A Contribution to the Petrography of Benguella, Based on a Rock Collection Made by Professor J

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XIV.—A Contribution to the Petrography of Benguella, based on a Rock Collection made by Professor J. W. Gregory. By G. W. Tyrrell, A.R.C.Sc, F.G.S., Lecturer in Geology, Glasgow University. Communicated by Professor J. W. GREGORY, F.R.S. (With One Plate.)

(MS. received June 2, 1915. Read June 28, 1915. Issued separately August 11, 1916.) Very little is known of the petrography of the Portuguese West African colony of Angola, and of its three provinces that of Benguella is probably least well known in this respect. Granites, gneisses, schists, limestones, and red sandstones have been recognised by the earlier observers of the geology of the region, and detailed descriptions of rocks from Angola, and especially of its northern province, Loanda, have been given by BERG* and HOLMES.t From Benguella itself J. P. GOMES\ has described r the collection of R. P. LECONTE, w hich contained granites, amphibolite, adinole with epidote veins, various schists, quartzites, diabase-porphyrite, and basic eruptives. Senhor J. P. BE NASCIAMENTO§ in 1912 recorded granite, diorite, basalt, and quartzite on the Benguella plateau. During his recent journey in Benguella, Professor GREGORY collected about a hundred and fifty specimens of igneous rocks or their metamorphic derivatives, and these offer a most interesting and varied study. This paper is concerned only with their petrographic character ; for their geological relations the reader is referred to the preceding paper by Professor GREGORY. For descriptive purposes the classification given below has been found to be most natural and convenient. The order in which the rocks are arranged also closely follows their sequence in time :— I. Basement gneisses and schists. II. Charnockite series. III. A series of hornfelsed porphyries and other hornfels. IV. Granites, granodiorites, and associated rocks. V. Rhyolite (dellenite). VI. An alkaline series, including nepheline-sodalite-syenite and other syenites, shonkinite, solvsbergite, and ouachitite. VII. Basic intrusions. I am much indebted to A. SCOTT, M.A., B.Sc, for two excellent chemical analyses of rhyolite and shonkinite respectively. * BERG, G., "Gesteine von Angola, Sao Thome, und St Helena," Tscher, min. u. petr. Mitth., xxii (1903), pp. 357-362. t HOLMES, A., " Contribution to the Petrology of North-western Angola," Oeol. Mag., dec. vi, vol. ii, pp. 228-232 ; 267-272 ; 322-328 ; 366-370, 1915. I GOMES, J. P., " Echantillons de roches recueillis entre Benguella et Catoco," Comtn. Dir. Trab. Geol. Portugal, vol. iii, fasc. 11, pp. 239-243, 1898. § NASCIAMENTO, J. P. DE, and MATTOS, A. A. DE, A Golonisagao de Angola, Lisbon, 163 pp., 1912. TRANS. ROY. SOC. EDIK, VOL. LI, PART III (NO. 14). 78 538 MR G. W. TYRRELL.

I. THE GNEISSES AND SCHISTS. The gneisses are predominantly quartzo-felspathic varieties with biotite as the principal ferro-magnesian constituent. They are medium- to coarse-grained, pink or grey, well-foliated rocks, with the biotite aggregated into thick clots or folia, or less often evenly spangling the rock. Hornblendic gneisses occur, but are much less common than the biotite-bearing varieties. In thin section the biotite-gneisses are coarse-grained aggregates of quartz and potash-felspar with folia of biotite, and show marked dynamo-metamorphic effects. The larger quartz crystals are broken up into a comparatively coarse granular mass of angular fragments; but the felspars have only suffered a fine peripheral granula- tion, the products of which, mingled with quartz, form a kind of groundmass in which the larger, more or less rounded, "eyed" crystals of felspar are set. The felspar is prevailingly microcline. Orthoclase is subordinate, and a little oligoclase is occasionally present. The felspars show the effects of crushing in the irregularity, curvature, and fracture of their twin striations. The biotite is of the common yellow variety, and occurs in large clots or folia consisting of small flakes felted together with minute grains of quartz, epidote, and occasionally a little muscovite. Felspar is absent from these aggregates. The above description refers particularly to a rock from the Bui River, S. of Ochilesa (185),* which may be taken as typical of the predominant biotite-gneisses. Other rocks of the- same type occurring at Bailundo (152) are very rich in " gitter" microcline. Rocks from the Lengwe Gorge (105, 106, 109) are decidedly richer in soda-lime felspars (oligoclase or albite-oligoclase), and show a development of minute, colourless, euhedral crystals within the felspars. These appear to be of two or three different kinds. Some are clearly small zircons, but others are referred to clinozoisite and epidote. In some types (e.g. (156), W. branch of Lovule River), the foliation is not so definite as in the above-described rocks. The texture is evenly granular and the biotite is uniformly distributed. The granulitic texture becomes more pronounced in a rock from the Benguella Railway, 55 kilometres, B. of San Pedro (114). In this specimen the biotite occurs as thin leaves which appear as narrow bands of biotite flakes in thin section, and produce a bacillar type of foliation. This rock has some resemblance to the Moine gneisses of the Scottish Highlands. Biotite becomes very scarce and the cataclastic texture very prominent, in a pink, granulitic, quartzo- felspathic gneiss from Kambengi (195). The rocks described above are all orthogneisses derived by dynamo-metamorphism from ordinary biotite-granites. These rocks grade into biotite-schists, e.g. at 55 kilometres, Benguella Railway (115). The texture becomes more even and finer-grained, the amount of quartz and biotite increases, whilst that of felspar diminishes, in comparison with the gneisses described above. Another mode of transition is into a * The numbers refer to trie registration numbers of the rocks in Professor GREGORY'S collection. A CONTRIBUTION TO THE PETROGRAPHY OP BENGUELLA. 539

