The Cheshire rock-salt, with very few exceptions, has hitherto been
ascertained to exist only in the valleys bordering on the river Weaver
and its tributary streams; in some places manifesting its presence by
springs impregnated with salt, and in others being known by mines
actually carried down into the substance of the salt strata. Between the
source of the Weaver and Nantwich, many brine springs make their
appearance; and occur again at several places, in proceeding down the
stream. At Moulton, a mine has been sunk into the body of rock-salt, and
a similar mine is wrought near Middlewich. At Northwich, brine springs
are very abundant; and there also many mines have been sunk for the
purpose of working out the fossil salt. In that vicinity a body of
rock-salt has been met with in searching for coal.
The brines in this district are formed by the penetration of spring or
rain waters to the upper surface of the rock-salt, in passing over which
they acquire such a degree of strength, that one hundred parts have
yielded twenty-seven of pure salt, thus nearly approaching to the
perfect saturation of brine. Their strength is therefore much greater
than that of the salt springs met with in Hungary, Germany and France.
The brine having been pumped out of the pits, is first conveyed into
large reservoirs, and afterward drawn off as it is needed, into pans
made of wrought iron. Here heat is applied in a degree determined by the
nature of the salt to be manufactured, and various additions are made to
the brine, with a view either to assist the crystallization of the salt,
or to promote the separation of the earthy particles, which exist in a
very small proportion. The importance of the manufacture of Cheshire
salt will be sufficiently obvious from the statement, that, besides the
salt made for home consumption, the annual amount of which has exceeded
sixteen thousand tuns, the average of the quantity sent yearly to
Liverpool for exportation, has not been less than one hundred and forty
thousand tuns.
The mine of rock-salt first worked was discovered by accident at
Marbury, near Northwich about a century and a half ago; and this bed had
been wrought for more than a century, when, in the same neighborhood, a
second and inferior stratum was fallen in with, separated from the
former by a bed of indurated clay. This lower stratum was ascertained to
possess a very great degree of purity, and freedom from earthy
admixture; on which account, and from the local advantages of Northwich
for exportation, the fossil salt is worked in the vicinity of that place
only. It occurs in two great strata or beds, lying nearly horizontally,
and separated, the superincumbent from the subjacent stratum, by several
layers of indurated clay, or argillaceous stone. These intervening beds
possess, in conjunction, a very uniform thickness of from thirty to
thirty-five feet, and are irregularly penetrated by veins of fossil
salt. There is every reason to believe that the beds of rock-salt at
Northwich, are perfectly distinct from any others in the salt district,
and form what are termed by mineralogists _incumbent bodies_ or _masses
of mineral_.
These enormous masses stretch a mile and a half in a longitudinal
direction from north-east to south-west; but their transverse extent, as
measured by a line at right angles from the former, does not exceed
forty-two hundred feet, somewhat more than three-quarters of a mile.
Without this area, the brine which is met with, is of a very weak and
inferior quality, and at a short distance disappears altogether. The
thickness of the upper bed varies from sixty to ninety feet; and a
general estimate made from its level, shows that its upper surface,
which is ninety feet beneath that of the earth, is at least thirty-six
feet beneath the low-water mark of the sea at Liverpool; a fact not
unimportant in determining the nature of the formation of this mineral.
The thickness of the lower bed has not hitherto been ascertained; but
the workings are usually begun at the depth of from sixty to
seventy-five feet, and are carried down for the space of fifteen or
eighteen feet, through what forms the purest portion of the bed. In one
of the mines a shaft has been sunk to a level of forty-two feet still
lower, without passing through the body of rock-salt. There is thus an
ascertained thickness of this bed of about a hundred and twenty feet,
and without any direct evidence that it may not extend to a considerably
greater depth.
