of the albite type-i.e., to a line perpendicular to the brachypinacoid. The lamella of the pericline type will also be extinguished symmetrically in sections from this zone. Sections showing this symmetrical extinction and yielding low angles are approximately parallel to the basal plane; those giving high angles should be selected. The maximum extinction-angles of the albite type of lamellæ in faces from this zone are as follows:-*

Were it possible to determine how each section has been cut, even albite and andesine could be distinguished from a difference in the direction of rotation; but Becke's simple method for the observation of relative refractive index effects this admirably, whenever sections of the felspar can be found abutting on the balsam at the edge of the slide (p. 142). Twins-Repeated twinning is extremely common, the lamelle being often very numerous. Both the albite type, with the brachypinacoid as the twin-plane and composition-plane, and the pericline type, with the "rhombic section as the composition-plane and practically the same axis of rotation as the former type, may occur in the same crystal, giving crosstwinning akin to the characteristic microcline-structure. In sections from the zone of the base and macropinacoid the traces of the lamella of the two systems are almost at right angles. Other types of twinning occasionally occur; the Carlsbad type is indeed common, one half showing, in addition, numerous lamella of the albite type, while the other shows few or no lamellæ. See Microcline.

Note. - Distinguished in general from orthoclase in ordinary sections by this character of repeated twinning. The discrimination of one plagioclase from another is a matter requiring considerable care, and the methods should be further studied in the larger text-books. A series of trials with Szabo's system of flame-reactions (p. 84) will often help when the mineral has been proved to be a plagioclase, and specific gravity tests with dense liquids are of great service.

The dull white altered felspars, full of recrystallised decomposition-products (mainly zoisite or garnet), that are common in many older diorites and gabbros, were formerly known as * Lévy, Détermination des Feldspaths, 1894, p. 31.

SAUSSURITE Such forms may be termed "saussuritic felspars." (Fig. 45.)

Pyrite (see Iron Pyrites).

=

PYRRHOTINE. Comp.-Fe, Sg, or Fe S Syst.-Hexagonal. Rather redder than iron pyrites. Hardness about 4, the particles cut out being easily attracted by the magnet. Opaque granular in sections.

Pyroxenes (see Augite, Diopside, Rhombic Pyroxenes, and Soda-pyroxenes. Also Bastite).

Quartz. Comp.-SiO2. Syst.-Hexagonal (Trigonal).

I. Commonly clear and colourless, with vitreous lustre. No cleavage; conchoidal fracture, thus resembling glass. Unscratched by knife. Grains (which should be looked for with the lens); short prism and double-pyramids (in some eurites); or longer prisms terminated by pyramids (when formed in cavities or in sedimentary rocks). Occurs also filling veins, and is often in such cases almost opaque white.

II. Form-In igneous rocks seldom shows crystal-outline. Commonly allotriomorphic or in corroded grains (fig. 32), but well bounded in some eurites. Micropegmatitic intergrowths with duller felspar may be expected. In metamorphic and in many plutonic rocks quartz forms aggregates of little granules, which are well revealed by the polariscope. Commonly cracked irregularly. In veins, and in residues of limestones, &c., shows good crystallographic outlines. Cleav.-None. Encl.-Liquidenclosures with moving bubbles commonly very abundant in the quartz of deep-seated or metamorphosed rocks, the lines along which they have developed often passing continuously across several grains (see fig. 19). These irregular strings and patches of minute enclosures may be taken for dull decomposition-products, such as kaolin, unless a high power is used. Glass-enclosures with fixed bubbles, and sometimes with the form of negative crystals, occur in the quartz of many lavas. Zon.-Exceedingly rare, except in crystals of sedimentary origin. Refr. Index-1.547, almost exactly that of the balsam. ColourColourless. See, however, Amethyst. No decomposition products. Pleo. Observable in the exceptional coloured varieties, which have little claim to be called rock-forming minerals. D. Refr.-About that of the felspars. The absence of decomposition, as compared with other minerals in the slide, is often brought out by the clear colours given by quartz between crossed nicols. Thin basal sections show in convergent light the ordinary black cross, the characteristic coloured central area, due to circular polarisation, being visible only in specially

cut thicker sections. rock-sections.

Opt. Sign-Positive.

Twins-None in

Note.-Compare and carefully contrast with orthoclase and plagioclases. See also Chalcedony, Opal, and Tridymite, and figs. 24 and 25.

Rhombic Pyroxenes. Comp.-(Mg, Fe) Si Og. The rock-forming examples of these were long confused with monoclinic pyroxenes, which they generally resemble to the eye. The boldly developed lustrous types familiar to mineralogists, such as the bronzite of the Kupferberg and the hypersthene of Paul's Island, Labrador, may now be regarded as altered forms of minerals possessing no such lustre. Hence Tschermak proposes for the rhombic pyroxenes a purely chemical grouping:

To these Prof. Judd,* reviving an older name of vom Rath, adds AMBLYSTEGITE; Fe O 25 to 35%.

=

I. Commonly closely resembling augite; sometimes as pale as diopside. The schillerised forms, bronzite and hypersthene of older authors, occur in some holocrystalline rocks. Monoclinic pyroxenes with similar lustres were also formerly classed under these names.