group of gneisses with very pronounced cataclastic textures, showing all gradations to mylonite. In the first stage the pseudo-porphyritic " eyed "..felspar becomes highly rounded, and the quartz is broken into coarse-grained aggregates of angular fragments. Both minerals are set in a minutely crystalline paste of crushed quartz and felspar (190, N. of Saccanjimba). In succeeding stages all the quartz is reduced to mylonite, and the crushed groundmass is thus increased at the expense of the intact crystals of felspar. The latter are much reduced in size, show angular outlines, and are traversed by lines of mylonised material (173, W. of Ochilesa). The ferro- magnesian constituents, such as biotite, have been crushed out of recognisable existence, and appear now as aggregates of chlorite and limonite. A few large crystals of anthophyllite, now in process of alteration to talc, have been formed in the Ochilesa rock. The three hornblendic gneisses in the collection differ considerably from those described above. They occur towards the lower end of the Lengwe Gorge (1056), N. of the Saccanjimba Mission (189), and in the river W. of Kambengi (197). In thin section they consist of a coarse foliated aggregate of hornblende and plagioclase felspar, with subordinate quartz, orthoclase, and biotite. The ferro-magnesian minerals bulk more largely in these rocks than in the biotite-gneisses. The hornblende is a strongly pleochroic variety, of strong absorption, and with colour extremes of bluish-green and yellowish-brown. Small felts of biotite and epidote cluster with the larger hornblende crystals. The felspars mostly belong to andesine, and contain colourless needles similar to those described above in the biotite-gneisses of the Lengwe Gorge. Cataclastic structures are also frequent. These rocks are typical diorite-gneisses. Only one basic hornblende rock occurs in the collection. This comes from the river W. of Kambengi (195). It is a fine-grained, dark-green, non-foliated rock, consisting, in thin section, of an evenly granular mass of hornblende crystals of the usual bluish-green colour. The small and sparse interspaces are filled with quartz and decomposed felspathic material. The rock is a granulitic amphibolite.

II. ROCKS OF THE CHARNOCKITE SERIES. These are dark-greenish or greyish fine-grained rocks, in which none of the minerals, save an occasional flake of biotite, are macroscopically identifiable. The outstanding microscopical characters of these rocks are their thoroughly granulitic texture, their pyroxenic composition, and their ideal freshness. The mineral constituents, occurring in varying proportions throughout the series, are a pale-green monoclinic pyroxene, hypersthene, plagioclase, orthoclase, quartz, hornblende, biotite, and magnetite. This assemblage is identical with that of the charnockite series of Southern India.* The invariable presence of hypersthene, the granulitic texture,

• HOLLAND, "The Charnockite Series," Mem. Geol. Surv. India, vol. xxviii, part ii, p. 124, 1900. 540 MR G. W. TYRRELL.

and the almost exact correspondence of petrographic types, make the identity of the two series even more certain. Rocks referable to three of the four divisions into which HOLLAND divides the charnockite series are to be found in this collection, as well as a rock which may be described as a charnockite-porphyry. A rock comparable to charnockite in the narrow sense is found to the W. of Ochilesa (177). It consists mainly of quartz, oligoclase, orthoclase, with subordinate biotite, hypersthene, and monoclinic pyroxene. Both quartz and orthoclase show strain-shadows, and the twinning of the oligoclase is often discontinuous, wedging out-in short distances and then reappearing. The soda-lime felspar is rather more abundant than in the Indian charnockite, and considerably ex- ceeds the orthoclase in quantity (see Table I, l). The biotite occurs in large ragged flakes, and is of a dark-brown, strongly pleochroic variety. Hypersthene is uniformly distributed in typical irregular, prismatic, cross-fractured crystals, with distinct pleochroism and faint lamellar twinning. There is also a pale-green monoclinic pyroxene. This mineral appears in all stages of alteration to a fibrous amphibole which has a distinct pleochroism from yellowish-green to a glaucophane-like blue. The amphibole occasionally forms a narrow zone grown around the hypersthene crystals. The texture of this rock is not so thoroughly granulitic as in the more basic rocks of the series. It may be described as intermediate between granitic and granulitic. In the hand specimen this rock has a distinct gneissic banding, which is, however, not apparent in thin section. A rock from the N. of Etunda (193) is referable to HOLLAND'S intermediate division of the charnockite series. Compared with the foregoing, quartz and orthoclase have so diminished in quantity as to become merely interstitial to the abundant plagioclase, whilst the amount of ferro-magnesian minerals remains stationary. Magnetite shows a large increase (see Table I, 2). Monoclinic pyroxene does not appear in this rock. The hypersthene is thoroughly anhedral, and has an intense pleochroism from pink to bluish-green. The rock also contains a little pink garnet which forms the centres of irregular aggregates of hypersthene and biotite. There is a tendency for the ferro-magnesian minerals to segregate into narrow linear areas, giving rise to a rude banding.* The water-clear plagioclase is richer in the anorthite molecule than in the charnockite proper, and is an andesine of composition Ab4An3. The texture of this rock is thoroughly granulitic. Its normal eruptive equivalent would probably be called quartz-mica-hypersthene-diorite or quartz-mica-norite. From the district S. of Andulo (194a), and from Ochilesa (176b), come rocks strictly comparable with those called " hornblende-augite-norite" by HOLLAND.! These are ideally fresh rocks consisting of a thoroughly granulitic aggregate of water- clear plagioclase (AJbiAnj) with pale-green augite, hypersthene, hornblende, iron-ores, and a little biotite. In the rock from Andulo (194a) the hornblende is a brownish-

* Gf. " Charnockite of the Ivory Coast," LACROIX, Comptes Bendus, cl (1910), p. 20. t HOLLAND, op. cit., p. 157. A CONTRIBUTION TO THE PETROGRAPHY OF BENGUELLA. 541 green, strongly pleochroic variety, and is disposed about the augite in such a way as to indicate that it is due to molecular instability of the pyroxene during or after crystallisation. The hornblende of the Ochilesa rock (1766) is of a reddish-brown tint. Some of the hypersthene shows a close lamellar twinning and a small amount of schillerisation. The rounded grains of iron-ore are embedded in the pyroxenes, and in the augite they form centres from which amphibolisation has started. These rocks are hornblende-hyperites approaching melanocratism, in which the amount of plagioclase and quartz is estimated to be only half that of the ferro-magnesian minerals (see Table I, 3, 4). They belong to HOLLAND'S basic division of the charnockite series. A similar rock (192) occurs near Andulo, and is distinguishable only by its finer grain. HOLLAND'S ultrabasic division of the charnockite series is not represented in this collection, but a thoroughly melanocratic hornblende-hyperite from Ochilesa (176a) forms an approach to that division. The proportion of ferro-magnesian minerals to plagioclase in this rock is estimated at three to one (see Table I, 5) ; otherwise the rock differs little from the hyperite described above. This completes the description of the series comparable with the charnockites of India. One other rock undoubtedly belongs to this series, but has no Indian equivalent. This rock is from the W. of Chieuca (164), and is interstratified with well- foliated gneiss. It may be described as a charnockite-porphyry. It consists of small phenocrysts of quartz, oligoclase, orthoclase, augite, hypersthene, and biotite, pro- fusely scattered over a minute, evenly granular, macrocrystalline groundmass composed mainly of quartz grains with subordinate water-clear felspar. The augite is well schillerised, and has fringes of greenish-brown hornblende. Biotite occurs in large clots consisting of small flakes felted with granules of augite and magnetite. It has exactly the appearance of the new-formed flakes in a hornfelsed rock, and the whole aspect of the rock suggests that it is secondary. Oligoclase forms the largest and most abundant set of phenocrysts. The rock appears to be the hypabyssal equivalent of the Ochilesa charnockite. The granulitic texture, the extreme freshness, and the presence of abundant new-formed biotite, indicate that this rock has suffered some thermal metamorphism. It is linked by these characters, as well as others, to the hornfelsed porphyries described in the next paragraph. The quantitative mineralogical composition or mode of this series has been obtained by the Rosiwal method of measurement on five of the rock types, with the results shown in Table I.

