166 ROCK 
HOCK-SALT, s. Mineral salt.—Two pieces of roclc-salt; 
one white, the other red. Woodward. 
This useful mineral forms large beds and masses in many 
parts of the world, and even composes entire mountains, 
(t occurs in large columnar or in spheroidal concretions, and 
also crystallized in cubes. Rock-salt is subdivided by 
Werner into two kinds, foliated and fibrous. The more 
common colours of foliated rock-salt are, white, grey, red¬ 
dish-brown, and red; but sometimes it is violet, sky-blue, 
and green, and is more or less transparent or pellucid: it 
breaks into cubical fragments, which have a vitreous lustre: 
the structure is indistinctly foliated. In fibrous rock-salt the 
fibres are generally small and curved; in other respects it 
differs little from the former. The taste of both is like that 
Of common salt. The red varieties are coloured by earthy 
matter and oxyd of iron; the white and transparent are 
extremely pure, being composed almost entirely of muriatic 
acid and soda, or, according to Davy, of chlorine and 
sodium. In the purest kind also, there is scarcely any trace 
of water of crystallization. According to Henry, pure 
transparent rock-salt, calcined for half an hour in a low red 
heat, equal to four or five degree; of Wedgewood’s pyrometer, 
lost absolutely nothing of its weight. It is remarkable, also, 
that if free from any adventitious moisture, it may be sud¬ 
denly and strongly heated with scarcely any of that sound 
called decrepitation, which is produced by a similar treat¬ 
ment of all the varieties of manufactured common salt. The 
specific gravity of the purest specimens of rock-salt is about 
2.170, of the less pure about 2-130. 
Rock-salt is widely distributed over the globe; it appears 
principally in the lower secondary strata. It is most fre¬ 
quently accompanied with sulphate of lime or gypsum, and 
by beds of clay impregnated with salt. Beside the beds of 
rock-salt which are known, numerous brine-springs in va¬ 
rious parts of the world attest the existence of this mineral 
deep under the surface, as it is evident these springs perco¬ 
late beds of salt, or strata impregnated with it. Several brine- 
springs have recently been discovered in the deep coal-mines 
of Northumberland and Durham. In the coal-mines near 
Ashby-de-la-Zouch, in Leicestershire, there are springs of 
brine 245 yards below the surface; and though these springs 
are in the centre of the island, they are 140 yards below the 
level of the sea. How much deeper their source may be, has 
not been ascertained. 
The most obvious hypothesis respecting the formation of 
rock-salt is, the one which supposes that it was deposited from 
the sea, or by the desiccation of salt lakes which formerly 
covered the present continents. Against this it has been 
objected, that the composition of rock-salt is much more 
pure than the contents of sea-water, which contains a 
quantity both of muriate and sulphate of magnesia, sul¬ 
phate of . soda, and of sulphate of lime. Rock-salt is 
also found at great heights above the present level of the sea. 
These objections will, in a considerable degree, be invali¬ 
dated by the consideration that whatever impurities there 
may be in sea-water, if the process of evaporation go on 
very slowly the salt will be crystallized nearly pure. Of this 
we have an instance in the species of salt made at Lyming- 
ton, in Hampshire, called salt cat, which is gradually formed 
in the course of ten or twelve days, by spontaneous evapo¬ 
ration of the liquor which drains from the common salt. 
This salt is so pure (though evaporation from the most im¬ 
pure part, the mother water, or residue of sea-water), that 
1000 parts contain only 12 of foreign impurities, or little 
more than one per cent. Thus if the desiccation of lakes 
or basins filled with salt water were very gradual, as it 
must be, except in the vicinity of subterranean fires, the mu¬ 
riate of soda or rock-salt would be crystallized before the 
other salts, which being more deliquescent might be sepa¬ 
rated and washed away. In this manner the sulphate of 
lime or gypsum, which exists in sea-water, and accompanies 
roclc-salt, may also have been deposited, and being nearly 
insoluble would remain. 
