80 _ GEOGNOSY. oe 
monosilicate of peroxide and protoxide bases, these being chiefly 
alumina, iron, chromium and manganese; the proportion of silica’ 
ranging between 36 and 41 per cent. Under the microscope, garnet 
as a constituent of rocks presents three-sided, four-sided, six-sided, 

eight-sided (or even rounded) figures, according to the angle at which _ 
the individual crystals are cut; usually clear, but full of flaws and 
often of cavities; passive in polarized light. The common red and 
brown varieties occur as essential constituents of eclogite, garnet 
rock; and as abundant accessories in mica-schist, gneiss, granite, &e. 
Tourmaline (Schorl). Rhombohedral, frequently in prisms and 
needles, also massive, compact, and columnar; generally black, with 
vitreous lustre. H. 7—7'5. Gr. 2:°94—3°3. Composition remarkably 
complex and varied, including silica (86—40 per cent.), alumina 
(29—40), magnesia (0°5—12), boric acid (85—9), with smaller propor- 
tions of phosphoric acid, ferrous oxide, manganous oxide, lime, potash, 
soda, lithia, fluorine and water ; pleochroism strongly marked. With 
quartz forms tourmaline-rock associated with some granites; occurs 
also diffused through many granites, gneisses, schists, crystalline 
limestones, and dolomites. 
Zircon. Tetragonal, in prisms, pyramids, or rounded crystalline 
grains; colourless to red, yellow, or brown; transparent to opaque ; 
vitreous lustre. H.7:5. Gr. 4:4—4°7. Composition—one molecule of 
silica and one of zirconia (=i O, 33°2, Zr O, 66°8) with a little 
oxide of iron as colouring matter. In polarized light gives bright 
colours between crossed Nicols. Occurs as a chief ingredient in the 
zircon syenite of Southern Norway; sparingly in other syenites, 
eranites, gneisses, crystalline limestones and schists, in eclogite, as 
clear red grains in some basalts, and also in ejected volcanic blocks. 
Titanite. Monoclinic in thin wedge-shaped crystals (sphene) ; 
yellow, green or brown to black; vitreous to adamantine lustre. 
H.5—5:5. Gr. 3:4—38-6. Composition—silica 30°61, titanic acid 40°82, 
lime 28°57. Between crossed Nicols gives dark yellowish-brown tints. 
Dispersed in small crystals in many syenites, also in granite, gneiss, 
and in some volcanic rocks (basalt, trachyte, phonolite). | 
Zeolites. Under this name is included a characteristic family of 
minerals, which have resulted from the alteration and particularly 
from the hydration of other minerals, especially of felspars. They 
are thus secondary products, and not original constituents of rocks. - 
They are marked by the following general characters: usually colour- 
less, transparent, or translucent, with a vitreous lustre which often 
becomes pearly on cleavage faces; H. 4—5°5; Gr. 1:°9—2°5; occur 
in cavities of rocks, both as prominent amygdules and veins, and 
in minute interstices only perceptible by the microscope. In these 
minute forms they very commonly present a finely fibrous divergent 
structure. ‘They are hydrous aluminous silicates with variable propor- 
tions of lime, potash, soda, or baryta. A relation may often be traced 
between the containing rock and its enclosed zeolites. Thus among 
the basalts of the inner Hebrides the dirty green decomposed. ‘ 