[TABLE 542 MR G. W. TYRRELL.

TABLE I.

1. 2. 3. | 4. 5.

1 Quartz .... 39-2 19-8 2-5 1-7 23 Orthoclase . 8-6 4 3 . . * . . . Oligoclase (Ab8Anj) 28-0 ...... Andesine (Ab4An3) 490 Labradorite (AbjAnj) . 304 324 22-8 Biotite.... 10-7 8-6 ... Pyroxenes 12-6 11-8 491 345 39-5 Hornblende . ... 16-6 : 29-5 28-6 Magnetite •6 5-9 1-4 ! 1-7 6-8 Apatite •3 •6 ... ! -4 Felsic/Mafic ratio . / 75-8 731 329 I 33-9 25-1 1 24-2 26-9 671 I 661 74-9

Summation in each case, 100. 1. Charnockite (177), Ochilesa. 2. Charnockite, intermediate variety (193), N. of Etnnda. 3. Hornblende-hyperite (194a), S. of Unyamba River, Andulo. 4. Hornblende-hyperite (1706), Ochilesa. 5. Melanocratic hornblende-hyperite (176a), Ochilesa.

By calculating the ratios of the felsic and mafic constituents,* it is seen that the series falls into two well-marked groups, one dofelsic, consisting of the charnockites proper and the intermediate varieties ; and other domafic, consisting of the hyperites. No rocks intermediate between these groups occur in the collection. The first group is mineralogically distinguished by the abundance of quartz and the presence of orthoclase and biotite.. In the domafic group, quartz, while still present, is in extremely small quantity, orthoclase and biotite have disappeared, and pyroxenes and hornblende constitute the bulk of the rocks. Whilst belonging to the ancient. basement of the Benguella plateau, the char-

* The terms felsic and mafic (CROSS, IDDINGS, PIRSSON, and WASHINGTON, Journ. Oeol., xx, 1912, p. 560) are used to denote two main groups of rock-forming minerals, one containing quartz, felspars, and felspathoids, and the other pyroxenee, amphiboles, micas, olivine, iron-ores, etc. Variations in the relative proportions of these groups give rise to the important variations in igneous rocks denoted qualitatively by BROGGER'S terms, leucocratic and melanocratic. A quantitative meaning may be imparted to the terms felaic and mafic by attaching the prefixes do- and per-, with exactly the same connotation in respect to the mode (actual mineralogical composition) as the analogous terms persalic, dosalic, etc., have in respect to the norm (standard mineral composition) of the American Quantitative Classification. Perfelsic Felsic constituents 7 Mafic constituents •"* 1 Dofelsic

Mafelsic

Domafic *•

Permafic . A CONTRIBUTION TO THE PETROGRAPHY OF BENGTJELLA. 543 nockite series is unquestionably intrusive into the gneisses and schists of the basement. It is interesting to record that LACROIX has described a very similar series of charnockites from the Ivory Coast.* These rocks cover a great area in a region on the frontiers of French Guinea, the Ivory Coast, and Liberia. They form a series ranging from hypersthene-granite very poor in ferro-magnesian minerals, through quartz- norites, to norites with 50 per cent, of hypersthene. These rocks are identical with those described above with the single exception that hornblende does not occur to any extent in the more basic types. The country rock is stated to be granitic.

III. HoRNFELSED PORPHYRIES AND OTHER HoRNFELS. In this section is described a curious and puzzling group of rocks, which, though unmistakably igneous rocks of an acid character, appear to have suffered considerable thermal metamorphism. The evidence for this lies in their perfect freshness, the recrystallisation of the groundmass into an evenly granular, water-clear mosaic, and the presence of abundant new-formed biotite, characters superposed upon their original igneous characters. The rock described as charnockite-porphyry, although of somewhat different mineral composition, has exactly the same characters as these rocks, and must be included in their discussion. It might also be remarked that the rocks of the charnockite series themselves present features, such as granularity, ideal freshness, and presence of biotite, which might be interpreted as affording evidence of thermal metamorphism. On the other hand, the mineral composition of these porphyries, especially the common occurrence of magnetite-cassiterite intergrowths, links them to the granites described later. Finally, associated with these porphyries, and indubitably of thermometamorphic origin, are rocks determined as scapolite- epidote-hornfels and cordierite-biotite-hornfels. In hand specimens these rocks may be described as " dark porphyries." They generally show a fine-grained, but obviously crystalline groundmass of a dark-bluish colour, with the peculiar saccharoidal texture of hornfels and granulites, and contain dark phenocrysts of felspar. One or two examples are light-coloured, and are aplitic or halleflintas-like in appearance. The most typical example of these rocks is a specimen from N.E. of Bailundo (159). In thin section this shows phenocrysts of felspar together with peculiar clots or felts of hornblende, biotite, magnetite, and cassiterite, embedded in a microcrystalline, granulitic mosaic consisting mostly of quartz, and necked with anhedral shreds of biotite. A plagioclase felspar forms by far the most abundant set of phenocrysts. It is extremely zonal and is characterised by minute, indefinite, bent, and discontinuous twin-lamellse, which renders the exact determination of its composition

* A. LACROIX, "Sur l'existence a la Cote d'lvorie d'un series p^trog. comparable a celle de la charnockite," Comptes Rendius, cl (1910), pp. 18-22. 544 MR G. W. TYRRELL.

