Part Il. § vi] CRYSTALLINE ROCKS—STRATIFIED. 111 
even in tropical regions. Towards the poles it descends to the sea- 
level, where large pieces of it break off and float away as icebergs, 
Water-ice is formed, Ist, by the freezing of the surface of 
fresh-water (river-ice, lake-ice), or of the sea (ice-foot, floe-ice, pack- 
ice); this is a compact, clear, white or greenish ice. 2nd, by the 
freezing of the layer of water lying on the bottom of rivers, or the 
sea (bottom-ice, ground-ice, anchor-ice) ; this variety is more spongy, 
and often encloses mud, sand and stones. 
Rock Salt (Sel gemme, Steinsalz) occurs in layers or beds from 
less than an inch to more than six hundred feet in thickness. The 
‘salt deposits at Stassfurt, for example, are 1197 feet thick, of which 
the lowest beds comprise 685 feet of pure rock salt, with thin layers 
of anhydrite 4} inch thick dividing the salt at intervals of from one 
to eight inches. The more insoluble salts are found in the lower 
parts of the saliferous series and disappear towards the top. When 
purest, rock salt is clear and colourless, but usually is coloured red 
(peroxide of iron); sometimes green, or blue. It varies in structure, 
being sometimes beautifully crystalline and giving a cubical cleavage ; 
laminated, granular, or less frequently fibrous. It always contains 
some admixture, either mechanical (clay, sand, vesicles of com- 
bustible gas, sometimes present in large quantity, or saline water) 
or chemical (chlorides of magnesium, or of calcium, &¢.). _ Occasion- 
ally remains of minute forms of vegetable and animal life, bituminous 
wood, corals, shells, crustaceans, and fish teeth are met with in it. 
Microscopic examination shows it to contain minute cubical cavities 
filled with a solution of salt. Owing to its ready solubility, it is not 
found at the surface in moist climates. With its associated seams of 
gypsum, anhydrite, red clay, &c., it forms series of strata several 
thousand feet thick, as in Gallicia. It has been formed by the 
evaporation of very saline water in enclosed basins—a process going 
on now in many salt-lakes (Great Salt Lake of Utah, Dead Sea), 
and on the surface of some deserts (Kirgis Steppe). In different 
parts of the world deposits of salt have probably always been in 
progress from very early geological times. Saliferous formations of 
Tertiary and Secondary age are abundant in Europe, while in America 
they occur even in rocks as ancient as the Upper Silurian period, and 
among the Punjab Hills in still more‘ancient strata. 
Limestone (Calcaire, Kalkstein).—Essentially a mass of calcium 
carbonate, sometimes nearly pure, and entirely or almost entirely 
soluble in hydrochloric acid, sometimes loaded with sand, clay, or other 
intermixture. Few rocks vary more in texture and composition. It 
may be a hard flinty close-grained mass, breaking with a splintery 
or conchoidal fracture ; or a crystalline rock built up of fine crystals 
of calcite and resembling loaf sugar in colour and texture; or a dull 
earthy friable chalk-like deposit; or a compact massive finely- 
granular rock resembling a close-grained sandstone or freestone. 
The colours, too, vary extensively, the most common being shades of 
blue-grey and cream-colour passing into white. Some limestones 