II. Form-Nearly isomorphous with monoclinic pyroxenes; hence the frequent confusion with them. Prism-angle 88°. Habit and sections similar to augite (fig. 36). Cleav.-Prismatic, thus also intersecting nearly at right angles. Enc.-Schillerised forms fairly common, but not to be expected in more modern lavas. Refr. Index-1·66-1·70. Colour-Like monoclinic pyroxenes; colourless, yellowish-brown, at times greenish. Hypersthene and AMBLYSTEGITE, owing to the striking pleochroism, often appear greenish or pink-red in the same slide. Decompose to green products. See Bastite. Pleo.-Enstatite is too pale to exhibit pleochroism; bronzite and the more ferriferous members of the series show bluish-green when the shorter diagonal of the nicol is parallel to the vertical axis, brown when it is parallel to the macrodiagonal, and a fine red-brown, when it is parallel to the brachydiagonal. This effect is naturally very striking in AMBLYSTEGITE. D. Refr.-Somewhat weaker than in the monoclinic pyroxenes, and even approaching quartz in the case of enstatite. Optic axial plane in the brachypinacoid.

* Geol. Mag., 1895, p. 173. Also Quart. Journ. Geol. Soc., vol. xli., (1885), p. 371.

Extinct.-Rhombic; thus markedly distinct from the monoclinic pyroxenes, with the exception of those extremely rich in soda. The direction of the principal axis is that of vibration for the slowest ray.

Note.-Intergrowths with diallage have been observed, the brachypinacoid of the rhombic form being applied to the clinopinacoid of the monoclinic. Distinguish the rhombic pyroxenes by the straight extinction and, if possible, by the pleochroism. In some remarkable granular rocks the pink rounded sections of hypersthene resemble in ordinary light the garnets occurring in the same slide. The nicols at once prove the latter to be isotropic.

In the absence of microscopic sections, rocks must often be called "pyroxene-andesite," "pyroxene-diorite," &c., until accurate determination can be made.

Compare carefully with diopside, augite, and soda-pyroxenes.

For altered forms see Bastite and Serpentine.

RIEBECKITE (see Soda-Amphiboles).

RUTILE. Comp.-Ti Og. Syst. -Tetragonal.

I. As a rock-forming constituent rutile is generally invisible until the microscope is applied. May appear as hard rich brown or black specks. Its high specific gravity (4·2) makes it separable by dense liquids.

II. Form-Granules and aggregates, with here and there recognisable prismatic forms. Minutely distributed in the altered minerals of some gabbros; also in practically all argillaceous and chloritic metamorphic rocks, and also in clays. Shows typically minute geniculated twins, sometimes heartshaped. Refr. Index-Extremely high (=2712). The tiny crystals and grains thus stand out with strongly marked margins. Colour-Yellow-brown to red-brown; sometimes almost black. Twins-As above stated; highly characteristic.

SANIDINE (see Orthoclase).

SAUSSURITE (see paragraph at end of Plagioclases).

SCAPOLITES. Comp.-A series of minerals with meionite at one end and marialite at the other, being silicates of alumina, lime, and soda, with some chlorine, the lime preponderating largely over soda in the meionite molecule. Syst.—Tetragonal.

I Crystallised in cavities of some lavas; also often occur associated with altered felspars, the products of which are recrystallising. Colourless to white or grey. Soluble in hydrochloric acid, leaving a residue of non-gelatinous silica.

II. In sections a number of clear colourless granules sometimes appears in the place of felspathic constituents. They are liable to be mistaken for secondary felspars or even quartz. Form-Commonly granular. Cleav.-Often not distinct. Refr.

Colour-Colourless.

Index-About 1.57. D. Refr.-Considerably stronger than felspars in the highly calciferous varieties, being near olivine in the case of meionite. In DIPYRE, however (the common gra ular type in holocrystalline rocks), the double refraction is fairly weak (= about 014). Uniaxial figure in convergent light. Opt. sign-Negative.

Note. For an account of the relations of the scapolites and plagioclases, and references to the literature of the subject, see Prof. Judd, "On the processes by which a plagioclase felspar is converted into a scapolite,' Min. Mag., vol. viii. (1889), p. 186.

SERICITE (see Muscovite).

Serpentine. Comp.-H, (Mg, Fe), Si, Og. Syst.-Rhombic.

I. Soft grey-green, green, black, or red areas among decomposing ferro-magnesian minerals, or even building up the mass of a rock. Hardness about 3. Sometimes crystallised in pale or golden-yellow fibres in veins running across the rock (chrysotile).

II. Form-Pseudomorphous, in patches, or in veins and cracks. Minute tufts and fibres often developed. Does not look so uniform as chloritic areas. The serpentine has often the ovoid form of olivine granules, the cracks of the latter being at times marked by bands of magnetite. Refr. Index-Low; close to that of the balsam. Colour-Yellow, yellow-green, or blue-green. Most commonly a yellowish-green. Colourless highly refracting areas of olivine are often left surrounded by the serpentine. Pleo.-Distinct, in shades of green. D. Refr.-Close to that of ordinary plagioclases. Shows tufts and fibres in polarised light, the serpentinous areas being made up of a number of needles. These needles, picked out by the use of the polariscope, frequently seem at or nearly at right angles to one another, and it is often stated that the serpentine in such cases has been derived from pyroxene; but the structure is extremely common in company with others referred as certainly to olivine. It is possible that in some cases described the rectangular effect is due to the illusion referred to on p. 149. (Figs. 31 and 38.)

SILLIMANITE. Comp.-Al, Si O. Syst.-Rhombic. Common in bunches of minute colourless prisms in schists altered by contact with granite. D. Refr.— 02.

Note.-Infusible, and gives good alumina reaction with cobalt nitrate. Minute fragments of rock may be thus treated and examined later under microscope.

SMARAGDITE (see Hornblende).

SODA-AMPHIBOLES. Comp.-All may be represented as Na (Fe, Al2) Si4 012, with some (Fe, Mg) Si Og. Like common hornblende in most respects. ARFVEDSONITE, with little Alg cannot be distinguished from common hornblende by simple