difficult. From the imperfect determinations possible it appears that the range of

composition is from Ab45An55 to Ab75An25, but the bulk of the felspar is probably near the latter composition. In thin section the felspars have a bluish tinge owing to the abundance of minute, dust-like inclusions. They appear to have suffered magmatic corrosion, as their angles are rounded off, and they have been worn into curious irregular shapes. The next most abundant phenocryst is a deep-green, strongly pleochroic hornblende forming irregular plates which poikilitically enclose numerous rounded grains of quartz. Frequently the crystals contain a core consisting of irregular grains of quartz and magnetite, devoid of the green hornblende. The plates frequently retain optical continuity, but occasionally they have been broken up into individual rounded grains intermixed with the quartz mosaic. Next to the hornblende in abundance are the magnetite-cassiterite intergrowths. These generally consist of a highly irregular core of magnetite invested by a thin sheath of cassiterite. The latter mineral is easily recognised by its uniaxial character, its well-marked twin-lamellation, and decided pleochroism from yellow to reddish-brown. Biotite is evenly spread as minute anhedral shreds amidst the quartz mosaic, but is more prominent as a constituent of curious clots or felts of the coloured minerals, which are a feature of the rock. These clots consist of the magnetite-cassiterite intergrowths, with numerous flakes of biotite, and the poikilitic hornblende, which occasionally envelops the other constituents. The groundmass consists of an even mosaic of quartz grains with subordinate water-clear untwinned felspar. The whole aspect of this rock, as described above, is suggestive of recrystallisation at a not very high temperature. It is clearly a hornfelsed porphyry belonging to the granodiorites, although quartz does not occur among the phenocrysts. A rock from N. of Huambo (148) differs from the above in several particulars. Amongst the phenocrysts quartz is abundant, and orthoclase is-at least as abundant as oligoclase. Hornblende is absent, but large flakes of muscovite enter as a constituent of Very closely felted clots consisting mostly of biotite flakes. The groundmass is of the same nature as that of the Bailundo rock, but is of much denser texture. This rock is clearly a granite-porphyry. In another specimen from the same locality (145) the secondary biotite is very abundant, and epidote enters the biotite felts as a prominent constituent. From 307^ kilometres on the Benguella Eailway (124) comes a specimen of the same nature, but the biotite forms wavy, sub-parallel streaks, which, in thin section, give a semi-foliated aspect to the rock. A rock from N. of Cruz's (233) clearly belongs to the same group as the above. It contains, however, no quartz phenocrysts, and only a few of felspar. The latter are highly altered and are now little more than masses of granular epidote. The new-formed biotite is not aggregated into clots, but is scattered evenly over the quartzose groundmass. A CONTRIBUTION TO THE PETROGRAPHY OF BENGUELLA. 545

Scapolite-epidote-hornfels.—This rock, which occurs E. of Chingwari (199), is a dark porphyry of appearance similar to that of the rocks described above. It consists mainly of a mosaic of quartz, biotite, and epidote, in which the first-named is perhaps the most abundant constituent. Magnetite and blue tourmaline form subordinate members of the mosaic. Embedded in this groundmass are occasional phenocrysts of andesine, witnessing to the original igneous character of the rock. These crystals frequently enclose the biotite and epidote of the groundmass. There are also numerous irregular areas of scapolite which are crowded with grains of the mosaic groundmass. The mineral is easily recognised by its bright second-order polarisation tints, its cleavage, straight extinction, and uniaxial character. Whilst the poikilitic areas frequently retain optical continuity, they are occasionally broken up into grains with different orientations. The abundant biotite presents no unusual optical features, and is very closely intergrown with epidote. This rock is clearly the pro- duct of combined contact-metamorphism and pneumatolysis. Cordierite-biotite-hornfels.—This rock occurs seven miles E. of Candumbo (230) and in hand specimen is a dark-bluish, finely crystalline granulite. In thin section it is seen to consist of a thoroughly granulitic groundmass of cordierite, quartz, together with abundant biotite, muscovite, and magnetite. The cordierite forms rounded sections, occasionally showing lamellar twinning, and the frequent development of a strong yellow pleochroism around minute inclusions. It is undergoing an incipient alteration to small flakes of white mica. The biotite is intergrown with muscovite. The magnetite is evenly distributed in minute subhedral grains throughout the rock. Occasional areas of turbid material with traces of multiple twinning suggest the presence of lime-soda felspar. The original nature of this rock is not so evident as in the foregoing. It was probably an impure sandstone or greywacke, and was almost certainly not of igneous origin, as it is devoid of the felspar phenocrysts characteristic of the types above described. The rocks described in this section appear to form part of the ancient basement of the Benguella plateau, into which the batholiths of tin-bearing granite described later were injected. They are interstratified with and injected into gneiss, and into the quartzites, quartz-schists, and sillimanite-schists of the Bailundo series. They formed a series of acid igneous rocks associated with various sediments. The presence of minerals such as cassiterite, tourmaline, and scapolite, suggest that the alteration they have undergone was due to pneumatolysis as much as to contact-metamorphism. The cassiterite-magnetite intergrowths of the Bailundo rock are identical with those occurring in the Benguella granites (p. 547), suggesting that this mineral association, together with scapolite and tourmaline, were pneumatolytic introductions derived from the Benguella granite batholiths, and that the peculiar modifications these rocks have undergone were consequent upon the intrusion of these great masses of molten material. The characters of this group of rocks suggest comparison with the " leptites" TRANS. ROY. SOC. EDIN., VOL. LI, PART III (NO. 14). 79 546 MR G. W. TYRRELL. and granulites of Finland and Sweden, which include rocks derived from various porphyries, along with cordierite, andalusite, and scapolite-bearing rocks.* Their characters are supposed to be due to " immersion " in a great granite batholith. It may also be noted that scapolite rocks occur along with the charnockite series in Peninsular India, t

IV. GRANITES, GRANODIORITES, AND ASSOCIATED ROCKS. These are fine- to coarse-grained rocks of white, pink, or grey colour, and of comparatively uniform texture. In only one specimen is there any tendency for felspar to assume a porphyritic development. Hornblende is macroseopically prominent in some of the granodiorite specimens. Under the microscope two quite distinct types may be distinguished. The most abundant group, that of the granites proper, are distinguished by an early generation of euhedral striped felspar approaching albite in composition, and usually much altered with the development of white mica. These crystals are prominent as white opaque areas in hand specimens and slides. Rocks of this type occur on the Benguella Railway at Kunhungamua (138), Portella Coroteva (116), and Caimbango (123), in the district of Quingenge (248, 249, 252), on the Cuiva River (242), and W. of Chingwari (231). They consist of a coarse hypidiomorphic aggregate of fresh microcline and quartz, enclosing the above-mentioned early crystals of albite. The quartz also occurs as rounded blebs embedded in the microcline. The only ferro- magnesian constituents are biotite, partially altered to chlorite and epidote, and a little magnetite. The quantitative mineral composition of the Quingenge rock, as estimated by the Rosiwal method, is given in Table II. It may stand as a good average sample of the Benguella granites, which may therefore be said to consist of about 50 per cent, of microcline, 10 of albite, 35 of quartz, and 5 per cent, of biotite and iron-ores. The textures are typically granitic and vary in grain-size from coarse (Benguella Railway) to fine (Chingwari and the Cuiva River). The second type is a coarse granitoid rock indistinguishable in general appearance from those described above. In thin section, however, they are seen to contain no albite, the place of which is taken by abundant fresh plagioclase felspar of average

