the potash
salt deposits of Stassfurt, beds of common salt 300 to 500 feet in
thickness are found, and beds even thicker are known in other
localities. When we come to investigate the volume of salts deposited
from a given volume of sea water, we find it to be so small that for the
formation of 500 feet of salt over a given area, an equivalent area of
water 25,000 feet deep would be required. It has therefore been one of
the puzzling problems of geology to determine the exact physical
conditions under which deposition of these beds took place.

One of the most prominent theories, the "bar" theory, suggests that
deposition may have taken place in a bay separated from the sea by a
bar. Sea water is supposed to have been able to flow in over the bar or
through a narrow channel, so that evaporation in the bay was about
balanced by inflow of sea water. Thus the salts of a very large quantity
of sea water may have accumulat