30 STRUCTURAL AND FIELD GEOLOGY 
(2-2 to 2-4) being considerably less than those of anhydrite. 
It may be variously coloured, but is usually transparent or 
white. It is soluble in hydrochloric acid. Before the blow- 
pipe it becomes opaque or white, exfoliates, and fuses to a 
white enamel. Crystals, lenticular concretions, and inter- 
rupted layers of gypsum often occur in clays. Frequently it 
appears as granular and compact masses, arranged as layers 
and thick beds in argillaceous strata, where it is commonly 
associated with rock-salt and anhydrite. Now and again it 
forms the cement or binding material of sandstone. Selenzze 
is the name given to crystallised gypsum; it shows perfect 
cleavage—the lamine being flexible but not elastic. The 
very fine-grained cryptocrystalline kinds are usually termed 
Alabaster, and the fibrous varieties Satzz Spar. 
Barytes or Heavy Spar (barium sulphate) crystallises in 
the orthorhombic system. Fibrous varieties are common. It 
is not, properly speaking, a rock-former, but is usually met 
with as a secondary mineral in veins and other cavities. It 
is commonly associated with ores (especially sulphides) in 
lodes. Its hardness (3 to 3-5) slightly exceeds that of calcite, 
but its greater specific gravity (4-3 to 4-6) and its resistance 
to acids at once distinguish it from the latter. Barytes 
decrepitates and fuses with great difficulty before the blow- 
pipe, colouring the flame yellowish-green. 
Vil PHOSEEAIES 
Apatite (phosphate of lime, containing either fluorine or 
chlorine: hence, chemically, two kinds are recognised—fluor- 
apatite and chlor-apatite). This mineral crystallises in the 
hexagonal system, usually as six-sided prisms. Hardness= 
5; specific gravity = 3-17 to 3:23. It is soluble in hydrochloric 
acid, and fusible with difficulty before the blowpipe. It 
occurs as a frequent but usually a microscopic accessory 
ingredient of very many eruptive rocks and crystalline schists, 
commonly in the form of long, slender, hexagonal prisms or 
needles. It is a frequent inclusion in all the essential con- 
stituents of eruptive rocks. Next to magnetite, it has the 
widest distribution of all accessory rock-constituents. Fine 
crystals occur in the drusy cavities of some granites, as like- 