Although two distinct beds, only, of fossil salt have been met with at
Northwich, it has been ascertained that the same limitations do not
exist throughout the whole of the salt district. At Lawton, near the
source of the river Wheelock, three distinct beds have been found,
separated by strata of indurated clay: one at the depth of one hundred
and twenty-six feet, four feet in thickness; a second, thirty feet
lower, twelve feet in thickness; and a third, forty-five feet further
down, which was sunk into seventy-two feet, without passing through its
substance. The intervening clay, the structure of which is very
peculiar, is called the shaggy metal, and the fresh water which passes
through its pores has the expressive appellation of Roaring Meg. This
epithet will not appear too strong, when it is mentioned that in a mine
in which the section of strata was taken, and where the “shaggy metal”
was found at the depth of about eighty feet, the quantity of water
ascertained to issue from its pores in one minute, was not less than
three hundred and sixty gallons; a circumstance which greatly enhances
the difficulty of passing a shaft down to the body of rock-salt.
In many of these beds of argillaceous stone, a portion of salt,
sufficiently strong to affect the taste, is found to exist; and this
saltness increases, as might be expected, in proportion as the body of
rock-salt is approached. In the strata or layers immediately above the
rock, which in all the mines are perfectly uniform in their appearance
and structure, this is particularly remarkable, notwithstanding there
are not, in these strata, any veins of rock-salt connected with the
great mass below. On the contrary, the line between the clay and
rock-salt is drawn with great distinctness in every instance, without
presenting any of those inequalities which would arise from a mutual
penetration of the strata. Not any marine exuviæ, or organic remains,
are found in the strata above the rock-salt; and the almost universal
occurrence of gypsum, in connection with beds of fossil salt, is a fact
still more deserving of observation, because it appears, not only in
these mines, but also in the salt mines of Hungary, Poland and
Transylvania; on which account Werner, in his geognostic system, assigns
to the rock-salt and fletz gypsum a conjunct situation.
The fossil salt extracted from the Northwich mines is of different
degrees of purity, and more or less blended with earthy and metallic
substances. The purer portion of the lower bed yields a rock-salt,
which, being principally exported to the Baltic, obtains the name of
Prussian rock. The extent of the cavity formed by the workings, varies
in different mines, the average depth being about sixteen feet. In some
of the pits, where pillars from eighteen to twenty-four feet square form
the supports of the mine, the appearance of the cavity is singularly
striking, and the brilliancy of the effect is greatly increased when the
mine is illuminated by candles fixed to the sides of the rock. The scene
thus formed almost appears to realize the magic palaces of eastern
poets. Some of the pits are worked in aisles or streets, but the choice
here is wholly arbitrary. Among the methods employed in working out the
rock-salt, the operation of blasting is applied to the separation of
large masses from the body of the rock, and these are afterward broken
down by the mechanical implements in common use. The present number of
mines is eleven or twelve, from which there are raised, on an annual
average, fifty or sixty thousand tuns of rock-salt. The greater part of
this quantity is exported to Ireland and the Baltic, the remainder being
employed in the Cheshire district, in the manufacture of white salt, by
solution and subsequent evaporation.
The general situation occupied by the rock-salt in Cheshire is very
similar to that of the Transylvanian and Polish mines, the beds of this
mineral being disposed in small plains, bounded by hills of
inconsiderable hight, forming a kind of basin or hollow, from which
there is usually only a narrow egress for the waters. The situation of
the Austrian salt mines near Saltzburg is, however, very different. The
mineral there appears to be disposed in beds of great thickness, which
occur near the summits of limestone hills, at a great elevation above
the adjoining country. This is a singular fact; and if the hypothesis be
allowed that rock-salt is formed from the waters of the sea, it is
necessary to suppose the occurrence on this spot of the most vast and
surprising changes!
Though there are no salt _mines_ in the United States, there are salt
_springs_ in several places. By far the most important and valuable of
these, are in the neighborhood of Syracuse, in the state of New York.
The land containing these springs, is owned by the state, and is leased
free of rent, to be used only for the manufacture of salt. The wells are
dug, and the water pumped up at the expense of the state, and the
manufacturer pays a duty of one cent on each bushel he makes. Some of
the wells are sunk to the depth of four hundred feet. Fine salt is
prepared by boiling; and coarse by solar evaporation. In 1850, the
number of salt manufactories in this vicinity was one hundred and
ninety-two; and the quantity of salt produced in 1853, amounted to more
than five million bushels. The salt of this region has been thoroughly
tested, and found to be fully equal to any of foreign manufacture.
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