The occurence of rock-salt deep under the earth, or 
SALT. 
high above the level of the sea, can scarcely form an objec¬ 
tion to its formation from Sea-water; for it is admitted by 
all geologists, and is proved by undoubted facts, that the 
ocean once covered our present continents. Now by what¬ 
ever process the dry land was raised above the sea, whether 
by the elevation of the former, or the depression of the beds 
of the latter, extensive hollows and closed valleys must have 
formed lakes of salt water, from which the salt might be 
deposited by evaporation. Some of these valleys or hollows 
would occur in elevated situations. With respect to the 
beds of rock-salt placed under other strata, however diffi¬ 
cult it may be to explain the formation of the secondary 
strata, the existence of organic remains in them prove that 
each stratum was once the uppermost part of the globe, and 
the strata by which it is covered were deposited upon it in 
successive and probably at distant periods. Nor is the diffi¬ 
culty greater with respect to the strata covering rock-salt,, 
than the strata covering coal and beds of coal-shale abound¬ 
ing in vegetable impressions. No organic remains have in¬ 
deed been discovered in the strata over the rock-salt of 
Cheshire, but they are commonly met with at greater depths, 
over the rock-salt beds in Poland, and in other parts of 
Europe. The occurrence of rock-salt at the sides or feet 
of extensive mountainous chains, may perhaps illustrate its 
formation, as it is probable these extensive chains once 
formed the boundaries of inland seas or lakes, when the re-' 
lative level of the ocean and our continents was very dif¬ 
ferent from the present. 
Rock-salt is not mined in any part of our island, ex¬ 
cept Cheshire, though it was bored through at Droit- 
wich; and it exists, in all probability, in many of the 
western counties through which the red sand-rock extends; 
We have proofs of its existence from the brine-springs at, 
Droitwitch, in Worcestershire, at Lemington in Warwick¬ 
shire, and at Ashby Wolds, in Leicestershire; and also in 
the counties of Northumberland and Durham, on the eastern 
side of England. The springs at Droitwitch furnish a brine 
as strong as those of Cheshire. A description of the rock- 
salt of Cheshire having been given under that article, 
we shall proceed to give a short account of the most im¬ 
portant repositories of this useful mineral in other parts of 
the world. 
Salt is very abundant in Africa; all the plains and sandy 
desalts are impregnated with it, and the greater part of 
the springs in these desarts are so saline, that it is not possi¬ 
ble to drink the water. To the south of Abyssinia, at the 
feet of the mountains which separate that country from that 
of the Gallas negroes, salt exists in dry and solid masses. 
The summit of the mountains which border the desart to 
the west of Cairo, presents an immense plain covered with a 
mass of salt. According to Horneman it is spread over so 
large a track of surface, that no eye can reach its termination ' 
in one direction; its breadth extends several miles. To the 
west of the desart of Sahara are the great salt rocks of 
Tegaza, on the south-east frontier of the desart of Zuen- 
ziga, a little distance from Cape Blanc. They are worked" 
by the Moors., These salt mines furnish the white and co¬ 
loured salt, which is carried by caravans to Casnah and 
Tombuctoo, to supply the Negro states; for it does not 
appear that there are any salt mines in Negro-land properly 
called. The mines of salt spread in that • part of Africa. 
which the ancients called Libya, have been well indicated by 
Herodotus, and it is in this country that he has described, 
buildings constructed of rock-salt, like those in Caramania 
and Arabia. Other salt mines, according to Park, are 
found on the southern frontier of the great desart Sahara. 
Their produce is also sold to the Negroes on the borders of 
the Niger and the Jolibe. In the kingdom of Tunis, mount 
Had Delfa is entirely composed of very compact salt of a 
red and violet colour. The lake des Marques, and the plains 
near it, also contain much salt. There are mines of rock- 
salt in the country of Bamba, in the kingdom of Congo.. 
On all the western coast of Africa there are salt lakes and 
marshes. In the neighbourhood of the Cape of Good Hope, 