composition Ab7An3 (acid andesine). They are further distinguished by the presence of hornblende along with biotite (although the latter remains the dominant coloured constituent), and by a larger proportion of the ferro-magnesian constituents. These rocks correspond to the Continental tonalites and to the American granodiorites. They occur at kilometres 135f, 223, and 238 on the Benguella Railway (119, 121, 123), and at a locality between Quingenge and the Cuiva River (244). The rock from kilometres 223 on the Benguella Railway is a fresh typical sample of this type. It consists mainly of quartz and oligoclase-andesine in hypidiomorphic relations, with * P. ESKOLA, " Petrology of the Orijarvi Region in South-western Finland," Bull. Comm. Geol. de Finlande, No. 40, 1914, pp. 14, 15, 131. t HOLLAND, op. cit., p. 232. A CONTRIBUTION TO THE PETROGRAPHY OF BENGUELLA. 547

subordinate alkali felspar, mostly microclme, with a little orthoclase. The ferro- magnesian constituents are biotite, and a green hornblende, which, in the position of greatest absorption, shows a distinct tinge of blue. Magnetite, cassiterite, and apatite occur sparingly, and in one place are associated in a small intergrowth like those in the hornfelsed porphyry from Bailundo (see p. 544). Cassiterite also occurs in the rock from kilometres 238, Benguella Railway, where it is associated with little clots of biotite. Epidote and chlorite occur sparingly as secondary products derived from the alteration* of biotite. The quantitative relations of the minerals, as determined by the Rosiwal method, are set out in Table II. A typical Benguella granodiorite therefore contains about 25 per cent, of quartz, 15 per cent, of alkali felspar, 50 per cent, of oligoclase-andesine, and 10 per cent, biotite, hornblende, and iron-ores. The typical American granodiorites are somewhat richer in the mafic constituents.* Table II shows the contrast between the two types of granitic rocks in Benguella. The granites proper are rich in microcline and albite, whereas the granodiorites are poor in alkali felspar, and contain nearly 50 per cent, of oligoclase-andesine. Along with this they are decidedly poorer in quartz, and richer in the mafic constituents. The granodiorites contain cassiterite, but this mineral is in too small quantity to come under the Rosiwal measurement, and its amount must therefore be considerably less than 1 per cent. TABLE II.

1. 2.

Quartz .... 359 26-5 Microcline and orthoclase 47 3 15-4 Albite (Ab) . 105 Andesine (Ab7A.n3) . 484 Biotite and chlorite . 3-9 5-5 Hornblende 1-6 Magnetite 2-4 1-5 Epidote . 11 Felsic/Mafic J 93-7 90-3 1 63

1. Biotite-granite, E. of Quingenge, Benguella (248). 2. Biotite-granodiorite, kilometres 238, Benguella Railway (123).

Of especial interest in this connection is the occurrence of a stanniferous selvage to the Kunhungamua granite. This rock (139) is an even-grained microgranite consisting of quartz and orthoclase, with the latter distinctly in greater quantity. The orthoclase frequently has the mottled and moire appearance which indicates a considerable content of the albite molecule. The dominant ferro-magnesian constituent is a greenish-brown biotite, but there are also a few large ragged patches of a bleached * "Geology and Ore Deposits of Butte, Mont.," U.S. Geol. Surv., Prof. Paper No. 74,. 1912, p. 36. 548 MR a. W. TYRRELL. green hornblende. Cassiterite occurs rather abundantly in spongy anhedral crystals which enclose quartz and felspar. It is yellowish-brown in colour, distinctly pleochroic, and shows a fine lamellar twinning. It is commonly associated with small grains of magnetite. No other mineral of the tin association is found, except, perhaps, one faint purplish section doubtfully referred to axinite. The only specimen which may be regarded as the hypabyssal equivalent of the granites is a coarsely porphyritic rock from E. of Lepi (224), which shows numerous large white felspars in a dense greyish groundmass. In thm section the felspar phenocrysts are seen to form at least half the rock. They are much altered, with the formation of white mica and epidote. Hence they belong to an acid soda-lime felspar (probably oligoclase), a conclusion strengthened by the occasional retention of multiple twinning. These phenocrysts frequently have a ragged external zone of orthoclase which interpenetrates with the groundmass. The only other phenocrysts are large, partially chloritised crystals of hornblende. This mineral also appears in the dense groundmass, which, in the main, is a fine micrographic intergrowth of quartz and alkali felspar. Magnetite and large apatite crystals form rather abundant accessories. This rock is a granodiorite-porphyry, and is clearly to be associated with the Benguella granodiorites. It differs entirely in appearance from the hornfelsed and granulitic porphyries described in a previous section.

V. RHYOLITE (DELLENITE). An extensive series of lava-flows belonging to this group occurs in the neighbourhood of Lepi and Quingenge. In hand specimens they are dense flinty rocks, generally light-grey in colour, occasionally dark-grey to black, with numerous, small, inconspicuous phenocrysts of quartz and felspar. In thin section they show numerous small phenocrysts of quartz, orthoclase, oligoclase, and biotite, in a dense, cryptocrystalline groundmass.. The quartz ranges from 1 to 3 mms. in diameter, and is subhedral, rounded, and embayed by corrosion. The felspars are euhedral, averaging 2 mms. in diameter, and mostly belong to orthoclase. In some of the rocks, however, oligoclase becomes a prominent constituent, and nearly equals the orthoclase in quantity. Its composition ranges from oligoclase

(AbsAni) to oligoclase-andesine (AbsAn3). The biotite is bleached and decomposed, and is often represented by chloritic decomposition-products and epidote (257, 260, 261, Quingenge). The groundmass is dense, uniform, cryptocrystalline to glassy, and is besprinkled with minute unidentifiable microlites. A flow-orientation is indi- cated by faint differences of colour and texture in different parts of the groundmass. The proportion of phenocrysts to groundmass is estimated at one to two, and quartz is about equal to the felspars amongst the phenocrysts. In some rocks (245, Quin- genge) the groundmass has apparently undergone silicification, which has also involved most of the felspars, leaving " ghosts" of the latter and much diffused 'sericitic matter. A CONTRIBUTION TO THE PETROGRAPHY OF BENGUELLA. 549

Some of the specimens from Lepi are compacted rhyolite-tuffs (225, E. of Lepi). These contain numerous fragments of quartz, oligoclase, orthoclase, and microcline, along with numerous angular fragments of microcrystalline quartz-porphyry, in a dense, cryptocrystalline groundmass. A rock generally similar to the rhyolites described above, from W. of Cruz's, Quingenge (234), is much poorer in phenocrysts, and is devoid of phenocrystic quartz. A chemical analysis of one of these rocks (258, Quingenge) has been made for me by A. SCOTT, M.A., B.Sc, with the result set out in Table III, 1.

