alignment of the long axes of the colourless phenocrysts and of the small xenoliths, and by the faint streaking of the vitreous groundmass.

The xenoliths in the quartz porphyry were not recognised by Paterson, and when seen in micro-section are rarely more than a millimetre long, and consist chiefly of fine-grained argillite probably derived from the pebbles coming from the higher members of the Kakanui rocks in the Horse Range, which were incorporated in the lowest portion of the Coal Measure conglomerates. Less often they are of fine-grained quartzose schists which have attained about the Chl 2 grade of metamorphism according to Turner's (1938) classification, and were probably derived from the more metamorphosed lower portions of the Kakanui semi-schists. No xenoliths of the Kakanui greywacke or of the typical Chl 4 or even Chl 3 schists of Central Otago, exposed south of the Shag Valley, and in the faulted inlier on the north-eastern side of the intrusion, have yet been recognised in the porphyry.

The argillite xenoliths may form nearly 5% of the bulk of the porphyry. P.8662 is the only slide free from them. They are quite inconspicuous in micro-sections, since their colour is almost that of the glassy matrix, though they are sometimes paler through lack of the faint limonitic staining of that matrix. Their grain-size (∠ 0.002 mm.) is so minute that mutual compensation of the birefringent granules renders them so nearly isotropic that a gypsum plate is necessary to show their granularity. Some are nearly rectangular (∠ 1.0 × 0.4 mm.) and bounded doubtless by close-spaced joint-fractures, though the majority are smaller, have rounded or less regular boundaries. Sections cut transverse to the bedding planes of the argillite show thin close-spaced lamellae containing flakes of pale yellow-green clinochlore, in a few cases as much as 0.15 mm. long. Parallel to these are rarely very thin segregation veinlets of quartz with optic axes perpendicular to the bedding planes. Sections cut parallel to the bedding plane show only a minute mosaic structure, though rarely (5923) there may be scattered rounded granules of quartz up to 0.1 mm. in diameter. In addition to such siliceous xenoliths there are small scanty turbid brown chips of more kaolinitic material.

Schistose xenoliths are more varied, and two groups of these may be distinguished. In some (5923, 5925) [see Fig. 2] the grain-size (0.02–0.03 mm.) is much smaller than all but the most finely granular or lowest grade Chl 2 of the quartz-albite-chlorite-muscovite schists and the structure more hornfelsic than schistose. The dominant minerals are quartz with albite (?) and biotite and little or no muscovite, suggesting that the rock has undergone some thermal metamorphism and that combination of chlorite and muscovite has occurred, as in the case of the more coarsely granular and relatively high-grade schists of the biotite zone (Turner, 1938), though the heat, not of plutonic magma, but of the quartz-porphyry magma may have been effective here. Small xenoliths of hornfelsic to semi-schistose quartz-rich rock free from biotite (e.g., Fig. 5922) may have a like origin.

Fig. 2.

• 5925. Quartz porphyry with glassy matrix (wide-spaced dots) containing irregular vesicles (V), phenocrysts of quartz (Q), plagioclase (P), and biotite. Small xenoliths (X) of argillite and semischist. Only a few of each are lettered.

• 5091. Quartz porphyry with large xenolith of schist.

• 5922. Quartz porphyry with abundant bauerite pseudomorphs (B¹) after biotite, and a small hornfelsic xenolith.

Schistose xenoliths of the second group have more strongly marked schistosity (e.g., in 5091, Fig. 2), and cut transversely show an approach to augen- or mortar-structure with relatively large (∠ 0.05–0.40 mm.) residual, marginally granulated and optically strained grains of quartz and feldspar set in finely crushed but partly recrystallised minerals divided by impersistent, crumpled and irregular laminae composed of minute flakes of muscovite and very

little chlorite, and rendered turbid by carbonaceous and limonitic particles. Large (∠ 0.3 mm.) flakes of muscovite may lie transversely across the schistosity or the lamellation. In (5936), the dark grey angular xenoliths contain porphyroblasts (∠ 0.15 mm.) of biotite lying obliquely across the schistosity plane of a fine-grained matrix containing relatively large (∠ 0.05 mm.) angular grains of quartz and feldspar, and extremely abundant flakes (about 0.02 mm.) of weakly pleochroic biotite grouped in laminae alternating with laminae in which almost colourless to pale greenish-brown mica is abundant. The rock-structure approaches that of rocks in the Chl 2–3 grade of metamorphism, the highest exhibited by the older rocks in the Kakanui Range. The largest xenolith (5 × 5 mm.) seen in the quartz porphyries is of this nature (Fig. 2, 5091).

The quartz porphyry has an almost to completely istotropic matrix making over 80% of most rocks, faint buff in colour with refractive index 1.508–1.513 ± 0.001 as determined by Dr. Hutton, such as may distinguish rock-glasses with 70–67% of SiO₂ according to Tilley's (1922) and George's (1924) observations. Difficulty in measuring this index was occasioned by the presence of crypto-crystalline material and minute indeterminate feldspar laths. The presence of kaolinitic material and limonitic staining also decreases the translucency. There may occur also indeterminate colourless almost isotropic sharply-bounded prismoids (∠ 0.03 × 0.01 mm.), or fragments with embayed margins sometimes recalling “bogen” structure in vitric tuffs, and others larger and less regularly shaped, the last possibly being what Paterson termed “cavities filled in by zeolites”. These have a lower refractive index than the glass and contain irregular platelets which rarely are almost as birefringent as quartz. The suggestion is tentatively advanced that they consist in the main of opal in part changed to chalcedony.

The most abundant phenocrysts are quartz, more or less corroded, and in some cases derived from bi-pyramidal crystals up to 2 mm. long. Others are sharply angular and of varying form, including many small splinters with elongation less often perpendicular than parallel to the optic axis. These, and the irregularly-bounded fragments occasionally show optical strain.

Much less abundant but varying in amount and smaller (∠ 0.8 mm.) are phenocrysts of oligoclase about An₂₁ rarely showing a core of An₂₉, sometimes in idiomorphic prismoids but nearly always with fractured boundaries, occasionally seeming to have been dragged out a little into sub-individuals by movements in the stiffening magma. A few irregular and fractured grains doubtfully referred to sanidine are present.

Biotite in hexagonal plates up to 20 or 30 mm. in diameter forms 1–3% of most rocks. It is generally fresh, strongly pleochroic, with slight marginal zoning or borders darkened by reaction with the glassy ground-mass. The highest colour is a deep yellowish or greenish brown. The optic axial angle is very small and slight chloritisation occurs in some cases. Many transverse sections of biotite display bending, slight corrugation or fraying out, especially of the smaller flakes. The commonest alteration is an incipient