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~ 
ParrIL §ii.]  ROCK-FORMING MINERALS. ——~ 79 
‘colourless centre. Occurs in minute forms in most phonolites, also 
in large crystals in some sanidine volcanic rocks. 
Both hauyne and nosean are volcanic minerals associated with 
the lavas of more recent geological periods. 
Epidote. Monoclinic, in elongated prisms, also granular, fibrous, 
and massive, usually of a peculiar and characteristic yellowish-green 
eolour. H.6—7. Gr. 3:32—3-°50. Composition—silica 36—40, 
~ alumina 18—29, ferric oxide 7—17, lime 21—25. Under the 
microscope, appears as a constituent of rocks in yellow needles and 
threads, often divergent ; with distinct pleochroism and remarkably 
bright limpid yellow and orange polarization tints. Occurs in many 
crystalline, chiefly hornblende-bearing, rocks, probably as a result of 
the alteration of the hornblende; largely distributed in certain schists 
and quartzites, sometimes associated with beds of magnetite and 
-_heematite. 
Vesuvianite (Idocrase). Tetragonal, in short often vertically 
striated prisms or compact aggregations, occurring in druses rather 
than in the body of a rock; yellowish, greenish to black. H. 6°5. 
Gr. 3:°34—3:44. Composition—Silica 37—39, alumina 13—16, ferric 
oxide 4—9, lime 33—87, alkalies less than 1 per cent., frequently 
with a little magnesia, ferrous oxide, and 2—8 per cent. of water. 
Occurs in ejected blocks of altered limestone at Somma, also among 
crystalline limestones and schists. 
Andalusite. Orthorhombic, often in large long prisms as well 
as In compact massive aggregates; white, grey, brown, red. H. 7:5. 
Gr. 3°05—3°35. Composition—silica 36:90, alumina 63:10. Found 
in crystalline schists. The variety Chiastolite, which occurs abun- 
dantly scattered through some dark clay-slates, is distinguished by 
the regular manner in which the dark substance of the surrounding 
matrix has been enclosed within the macles, giving a cross-like 
transverse section. These crystals have been developed in the rock 
after its formation, and are regarded as proofs of metamorphism. 
(Book IV. Part VIIT.) 
Dichroite (Cordierite, Iolite). Orthorhombic, usually in indistinct 
short prisms and crystalline grains, bluish in colour, with greasy to 
vitreous lustre and fracture like that of quartz. H.7—7-5. Gyr. 2:56 
—2°67.. Composition—silica 49—50, alumina 32—89, ferric oxide 
5—-9, magnesia 10—12, usually with a little manganous oxide, 
lime and water. Occurs in gneiss, sometimes in large amount 
(cordierite-gneiss), occasionally as an accessory ingredient in some 
granites ; also in talc-schist. Apt to be confounded with quartz, but 
usually gives marked dichroism with one Nicol prism, and pale grey- 
blue tints with the two prisms. Undergoes numerous alterations, 
having been found changed into pinite, chlorophyllite, mica, &c. 
Garnet. Isometric, usually in rhombic dodecahedrons and icosi- 
tetrahedrons, also granular and massive; mostly some shade of red, 
but also green, yellow, brown, and black; vitreous to greasy lustre, 
pellucid to nearly opaque. Composition various, but essentially a 
