I may further remark that the oligoclase is often free from this species of alteration, which is localized in the orthoclase, and, above all, in the microcline. Figure 4 illustrates the principal modes of quartzification.

PEGMATITES.

The pegmatites are found in Ceylon, and in the neighbourhood of Salem, at Petalia [53 miles (22 lieues) south-west from Salem], and at Perindorél: they are largely crystallized, composed of microcline of bright pink tints, or green (amazonstone), oligoclase, albite, and a black mica in large plates.

The pink microcline is often mixed with oligoclase; the two feispars being associated in the usual manner, with cleavages common to the two minerals. Together with quartz it forms graphic pegmatites. The cross-hatched twinning of the green microcline (amazon-stone) is extremely fine and close. Sections parallel to oP.(001) produce, in parallel polarized light, an appearance resembling that of a finely crumpled texture: sometimes, even, the bands are so narrow that there is but an uncertain extinction.

Where the bands are sufficiently wide, one may observe the characteristic extinctions of about 15° on oP.(001) and about 7° on ooPoo.(010). Inclusions of albite are very abundant.

At Petalia, near Salem, the oligoclase is white, and, together with white quartz, forms a fine graphic pegmatite. The extinctions in relation to the edge oP. ∞∞.(001)(010) are about 1° on oP.(001) and about 8° on ooPoo. (010). On a plate parallel to ooPoo.(010) the axes appear to the eye symmetrically placed with reference to the positive bisectrix c, which is nearly perpendicular to the plate. This oligoclase belongs to the second class of M. des Cloizeaux.2

It frequently encloses the little patches of microcline (Fig. 5)

1' Perunduré' (Keith Johnston's Royal Atlas) is about 45 miles south-west of Salem, in the Koimbatur district. It is spelled 'Peranduray' in the Preliminary Sketch of the Geology of India.-F. R. M.

2 Bull. Soc. Minér, VII (1884), 249.

In the same rock (gisement) a fine white albite is found which cannot be distinguished by the naked eye from the preceding felspar. It occurs in large translucent plates, which are often cavernous, and, in such cases, filled with geodes of transparent quartz, and little hexagonal lamina of white mica.

This mineralogical association is identical with that which occurs so frequently in the pegmatites of Norway, and notably at Gartà, near Arendal. The hemitropebands on the albite-type are large and regular.

The extinction-angle in relation to the trace of this twinning is about 4° on OP.(001) and about 20° on ooPoo.(010). The positive bisectrix is sensibly normal to ooPoo.(010).

Numerous inclusions are observable in this felspar, comprising either opaque inclusions, generally distributed in the cleavage-fissures, or those of white micas. The latter occur in the form of very thin spangles disposed in three directions; some very small ones lie in the planes oP.(001) and ooPoo.(010). The others, which are much larger, are parallel to the faces of a prism of 115°. In microscopic sections cut parallel to oP.(001) these laminae of mica may be seen in the form of long rods, intersecting at angles of 115° and 65°. The trace of the albite-twinning and the inclusions lying parallel to the trace of the cleavage ∞Poo.(010) [are parallel to] the bisectrix of the angle of 115°. The micaceous laminæ seem to mark the direction of a difficult cleavage. The angle given as 115° is a little variable, but is never as much as 117°. It therefore does not correspond to the prism co'P.P'.(110)(110), which has an angle of 120° 47', but to a prism of the same zone. The white mica which accompanies this albite forms little greenish-yellow rosettes. The plane of the optic axes is perpendicular to Poo. (010).

2E = about 72°.

De Bournon's collection includes a felspar which is opaque-white in mass, and remarkable for its pearly and silky lustre. When examined in microscopic sections, it is seen to possess the properties of normal orthoclase (orthose non déformé): the double refraction is more feeble than that of ordinary orthoclase. The cleavages OP.(001) and ooPoo.(01c) are very well marked, and their directions are often indicated by amorphous matter.

It has become very fragile, and merely by pressure with the finger breaks into little parallelopipeds. It contains opaque, extremely fine inclusions in considerable quantity.

We have observed an analogous mode of alteration in the felspar of a hornblendic gneiss forming the escarpments of Cape Palmas, at the mouth of the river Cavalla, on the coast of Maryland, to the south-east of Liberia (West Africa). This rock was collected, in 1885, by Dr. Jullien, who has been good enough to send it

to us.

In its exterior aspect it bears much resemblance to the hornblendic gneisses of Ceylon. The milky-white felspar is disseminated in grains through the dark green hornblende. In some largely crystallized beds the former mineral pre

dominates.

The microscope shows that the rock is composed of sphene, hornblende, oligoclase, orthoclase and quartz.

'A. Lacroix: "Sur l'albite des pegmatites de Norvège" Bull. Soc. Minér., IX (1886), p. 131.

The extinction angle on ooPoo.(010) is about 22°. The mineral is often fissured, and stained with yellowish ferruginous products.

The felspars, occurring in rounded grains, do not include any of the alterationproducts habitual to such minerals: they are often much cracked, but the fragments are limpid, with the double refraction unchanged. The sinuous fissures which traverse the crystals, all remain dark in parallel polarized light. When the mineral is examined in ordinary light, a fibrous substance may be observed on both sides of the fissures, penetrating the mineral from thence, in the same way as the micaceous alteration-products of cordierite. In some cases the entire crystal is invaded by this substance, when it no longer has any action on polarized light. The composition of this isotropic substance is unknown to us (Fig. 6).

To the left in polarized light; to the right in ordinary light (orthoclase, oligoclase, quartz and hornblende).

Besides these particular phenomena of alteration we may notice the occasional formation of secondary quartz, in little grains along the fissures just described.

