Similar strains disturb the optical characters of minerals when viewed with convergent light; so that the observations to which we now proceed must be made on a number of sections of the same mineral in a slide before accurate conclusions can be drawn. Uniaxial and Biaxial Crystals in Convergent Polarised Light.— The chief use of the sub-stage condenser in dealing with rocksections is the determination of the uniaxial or biaxial character of doubly refracting minerals. A mineral-section is selected with the low power which appears suitable from a consideration of the probable position of the optic axis or the optic axial plane. Having been moved into the centre of the field, a high power, preferably an eighth-inch, is brought to bear on it, and the condenser is adjusted so as to converge the rays within the crystal. The nicols are crossed and the eye-piece is removed, the eye probably requiring to be held at a little distance from the top of the tube. As already mentioned, a lens may be used above the objective that will bring the optic axial figure within the focus of the eye-piece, which is thus retained; but it is noteworthy that observers of great eminence have preferred the smaller but brighter results given by the simple observation down the tube.

Rotate the stage, and, if the section is at all favourably cut, a dark shadow will move across. Some minerals with a strong double refraction, such as epidote, will show in addition coloured rings even in thin sections. The thicker the section, the more of these iris-tinted rings will appear within the field.

The indications of the rings and shadows, subject to the cautions given under the last heading, may be stated as follows.

a. Uniaxial Minerals.-1. An isotropic section should, if possible, be selected-i.e., one perpendicular to the optic axis. In the case of quartz, owing to the rotation of the polarised ray to right or left by circular polarisation, even sections thus cut are not truly isotropic.

A section perpendicular to the optic axis will show a black cross, which is unchanged on rotation. Sometimes coloured circles may be seen round it. The arms of the cross are parallel to the vibration-planes of the nicols (fig. 21, a). The microscope should be tested on a good section of calcite, devoid of flaws since little errors in the construction of the condenser may cause the cross to divide at the centre during rotation as if the mineral were biaxial

2. If the section is oblique to the optic axis, the rays traversing it parallel to that axis may still be able to enter and emerge from the, obiective. In this case the centre of the black cross

will appear, but will not be in the centre of the field, and will shift its position as the stage is rotated, thus moving round in a circle (fig. 21, b).

3. If the section is still more oblique to the optic axis, the centre of the black cross will lie outside the field, and only one

of its arms will become visible at a time. Thus on rotation a black bar will move across the field, keeping in a vertical position, followed by one moving across in a direction at right angles to the former, thus keeping in a horizontal position; when this has disappeared, the third arm appears, moving like the first; and then the fourth, moving like the second. The absence of deflection in the dark bars is the point to be especially noted (fig. 21, c, d, and e).

Strain will destroy this regularity; some biaxial minerals, on the other hand, have so small an optic axial angle that the figure given by them scarcely deviates from the black uniaxial cross throughout a complete rotation.

B. Biaxial Minerals.-1. If cut so that the rays parallel to an optic axis reach the eye, the section shows a dark bar which swings round on the rotation of the stage, moving in an opposite direction. At the same time it becomes hyperbolic, straightening itself out in four positions during rotation. Coloured rings may possibly surround the optic axis, as above described.

Since the bar becomes straight when the trace of the optic axial plane is parallel to the vibration-plane of either nicol, and since the bar then lies along this trace, it may be of service as giving the position of the optic axial plane in the crystal.

When the section is oblique to the optic axis, o, through which the black bar passes, this moves round the centre of the field (fig. 21, f, g, and h).

2. If the section is more oblique to the optic axis--i.e., if it approaches more nearly to a direction perpendicular to one of the bisectrices the dark bar may escape from the field during rotation.

3. If the section is perpendicular or approximately perpendicular to one of the bisectrices, preferably the acute bisectrix, a black cross will appear when the optic axial plane is parallel to the vibration-plane of either nicol. Rotation at once causes a separation of the cross into two hyperbole, though in a few minerals this separation is very slight (fig. 21, i and j).

In the examination of thin sections, both the optic axes of the figure in the case we are considering generally lie outside the limits of the field, and hence, when the optic axial plane is at 45° to the diagonals of the nicols, both the hyperbolæ are carried out of sight. Their convex sides always face the bisectrix which emerges in the field, and the thinner arm of the black cross that can be formed by them lies along the trace of the optic axial plane.

