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Parr IL. § vi] CRYSTALLINE ROCKS—STRATIFIED. 115 
much less solubility in hydrochloric acid. It occurs sometimes in 
beds of original deposit associated with gypsum, rock-salt and other 
results of the evaporation of saturated saline waters; it is also 
found replacing what was once ordinary limestone. This process, 
in which carbonate of lime is replaced by carbonate of magnesia, is 
known as dolomitization (see Book III., Part I, Section iv., § i1.).! 
Dolomite forms huge mountain masses, as in the Dolomite Mountains 
of the Hastern Alps. 
Gypsum.—A fine granular to compact, sometimes fibrous or 
sparry ageregate of the mineral gypsum, having a hardness of only 
1:5—2 (therefore scratched with the nail), and unaffected by acids ; 
hence readily distinguishable from limestone, which it occasionally 
resembles. It is normally white, but may be coloured grey or 
brown by an admixture of clay or bitumen, or yellow and red 
by being stained with iron oxide. It occurs in beds, lenticular 
intercalations and strings, usually associated with beds of red clay, 
rock-salt, or anhydrite, in formations of many various geological 
periods from the Silurian (New York) down to recent times. The 
Triassic gypsum deposits of Thuringia, Hanover and the Harz have 
long been famous. One of them runs along the south flank of the 
Harz Mountains as a great band six miles long and reaching a 
height of sometimes 430 feet. 
Gypsum furnishes a good illustration of the many different ways 
in which some mineral substances can originate. Thus it may be 
produced, Ist, as a chemical precipitate from solution in water, as 
when sea-water is evaporated; 2nd, through the decomposition of 
sulphides and the action of the resultant sulphuric acid upon lime- 
stone; 3rd, through the mutual decomposition of carbonate of lime 
and sulphates of iron, copper, magnesia, &c.; 4th, through the 
hydration of anhydrite; 5th, through the action of the sulphurous 
vapours and solutions of volcanic orifices upon limestone and cal- 
careous rocks.” It is in the first of these ways that the thick beds 
of gypsum associated with rock-salt in many geological formations 
_ have been formed. The first mineral to appear in the evaporation 
of sea-water being gypsum, it has been precipitated on the floors of 
inland seas and saline lakes before the more soluble salts. 
Anhydrite.—The anhydrous variety of calcium sulphate occurs 
in saliferous deposits, but is less frequent than gypsum, into which it 
passes by taking up 0°2625 of its weight of water.* 
_ Tronstone.—Under this general term are included a number of 
iron ores in which the peroxide, protoxide and carbonate enter in 
various mixtures with clay and other impurities. They have 
_ generally been deposited as chemical precipitates on the bottoms of 
~ (1871) p. 188. 
1 On the mineralogical nature of dolomite see O. Meyer, Z. Deutsch Geol. Ges. xxxi. 
p. 445, Loretz, op. cit. xxx. p. 387, xxxi. p. 756. 
2 Roth. Chem. Geol. i. p. 553. 
3 See G. Rose on formation of this rock in presence of a solution of chloride of 
sodium. Neues Jahrb. Min. 1871, p. 932. -Also Bischof, Chem. und Phys. Geol. Suppl. 
EZ 
