290 



NATURE 



IFeb. 8, 1872 



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 1670 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 throngh 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 abiut five yards of the purest of it 

 is "got." This good portion lies at a depth of from 100 

 to 1 10 yards, according to the locality. In the neighbour- 

 hood of Winsford both beds are met with at a much 

 greater dep'h. The whole of the rock salt obtained i.^ got 

 now from the lower bed, and last year it reached nearly 

 150,000 tons, prob.ibly the largest cjuantity 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 1S71 

 upwards of 1,250,000 tons of white salt have been sent 

 from the various works in that countv. The whole of this 

 immense quantity has been manufactured frorn 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, teclinically called the Rock 

 Head. The fresh water dissolves the rock salt, and 

 beconifs 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 5.000,000 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 dtpressions 

 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 arc 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 too 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. Tn the imtnediate 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 cbyey stone and rock salt between lh=m 

 and the upaer 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. Itisrather 

 difficult to know what is its cause, as there is no rccoid of 

 any mines ever beins 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 s;;ndy nature, 

 as quicksands are kno^n to exist about Northwich — the 

 earthy and sandy matter of the immediately overlying 

 strata has been carried aw.uy 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 op ning a large crater-like pit from the surface. 



A Government insoector has been to the neighbour- 

 hood, an 1 his report is expe':ted very shortly. 



The whole neighbourhood of Northwich is well worthy 

 of more attention thin 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 lias done so much to raise the scientific 

 reputation of the University of Jena. A sunilar 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 31, 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 



