Page 290

NA TURE
[Feb. 8, i872
is twenty-five yards thick. Although brine springs had
been known and worked as early as the time of the
Norman Conquest or earlier, yet the bed of rock salt was
only discovered in i670 when searching for coal at Mar-
bury, about a mile to the north of Northwich. During
the last 200 years this rock salt has been worked, or to
speak more correctly, for more than a century the upper
bed was worked, when an agent of the Duke of Bridge-
water sank lower still, and, after passing through about
ten yards of hard clay and stone, with small veins of rock
salt running through it, the lower bed of rock salt was
discovered. This lower bed is between thirty and forty
yards thick, but only about five yards of-the purest of it
is "got." This good portion lies at a depth of from ioo
to i i0 yards, according to the locality. In the neighbour-
hood of Winsford both beds are met with at a much
greater depth. The whole of the rock salt obtained is got
now from the lower bed, and last year it reached nearly
I50,000 tons, probably the largest quantity ever obtained
in one year. It may as well be said that this mining of
rock salt has had nothing whatever to do with the subsi-
dences spoken of, though the wording of the note would
lead readers to expect the contrary. At present there is
no danger to be expected from the lower bed of rock salt.
The whole danger arises from the upper bed, as will be
seen from the following account :-The salt trade of
Cheshire is a very extensive one, and during the year i871
upwards of 1,250,000 tons of white salt have been sent
from the various works in that county. The whole of this
immense quantity has been manufactured from a natural
brine which is found in and around Northwich and Wins-
ford, as well as in several other smaller places. This brine
is produced by fresh water finding its way to the surface
of the upper bed of rock salt, technically called the Rock
Head.   The fresh water dissolves the rock salt, and
becomes saturated with salt. The ordinary proportion of
pure salt in the brine is 25 per cent. To obtain the quan-
tity of salt above mentioned, it would be necessary to
pump 5,ooo,ooo tons of brine. The pumping of brine is
incessantly going on, and as a natural consequence the
bed of rock salt is being gradually dissolved and pumped
up. As the surface of the salt is eaten away, the land
above it subsides. This subsidence is not spread over
the whole surface, but seems to follow depressions
in it, thus forming underground valleys with streams
of brine running to the great centres of pumping.
Wherever a stream of brine runs, there the subsidence
occurs, and in' many localities the sinking is very rapid
and serious, but fortunately is almost always gradual and
continuous. An immense lake, more than half a mile in
length, and nearly as much in breadth, has been formed
along the course of a small brook that ran into the river
Weaver, and this lake is extending continually. Besides
this gradual continuous sinking, which affects the town of
Northwich very seriously, causing the removal and re-
building of houses or the raising of them by screw-jacks
in the American fashion, the raising of the streets and so
on, there is a sudden sinking of large patches of ground,
leaving large deep cavities such as described in your
Note. These latter are more terrifying and dangerous.
They are in the majority of cases caused by the falling-in
of old disused mines in the upper bed of rock salt. These
old mines were worked so as to leave but a thin crust of
rock salt between the superincumbent layers of earth and
the mines. The roof of the mine is supported by pillars
of rock salt at intervals. Of course the weakest and most
dangerous point is the old filled-up shaft. As most of
these mines have been disused for nearly a century, the
position of the old shafts is unknown. When the brine
has eaten away the layer of rock salt left as a roof, the
whole of the earth lying above falls into the mine, and
an enormous crater-like hole, some loo feet or more in
depth, is formed, which in process of time becomes filled
up with water, the mine itself being choked with earthy
matter. In the immediate neighbourhood of Northwich
there are a great number of these rock pit holes, as they
are called, and it is nothing very unusual for one to fall
in.
  The rock miners, as they are called, were at work in the
lower mine last year when one of these sudden subsidences
occurred. They knew nothing of it. I have been myself
under this hole, and it was a fearful one to look at when
it first went in. There is no communication between the
upper and lower beds, and the miners have about thirty
yards of hard clayey stone and rock salt between them
and the upper old mines. The subsidence more particu-
larly alluded to in your Notes is not in the immediate
neighbourhood of Northwich, but rather midway between
Northwich and Winsford, near Marton Hall. It is rather
difficult to know what is its cause, as there is no record of
any mines ever being worked in that neighbourhood. The
general belief is that the rock salt, which undoubtedly
underlies the whole neighbourhood, has been gradually
dissolved, and that a sinking has commenced as at
Northwich; then that, owing to some peculiarity of the
particular overlying strata-probably to their sandy nature,
as quicksands are known to exist about Northwich-the
earthy and sandy matter of the immediately overlying
strata has been carried away by the brine streams till a
large hollow has been formed. This has continued till
the superincumbent mass could not be borne up any
longer, and thus suddenly fell in, filling up the lower cavity,
but opening a large crater-like pit from the surface.
  A Government inspector has been to the neighbour-
hood, and his report is expected very shortly.
  The whole neighbourhood of Northwich is well worthy
of more attention than it has received, and it is sur-
prising that our geologists have not been able to give a
better account of the rock salt formation than has yet
been done.
                                        THOS. WARD
                       NOTES
  WE are glad to be able to state that the severe sentence passed
upon M. E. Reclus has been changed, in consequence of the re-
presentations of the scientific men of this and other countries,
into the comparatively mild one of exile from France.
  WE understand that the Chair of Anatomy in the new German
University of Strasburg has been offered to, and declined by,
Prof Gegenbaur, who has done so much to raise the scientific
reputation of the University of Jena. A similar offer has also
been made to Gegenbaur's distinguished colleague, Haeckel, the
result of which is not yet announced.
  THE Master and Senior Fellows of St. John's College, Cam-
bridge, have elected Mr. J. B. Bradbury, M.D., of Downing
College, Linacre Lecturer in Medicine in the room of Dr.
Paget, who has been elected Regius Professor of Physic.
  THE Royal Commission on Scientific Instruction and the Ad-
vancement of Science recommenced their sittings yesterday.
  THE two Smith's Prizes of the University of Cambridge have
been this year awarded to the First and Second Wranglers re-
spectively.
  WE regret to learn that the Australian Eclipse Expedition has
proved a failure, through the unfavourable state of the weather at
the point of observation.
  IT is with great regret we have to record the death on Wed-
nesday, January 3i, at Torquay, of Dr. G. E. Day, F.R.S., late
Chandos Professor of Medicine in the University of St. Andrew,
at the age of 56. Our columns have borne frequent evidence of
the extent of Dr. Day's acquirements in many branches of