TABLE III.

2. 3. 4. 1

SiO2 . . 68-98 72-60 66-91 69-48 TiO2 . •23 •30 •33 A12O3 . 15-24 13-88 16-62 13-88 Fe2O3 . 2-83 1-43 2-44 2-67 FeO •43 •82 1 33 1-53 MnO . •05 •12 04 15 MgO. . 1-08 •38 1-22 •71 CaO 301 1-32 3-27 2 39 (Ba, Sr)O tr...... Na2O . 3-88 354 413 3-74 K2O . 3-53 403 2-50 4-44 •53 { H2O + . 1-52 113 119 H2O- . •39 ) •09 •06 P2OS • •08 ... CO2 nt. fd.

100-27 100-00 100-00 100-18

1. Rhyolite (dellenite), Toscanose-Lassenose (I', 4, 2', (3) 4), Quingeiige, Benguella (258). Anal., A. SCOTT. 2. Average rhyolite, quoted from DALY, Igneous Rocks and their Origin, 1914, p. 19. 3. Average dacite, ibid., p. 25. 4. Dellenite, Toscanose (V, '4, 2, 3), Dellen, Helsingland, Sweden. Quoted from IDDINGS, Igneous Rocks, vol. ii, 1913, p. 128.

Consideration of this analysis, combined with what is known of the mineral composition, shows that the Quingenge rocks are the lavaform equivalents of the grano-diorite rather than of the granite family, and belong therefore to the group called dellenite by BROGGER.* The toscanites of Italy are chemically, but not mineralogically, similar to the Benguella type. The averages of sixty-four analyses of rhyolite, and of thirty dacites, as calculated by DALY,t are given for comparison in Table III. The average rhyolite is richer in silica, poorer in lime, and has a slightly greater content of alkalies in which potash is predominant, than the Quingenge rock. On the other * See IDDINGS, Igneous Rocks, vol. ii, 1913, p. 104. t Igneous Bocks and their Origin, 1914, pp. 19, 25. 550 MR G. W. TYRRELL.

hand, the average dacite is poorer in silica, and has a decided predominance of soda over potash. In most respects, therefore, the Quingenge rhyolite is intermediate between average rhyolite and dacite. The analysis of the type dellenite from Sweden (Table III, 4) is very similar to that of the Quingenge rock. It is, however, richer in potash and poorer in lime. The Swedish rock contains hypersthene, and was originally called hypersthene-andesite ; but the analysis shows that this identification was completely astray. It is worth while comparing the norms of these rocks (Table IV). TABLE IV.

1 2.

Quartz .... 25-4 25-6 Orthoclase 20-6 26-1 Albite .... 33-0 30-9 Anorthite 13-6 8-3 Femic minerals, water, etc. 74 91

1. Rhyolite (dellenite), Quingenge, Benguella. 2. Dellenite, Dellen, Helsingland, Sweden.

The norms correspond very closely, especially in regard to the salic constituents, to the modes that would have been developed had the rocks been holocrystalline. The dellenite is shown to be somewhat richer in orthoclase and poorer in oligoclase than the Quingenge rock, but the figures are sufficiently close to justify the identification of the latter as dellenite. I have not been able to find records of ancient African rhyolites similar to those of Benguella, but some of the Tertiary rhyolites of Algeria approach them closely in chemical and mineral composition. In the American Quantitative Classification the dellenite of Quingenge falls into the subrang lassenose, but is transitional to toscanose. The type dellenite of Sweden falls into toscanose.

VI. THE ALKALINE SERIES. An interesting series of alkaline rocks occurs in Benguella in the district of Ochilesa, the extreme north-eastern point reached by Professor GREGORY. At Chieuca these rocks are found within a volcanic vent, associated with tuffs and igneous breccias composed of the same materials ; but there are also intrusive masses within the district which are not in actual connection with the vent. The principal alkaline types found in the collection are nepheline-sodalite-syenite and other types of syenite, shonkinite, solvsbergite, and ouachitite, which are treated in order below. 1. Syenites. A. Nepheline-sodalite-syenite.—This type occurs as an intrusion at Ochilesa (209). It is a thoroughly leucocratic or perfelsic, coarse-grained rock, consisting n ainly of A CONTRIBUTION TO THE PETROGRAPHY OF BENGUELLA. 551 broad, simply-twinned anhedrons of soda-orthoclase, which are irregularly mottled in polarised light, together with subordinate nepheline and sodalite. The nepheline is distinguished by its lower interference tints, and total absence of the mottling which characterises the orthoclase, as well as by refractive index and interference figure tests. The sodalite has occasionally a faint bluish tinge, and is crowded with minute bubbles and other inclusions. It is identical with a mineral which occurs in the shonkinite described later, and which has been determined by chemical means. The ferro-magnesian constituents consist of ragged anhedrons of a pleochroic greenish-brown hornblende, which envelops small tablets of biotite, and occasionally contains a small core of almost colourless pyroxene. Irregular grains of aegirine are frequently inserted around the margins of the crystals. Apatite and magnetite are also abundant as inclusions in the amphibole sections. The pleochroism of the mineral is from pale yellowish-green to a light brown-red which has a tinge of violet. There is always an irregular marginal zone in the crystals in which the colour becomes bluish-green. The extinction in most sections is in the neighbourhood of 17°, but may range up to 31°. The mineral is probably a soda-amphibole belonging to the katoforite-arfvedsonite series, or possibly to ksersutite. iEgirine also occurs as small anhedrons embedded in the felspars. An estimate of the relative proportions of the constituents is : soda-orthoclase, 80 per cent. ; nepheline and sodalite, 10 per cent. ; hornblende, augite, biotite, etc., 10 per cent. B. Sodalite-syenite.—A second variety of syenite occurs in the volcanic vent of Chieuca (163). It differs from the above in its much finer grain, in the substitution of a pale-purplish titanaugite for hornblende as its principal mafic constituent, and in a smaller proportion of nepheline and sodalite. The titanaugite encloses biotite tablets, apatite, and magnetite ; and is in all stages of alteration to granular segirine- augite. The mass of the rock, however, is soda-orthoclase, with a very subordinate amount of euhedral nepheline and sodalite. The latter mineral penetrates the felspars in channels directed by the cleavages, as though it had made its way by corrosion. Sphene is an important accessory constituent. Decomposed specimens of this type, in which the pyroxene has completely passed over to aggregates of fibrous amphibole, and the felspar completely " kaolinised," occur in the Ochilesa district (208a, 209e, 219/). C. Akerite.—An alkali-syenite containing neither nepheline nor sodalite occurs to the W. of Ochilesa (1726). It consists mainly of anhedrons of mottled soda- orthoclase, with oligoclase (AbaAni) in little inferior quantity. Biotite forms the principal ferro-magnesian constituent, but is accompanied by a little brown hornblende similar to that of the nepheline-sodalite-syenite of Ochilesa described above. These crystals may contain a core of nearly colourless pyroxene. Crystals of apatite and magnetite are common enclosures within the ferro-magnesian minerals. This rock has a close affinity with the akerite of Norway, although the principal ferro-magnesian constituent of the latter is augite. 552 ME G. W. TYRRELL.

