78 GEOGNOSY. - > iRegaat, a 
in broken crystals. It is liable to be traversed by fine fissures, which _ 
are particularly developed transverse to the vertical axis. It is more 
liable to alteration than almost any other mineral constituent of rocks. 
The change begins on the outer surface and extends inwards and 
specially along the fissures, until the whole is converted either into a 
green eranular or fibrous substance, which is probably in most eases 
serpentine (Fig. 6), or into a reddish yellow amorphous mass 
(limonite). 
Olivine forms an essential ingredient of basalt, likewise the 
main part of various, so-called olivine-rocks or Peridotites (as 
lherzolite and pikrite), and occurs in many gabbros. 
Leucite. ‘Tetragonal, in isolated icositetrahedrons of a greyish- 
white colour, semi-transparent. H. 55. Gr. 2:-40—2°50; infusible 
and unchanged before the blowpipe. Composition—silica 54°97, alu- 
mina 23°50, potash 21°53. Under the microscope sections of this 
mineral are usually eight-sided, and very commonly contain 
enclosures of magnetite, &c., conforming in arrangement to the ex- 
ternal form of the crystal. Leucite is a markedly volcanic mineral, 
occurring as an abundant constituent of many ancient and modern 
Italian lavas, and in some varieties of basalt. 
Nepheline. Hexagonal, in small prisms or in crystalline and 
oranular aggregates, usually clear and colourless with vitreous lustre. 
H. 55—6. Gr. 2°58—2-64. Composition—silica 41:24, alumina 
30°26, soda 17:04, potash 6°46. Presents under the microscope 
various six-sided and even four-sided forms, according to the angles 
at which the prisms are cut.'. Essentially a volcanic mineral, being 
an abundant constituent of phonolite, of some Vesuvian lavas, and 
of some forms of basalt. 
Under the name of Elzolite are comprised the greenish or reddish, 
dull, greasy-lustred compact or massive varieties of nepheline which 
occur in some syenites and other ancient crystalline rocks. 
Hauyne. Isometric, but usually in solitary crystalline grains of 
asky-blue to bluish-green colour ; this tint, probably due, as in lapis- 
lazuli, to a mixture of sulphur and sodium, is discharged by heating. 
H. 5—5‘5, Gr, 2-4—2°5. Composition—silica 34-06, alumina 27-64, 
soda 11-79, potash 4°96, lime 10°60, sulphuric acid 11:25. Occurs 
abundantly in Italian lavas, in basalt of the Hifel and elsewhere. 
Nosean. Isometric, in solitary rhombic dodecahedrons, grey, 
greenish-blue to black, often with a dull opaque border. H. 5:5, 
Gr. 2°28—240. Composition—silica 86°13, alumina 30°95, soda 
24°89, sulphuric acid 8°03, with a little chlorine, supposed to be 
due to a slight intermixture of the mineral sodalite. Under the 
microscope, one of the most readily recognized minerals, showing a 
hexagonal or quadrangular figure with a characteristic broad dark 
border corresponding to the external contour of the crystal, and 
where weathering has not proceeded too far, enclosing a clear 
1 . _ . . . ‘ . . . F . ® a Li d 
_' On tnicroscopic distinction between nepheline and apatite, see Fouqué et Michel- 
Lévy, op. cil. p. 276. 



