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i ae _ GEOGNOSY. [Book IL 
Damourite is merely a variety of muscovite with about 5 per 
cent. of water. It occurs among crystalline schists, and is regarded 
by Genth as one of the products of the alteration of corundum. 
Sericite, a talc-like variety of muscovite occurring in soft inelastic 
scales in some schists. 
Margarodite, a silvery, talc-like hydrous mica, which appears to 
have resulted from the hydration of muscovite, and to be widely 
diffused as a constituent of granite and other crystalline rocks. 
Paragonite, a scaly micaceous mineral forming the main mass of 
certain alpine schists; it is a hydrous soda mica (containing 6—8°49 
per cent. of soda). 3 
Biotite (Magnesia mica) occurs in six-sided plates or irregu- 
larly defined scales, usually dark coloured (green, grey, brown to 
black) with pearly lustre on the basal cleavage planes. H.2°5—3. 
Gr. 2°74—3:13. Composition variable, but marked by the occurrence — 
of 10 to 30 per cent. of magnesia. Occurs abundantly as an 
original constituent of many granites, gneisses, and schists; also 
sometimes in basalt, trachyte, and as ejected fragments and crystals 
in tuff. Its small scales, when cut transverse to the dominant 
cleavage, may usually be detected under the microscope by their 
remarkably strong dichroism, their fine parallel lines of cleavage, 
and their frequently frayed appearance at the ends. 
Biotite under the action of the weather assumes a pale, dull, soft 
crust, owing to removal of its bases. The mineral rubellan, which oe- 
curs in hexagonal brown or red opaque inelastic tables in some basalts 
and other igneous rocks, is regarded as an altered form of biotite. 
Hornblende (Amphibole). Monoclinic, in short stout or long 
slender prisms; also in bladed forms and needles, generally of a dark 
green or black colour (sometimes white). H.5—6. Gr. 2:9—3:3, 
Divided into two groups. 1st. Non-aluminous, consisting mainly of 
meta-silicates of magnesium and calcium, with 55 to 59 per cent. of 
silica, 21 to 27 magnesia, 11 to 15 lime, and minor proportions of 
the protoxides of iron and manganese. These include the white 
and pale green or grey fibrous varieties (tremolite, actinolite, 
anthophyllite, &c.), 2nd. Aluminous, containing silica 39-—49, 
magnesia 10—20, alumina 8—15, lime 10—15, ferrous and ferric 
oxides sometimes up to more than 20 per cent. These embrace 
the more abundant dark green, brown, or black varieties. Under 
the microscope hornblende presents cleavage angles of 124° 30’, - 
the definite cleavage planes intersecting each other in a well- 
marked lattice work, sometimes with a finely fibrous character 
superadded, It also shows a marked pleochroism with polarized 
light, which, as ‘I'schermak first pointed out, usually distinguishes 
it from augite.! 
The pale non-aluminous hornblendes are found among eneisses, 
erystalline limestones, and other metamorphic rocks, The dark 
varieties, though also found in similar situations, sometimes even 
' Acad. Wien, May 1869. See also Fouqué et Michel-Lévy, op. cit. pp. 349, 365. 