Nepheline-syenites appear to be widely distributed in Central Africa. BERG * has described a nepheline-sodalite-syenite from Senze do Itombe, and HOLMES f has recently described a nepheline-syenite from the same locality in north-western Loanda, the northern province of Angola. This rock contains segirine-augite as the principal ferro- magnesian constituent, but this mineral is frequently bordered by amphiboles which are identified with hastingsite or a mineral of the katoforite-arfvedsonite series.

2. Shonhinite. This remarkable rock occurs in the Chieuca volcanic vent (1706, l70g), and at Ochilesa (l72a, 2086, 208c). It appears to be the dominant type in this volcanic district. It is a fine-grained rock of bluish-grey colour and doleritic appearance, and is besprinkled with numerous small black crystals of hornblende ranging up to \ inch in diameter. In thin section the rock is ideally fresh, and consists mainly of orthoclase with interstitial sodalite, and a comparatively large quantity of mafic minerals which include titanaugite, barkevikite, and magnetite. Apatite is an important accessory. The texture of the rock is thoroughly granitoid or hypidiomorphic. Many of the augite and hornblende crystals are larger than those of the orthoclase composing the mass of the rock, but their porphyritic character is not so apparent in thin section as in the hand specimen. The felsic constituents poikilitically enclose numerous small crystals of titanaugite, magnetite, and apatite, which doubtless gives the rock its dark colour. The orthoclase forms elongated prisms which are simply twinned, or it builds more equidimensional subhedral crystals which contain numerous, evenly spaced, parallel rows of very minute bubbles, apparently of liquid. These rows are perpendicular to a direction of cleavage within the mineral, and are also parallel to a direction of extinction. A pale-bluish sodalite fills up many interspaces, and is frequently crowded with minute bubbles and inclusions. The presence of sodalite was confirmed by obtaining a silver chloride precipitate in a nitric acid digest of the rock, after removing apatite by means of a heavy liquid. The felsic minerals form a sort of coarse groundmass to numerous, pseudo-porphyritic, perfectly euhedral crystals of barkevikite and titanaugite, and also poikilitically enclose a swarm of small grains of the same minerals along with apatite and magnetite. The barkevikite and titanaugite are in roughly equal amount. The former occurs in fine, euhedral, red-brown crystals, occasionally with simple twinning, and narrow darker-tinted borders. In some of the rocks considerable resorption of the hornblende has taken place, the final appearance being of an irregular mass of magnetite mixed with granules of a greenish amphibole, the whole being surrounded by a dense corona of magnetite granules. The pyroxene has occasionally undergone a similar alteration, but to a lesser extent. The augite is of the purplish tint which is held to indicate a * Tscherm. min. petr. Mitth., vol. xxii (1903), p. 359. t Geol. Mag., dec. vi, vol. ii (1915), p. 323. A CONTRIBUTION TO THE PETROGRAPHY OP BENGUELLA. 553

high content of soda and titania. It is zonal, with hour-glass structure, faint pleochroism, and strong dispersion of the bisectrices. In some slides the smaller crystals, and the borders of the larger ones, have a greenish tinge, indicating some admixture of the segirine molecule. Apatite occurs abundantly in large dusty crystals with rounded corners. In some, an addition of fresh material has grown around them with sharp crystalline boundaries, in a manner which recalls the quartz crystals of the Penrith sandstone. A little sphene occurs in the usual wedge-shaped crystals ; and there is frequently a border of granular sphene fringing irregular masses of titaniferous iron-ore. The affinities of this rock are clearly with the type described as shonkinite by PIRSSON. In his latest work ROSENBUSCH * defines shonkinite as a hypidiomorphic granular rock of the alkaline series, characterised by the mineral combination potash- felspar and nepheline, and by the predominance of coloured constituents. This definition would make it the melanocratic facies of the nepheline-syenite group. IDDINGS t defines the rock as a phanerite characterised by alkali-felspar and equal or nearly equal amounts of mafic minerals, with small but notable amounts of felspathoids such as nepheline or sodalite ; and he treats it as the mesocratic facies of nepheline-syenites. In the original shonkinite of Square Butte, Montana, nepheline, along with sodalite, was regarded as a rare accessory mineral, and was, indeed, only detected by chemical means.j The rock was thus regarded as the mesocratic or melanocratic facies of alkali-syenites. The later work of PIRSSON showed, however, that his type shonkinite contained 5 per cent, of nepheline and 1 per cent, of sodalite. § The term shonkinite, in fact, has been used to cover rocks which belong to the mesocratic and melanocratic divisions of both alkali-syenites and nepheline-syenites. Recognising this, LACROIX has proposed to restrict the term to the melanocratic types of the alkali-syenite family,|| and to use another term for the equivalent rocks of the nepheline-syenite family.H In his latest description ROSENBUSCH himself speaks of " the typical, nepheline-poor shonkinite of Montana." ** The Ochilesa rock falls most naturally under the original definition of the term. The quantitative mineral composition of the Ochilesa shonkinite has been determined by the Rosiwal method, with the result shown in Table V, 1. It is

* Gesteinlehre, 3rd ed., 1910, p. 199. t Igneous Rocks, vol. ii, 1913, p. 230. I WEED and PIRSSON, Bull. Geol. Soc. America, vol. vi, 1895, p. 415. § PIKSSON, " Geol. and Petr. of Highwood Mountains, Mont.," U.S. Geol. Surv., Bull. 237, 1905, p. 104. || LACROIX, NOUV. Arch, du Mus., 1902, p. 179. IT LACROIX proposes to use the term theralite in this sense. This, however, ignores ROSENBUSCH'S plain intention of using this term for the plagioclase-nepheline rocks of plutonic habit. It is true that the original " theralite " of the Crazy Mountains, Montana, on which ROSENBUSCH erected the group, was erroneously described by WOLFF as carrying plagioclase, and is in fact a shonkinite in the sense of ROSENBUSCH and IDDINGS. LACROIX'S usage of the term theralite, however, merely perpetuates the original mistake in the diagnosis of the Crazy Mountain rocks. When ROSENBUSCH discovered the mistake, he promptly withdrew these rocks from the theralite group, and placed them under shonkinite. He then created as his type theralite the unquestionable plagioclase-nepheline rock of Duppau in Bohemia, thus preserving his term theralite with its original connotation. ** Loc. cit., p. 203. TRANS. ROY. SOU. ED1N., VOL. LI, PART III (NO. 14). 80 554 MR G. W. TYRRELL. there compared with the type shonkinite of Square Butte, Montana, and with a shonkinitic syenite from Middle Peak in the same district.