In

This occurrence of two dark curves sweeping across the field and uniting at every 90° of rotation to form a cross is one of the best features by which biaxial crystals can be determined. default of so good a figure, the curvature of the single bars that come into view must be noted, in opposition to the permanent straightness of those of uniaxial crystals. The typical figure given by biaxial minerals may be well studied in muscovite, and, commencing with a thick piece, the specimen should be thinned down until the hyperbolæ are accompanied by the merest trace of coloured rings.

In certain special cases, finally, where it is known that the section is perpendicular to the acute bisectrix, the optical sign of the crystal can be simply determined. The trace of the optic axial plane is set at 45° to the diagonals of the nicols; since it is one of the vibration-traces of the crystal-section, determine with the quartz wedge in plane polarised light whether it corresponds to the ray of greatest or least velocity (ses p. 147). If greatest, that is to say, if compensation occurs when the wedge is thrust along this direction, then the vibration-trace perpendicular to the optic axial plane is that of the slow ray in the section. Since this latter direction is always the vibration-direction of the ray of mean velocity in the crystal as a whole, the acute bisectrix, the normal to the section, must be the vibration-direction for rays of least velocity in the crystal, which is, therefore, positive. If the experiment gives a reverse result, the crystal is negative.

CHAPTER XVII.

THE CHARACTERS OF THE CHIEF ROCK-FORMING MINERALS IN THE ROCK-MASS AND IN THIN SECTIONS.

UNDER this heading are given the characters presented by common minerals as they occur in rocks, the order followed being alphabetical. The minerals of first importance are printed in thick type. Each description is divided into two parts:

I. The most striking characters of the mineral as it appears embedded in the rock-specimen, with one or two additional notes.

II. The characters it exhibits in microscopic sections. The abbreviations used are as follows:

Comp.-Chemical composition.

Syst.-Crystallographic system.

Form.-Ordinary form, or outlines in sections.

Cleav.-Cleavage.

Encl.-Enclosures.

Zon.-Zone-structure.

Refr. Index. -Average index of refraction (determined with yellow light).

Colour.-Colour as seen in ordinary light. Variations according to the face viewed (face-colours). Appearance of alteration-products.

Pleo.-Pleochroism as observed by means of the single nicol (axis

colours).

D. Refr.-Double refraction. This is strong" when the difference between the greatest and least index of refraction in the crystal is large (say '040), and "weak" when this difference is small (say '005). In the former case we have "high" colours, in the latter "low."

Extinct.-Positions of extinction.

Opt. sign.-Optical sign; including character of the acute bisectrix, and its position.

Twins. Characteristic twinning.

Actinolite.-A non-aluminous amphibole. Special points :-
I. Long prisms, distinctly green; often in radial bunches.
II. Colour Often colourless. Pleo.-Correspondingly feeble.

Note.-See Hornblende and Tremolite.

EGIRINE.-See Soda-Pyroxenes.
Albite. See Plagioclases.

AMBLYSTEGITE.-See Rhombic Pyroxenes.
AMETHYST.-Like quartz. Special points :-

I. Violet colour; occurs in cavities of rocks.

II. Pleochroic when the section is thick enough for the colour to appear.

Amphiboles.

See Actinolite, Anthophyllite, Hornblende, Soda-Amphiboles, Tremolite.

2

ANALCIME. Comp.-Some varieties = Na, Al, Si4 012 + 2H2O. Syst. Probably cubic.

I. In cavities of rocks; colourless glassy-looking or opaque white icositetrahedra, commonly in groups.

II. Refr. Index.-Near leucite (1.487). D. Refr.--Sometimes anomalous, in grey and greyish-white tints; sometimes forms an isotropic ground-mass between other minerals.

Note.-Fuses easily, and gelatinises in HCl. Compare leucite, which it resembles externally. If the substance is transparent in the mass, it is very probably analcime rather than leucite. Analcime may occur as a decomposition-product of nepheline.

ANATASE.-Comp.-Ti O2. Syst.—Tetragonal.

I. Occurs as brilliant blue-black to black modified pyramids, which, though commonly about 3 mm. long, catch the eye by their lustre on the surfaces of rocks. Is found sometimes on dissolving limestones or extracting the heavy minerals from sands. II. Refr. Index-Very high (2·52).

ANDALUSITE.-Comp.-Al, Si Og. Syst.-Rhombic.

5.

I. Sometimes seen as well marked and nearly square grey or pink prisms in schistose rocks, or as blue-grey heavy granular aggregates (specific gravity= 3·18).

II. Form-Small granular, as in Cornish granites, to rod-like, as in schists. Refr. Index-1.638. Colour-Colourless, or facecolours faintly pink or green. Pleo.-Remarkable, from palest