DISCOVERY 



323 



into this brine it would not be dissolved : conversely, 

 if more water is evaporated and taken away from the 

 brine, salt will crystallise out and wiU be deposited 

 as a bed of rock-salt. And last of all this bed comes 

 to be preserved for all time by a bed of clay, through 

 which fresh water cannot penetrate to redissolve the 

 salt ; this bed of claj- is formed from sediment from the 

 fresh-coming water. This fresh water probably very 

 largely travels over, without distiurbing, the heavier 

 brine. (A gallon, or -iG cub. ft., of water weighs 

 10 lb., whereas a gallon, or -16 cub. ft., of brine weighs 

 12 lb.) We may therefore imagine this process being 

 repeated througii untold ages, for the deposits are 

 upwards of 4,000 ft. thick : wet seasons being repre- 



8 in. throughout the bed. At this place also other 

 salts — CarnaUite. Kieserite, Polj-hahte — occur in thick 

 beds. In France there is a rich deposit of Potassium 

 Chloride associated with rock-salt, which is known as 

 sylvinite. Most of these salts have been preserved by 

 clay-seams in the manner indicated, but other salts, 

 such as those that are only very shghtly dissolved by 

 water, are found at the surface, an example being 

 Celestine, which occurs in patches in the Trias in 

 Gloucestershire. 



How is Rock-salt Obtained ? 



Rock-salt-winning is a very ancient industry. In 

 olden times it was obtained from the sea, by leading 



Fig. I.— WIEI,ICZKA SAI,T mines, GAIICIA. 

 The beautiful Francis Joseph Ballroom, made of salt. The chandeliers are made of polished salt crystals. 

 (By courtesy of Sir Isaac Pitman & Sons, Ltd.) 



sented b}- marls, clay, or sandstone seams ; and dr}' 

 seasons, by rock-salt seams. 



In England rock-salt appears in beds of I in. to 

 100 ft., of various colours ; the red being due to oxide 

 of iron ; the green, to chloride of copper ; the blue, to 

 sUicate of copper. It contains several other impurities 

 — clay, sand, bitumen, gypsum. Calcium and Mag- 

 nesium Chloride (the latter being responsible for its 

 deliquescence, i.e. its capacity for absorbing moisture). 

 There are cavities that are fiUed with brine and 

 various gases. FossUs are sometimes embedded in it. 

 Gypsum is often found in layers of various thicknesses ; 

 thus at Stassfurt there is a bed of pure rock-salt 685 ft. 

 thick, with thin layers of Anhydrite (gypsum without 

 water), a quarter of an inch thick, at intervals of i to 



the sea into shallow lagoons and evaporating the water 

 by the heat of the sun, thus imitating on a small scale 

 the natural process of the formation of a rock-salt bed. 

 This method is pursued in several parts of the world 

 at the present day, notably in China. The actual 

 mining of rock-salt is carried out on a large scale in 

 several parts of Europe, for example at \\'ieUczka, 

 near Cracow, in Poland. Here the underground 

 galleries have a length of some 65 miles ; some of these 

 are converted into show-places for visitors and lighted 

 b}^ electricity, provided also with houses, churches, ball- 

 rooms, restaurants, lakes, bridges, and even railway 

 stations. Fig. i gives a view of the Francis Joseph 

 Ballroom, everything, including the chandehers, being 

 composed of salt. It may be noted en passant that 