It is in pegmatites similar to those which have been described above that the moonstone' variety of orthoclase occurs in Ceylon; it is so well known that further remarks are unnecessary.

All these felspars contain, in greater or less quantity, fusiform inclusions of unknown nature, which have been described as occurring in the felspars of the granulitic gneisses.

The collection in the Museum includes a fine specimen, from Perindoré, of a bright brown phlogopite, with the two axes nearly coincident.

Muscovite, also, is found, in large plates, penetrated by numerous patches of quartz, and forming a sort of pegmatite with the latter mineral.

Leschenault notices the occurrence of beryl (aquamarine) in these rocks (Salem).2

BASIC GNEisses.

(a). Pyroxenic and hornblendic gneiss.

These gneisses are especially well developed in the neighbourhood of Kandy. They are intercalated with granulitic gneisses and leptynites, into which they frequently pass by the addition of quartz and the disappearance of pyroxene.

Nevertheless, this rock is sufficiently constant in its composition, and forms one of the most characteristic types of that region: it greatly resembles the lower pyroxenic gneisses of Odegarden, in Norway.

These gneisses possess a high specific gravity and a dark colour.

Sometimes they are composed almost entirely of dark-coloured minerals (éléments noirs), which produce a stratiform structure; sometimes, on the other hand, the abundance of granular felspar masks the orientation of the ferruginous minerals.

Black hornblende, dark red almandine garnet, and a yellowish-white felspar may be distinguished with the naked eye. As at Bamle, in Norway, pyroxene can only be detected with the microscope: as at the same locality, also, the rock is but slightly coherent, and breaks easily, although its constituent minerals are unaltered. From this point of view, therefore, the rock bears some resemblance to the gneisses enclosed in volcanic rocks.

In addition to the preceding minerals the microscope discloses apatite, pyroxene, magnetite, and a little quartz.

Apatite is of frequent occurrence, in hexagonal prisms, some of which attain the size of half a millimetre. The magnetite has no definite form. Quartz is present only as quartz de corrosion. The pyroxene is remarkable for its intense pleochroism. Sometimes in rounded grains, sometimes in irregular crystals elongated parallel to the edge of the zone ooPoo.∞∞.(100) (010), it possesses the cleavages ∞P. ∞P.(110)(110) which are sometimes very regular. In some cases there are also traces of cleavages parallel to ooPoo. (100) and oP.(001), usually indicated by a considerable number of little rod-shaped, greatly elongated, opaque inclusions, which are only visible under high powers.

Besides these cleavages, there are numerous fissures which are filled by an isotropic ferruginous substance. Inclusions of magnetite and hornblende, and, very rarely, those of apatite, may also be observed.

The pyroxene is very transparent in microscopic sections; the polarization-tints are limpid.

The pleochroism is intense, very similar to that observable in hypersthene. We have

£, Sea-green.

b, Bright pink.
a, Yellowish green.

The absorptions parallel to c and to a differ but little. It follows from this that

1 Op. cit.

' Was M. Leschenault's beryl from Paddoor or Kangiam in Koimbatur? The latter town is about 20 miles south of Peranduray. See Manual of the Geology of India, III, 520.- F. R. M.

с

sections parallel to Poo.(010), containing c and a, are but slightly pleochroic, showing green tints.

The colour observed when the rays traversing the plate vibrate parallel to b is nearly identical with that of the garnets, which are very abundant in the rock. The maximum extinction on ∞∞.(010) is about 45°.

The properties of the pyroxene nearly resemble those of the Arendal coccolite.

The hornblende is remarkable for its most intense pleochroism. It appears absolutely black to the naked eye, and occurs in crystals elongated in the direction of the zone oo Poo.00Poo.(100) (010), and sometimes attaining a size of 15 millimetres. The cleavages P.∞P.(110)(110) are very well marked, giving highly reflective surfaces.

The angle ooP.AooP. 124°15'

=

In microscopic sections these cleavages are very straight; the transversal fissures are much less frequent than in the pyroxenes. The pleochroism is as follows:

c, Brown-green; nearly black;

b, Greenish brown; nearly black;

a, Bright yellow,

with sensibly b> a.

:

The properties of the mineral are those of basaltic hornblende. The extinction angle on oooo.(010) is very small (3° or 4°).

The double refraction is strong.

but difficult to measure, on account of the colour of the substance.

ya about 0'31.

Biotite is rare, and occurs in little aggregations around the magnetite. The garnet is of the almandine variety of a very dark red in mass, it is bright pink in microscopic sections.

The felspar occurs in rounded grains without definite form. It is always remarkably unaltered, and free from microscopic inclusions. To the naked eye it is vellowish, this colour being due to ferruginous products, which insinuate themselves between the crystals, or into the numerous fissures by which the latter are intersected.

They are purified, and rendered absolutely colourless, by treatment for some minutes with boiling hydrochloric acid: the portion attacked includes a little silica, alumina, ferric oxide, and traces of lime.

The optical properties are those of an andesine. As the size of the crystals is generally less than half a millimetre, it is impossible to determine the extinction. upon the faces oP.(001) and ooPoo.(010) with precision.

After breaking some of the crystals, and collecting the cleavage-plates thus obtained on a glass slide, I was unable, with certainty, to recognize traces of a second cleavage in any of them.

Assuming that such is in the direction of the most frequent fissures, the extinction-angle referred to their trace does not exceed 2o. In the zone of symmetry normal to oooo.(010) the extinction-angles attain, but do not exceed, 20° on each side of the twinning plane on the albite-type.

Some crystals do not present a uniform extinction in parallel polarized light, but