TABLE V.

1. 2. 3.

Alkali-felspar* . 610 20 53-2 Nepheline ... 5 Sodalite 2-0 1 Biotite 8 40 Titanaugite 14-1 46 32-3 Barkevikite 13-4 • . * Olivine 10 1-7 Iron-ores t 7V5 6 7-9 Sphene •6 Apatite 1-4 4 •9

1. Shonkinite, Ochilesa, Benguella. 2. Shonkinite, Square Butte, Montana. Quoted from PIRSSON, U.S. Geol. Sun., Bull. 237, 1905, p. 104. 3. Shonkinitic syenite, Middle Peak, Highwood Mountains, Montana; ibid., p. 93.

The Square Butte rock is much richer in the mafic constituents than that of Ochilesa, which resembles the Middle Peak shonkinitic syenite much more closely. As is shown in the discussion of the chemical analyses, the type shonkinite of Square Butte tends now to become an extreme type as compared with the average of the rocks which have been grouped as shonkinites; and the Ochilesa rock is very close to the average shonkinite. It differs from all previously described shonkinites in containing abundant hornblende. It compares well with the Montana types in containing a little sodalite, and in being associated with sodalite-syenites. The chemical composition of the Ochilesa shonkinite has been kindly determined for me by A. SCOTT, M.A., B.Sc. The figures are set out in Table VI, 1 ; and are there compared with the type shonkinite of Square Butte, and with the average shonkinite. The composition of the latter has been derived from the average of twenty-one analyses of shonkinitic rocks found in IDDINGS' Igneous Rocks, vol. ii (1913), and in ROSENBTJSCH'S Gesteinlehre, 3rd ed., 1910. In the collection is included the shonkinites of Montana, Arkansas, Celebes, and the Katzenbuckel, Odenwald ; also the malignites-(nepheline-pyroxene-garnet-shonkinites) of Ontario and British Columbia ; and the marosites (biotite-augite-shonkinites) of Celebes. The range of rocks covered by this average corresponds to that covered by ROSENBUSCH'S definition and descriptions, and consequently includes nepheline-bearing types. Certain " microshonkinites" described by LACBOIX from the Los Islands, West Africa, are excluded, since they are far too rich in nepheline to come under the heading of shonkinite. * Mostly soda-orthoclase. t Magnetite and ilmenite. A CONTRIBUTION TO THE PETROGRAPHY OF BENGUELLA. 555

TABLE VI.

1. 2. 3.

SiO2 47-41 46-73 47-46 TiO2 . 1-86 •78 1-14 A12OS . 12-30 1005 14-28 Fe2O3 . 6-99 353 4-16 FeO 5-03 8-20 4-97 MnO . •14 •28 •19 MgO . 6-82 9-27 6-64 CaO 9-57 13-22 9-42 (Ba, Sr)O p.n.d. Na2O . 3-06 1-81 386 • K2O 4-05 376 465 H O + . 1-24 1-24 2 | 1-66 H2O- . •31 P,O5 . 1-62 1-51 1-02 ; •07 •18 Cl . . ) Rest ! F . •21 CO2 nt. fd. f -72 I

100-68 100-56 100-17 Less 0 = Cl, F. -10 •04

100-58 100-52

1. Shonkinite, Lamarose (III, 5', 2, 3), Ochilesa, Benguella. 2. Shonkinite, Shonkinose (III', 6, 2, '3), Square Butte, Montana. Quoted from PIBSSON, U.S. Geol. Sun., Bull. 237, 1905, p. 102. 3. Average shonkinite, Shonkinose ('III, 6, 2, 3).

The Square Butte shonkinite is richer in lime and magnesia, and poorer in alkalies, than the Ochilesa rock or the average shonkinite, thereby revealing its more mafic character. The analysis of the average shonkinite, however, is extraordinarily like that of the Ochilesa type. It is somewhat richer in alkalies and alumina, but the correspondence of the other constituents is very close. A notable feature of most analyses of shonkinite, including the Ochilesa rock, is their abundance in phosphorus pentoxide, which constituent commonly reaches 1"5 per cent., indicating about 3^ per cent, of apatite. The Ochilesa shonkinite falls into the subrang III, 5, 2, 3 (lamarose) of the American Quantitative Classification, but is intermediate towards the next subrang, III, 6, 2, 3 (shonkinose), in which the typical shonkinites are found. The average shonkinite falls almost centrally into the subrang shonkinose. Light-coloured veins of alkali-syenite-aplite (208c) occur in the shonkinite of Ochilesa. On the other hand, a boulder of sodalite-syenite from the volcanic vent of Chieuca shows a vein of microshonkinite or shonkinite-aplite. It is unnecessary to describe these rocks in detail, as they are merely fine-grained modifications of the rocks above described. 556 MR G. W. TYRRELL.

3. Solvsbergite. The volcanic vent of Chieuca is filled with tuff and agglomerate consisting of boulders, pebbles, and comminuted debris of shonkinite, sodalite-syenite, and grey or green compact rocks identified as solvsbergite. It is possible that the latter rock also occurs as small intrusive masses or as lava-flows. The principal rocks of the vent may be described as shonkinite-tuff (l70h), and as solvsbergite-tuff (170c, 170/). Many specimens of the solvsbergite were collected (170, 170a, 170e, 1626); and amongst these three varieties may be distinguished. One contains large phenocrysts of soda-orthoclase, oligoclase (AbaAni) diopside with mantles of segirine-augite, and small crystals of segirine-augite, in a dense, felted, trachytic groundmass consisting of minute laths of orthoclase, granules of green pyroxene and magnetite, with some indeterminate material. A few, small, partially resorbed crystals of barkevikite occur. In the second variety the barkevikite constitutes the only phenocrystic mineral apart from small crystals of aegirine-augite, and this type may therefore be called hornblende-solvsbergite. The groundmass here contains abundant segirine and a little nepheline, and the rock is therefore transitional to tinguaite. The barkevikite has suffered considerable resorption ; the crystals are always surrounded by a dense corona of granules of green pyroxene. Frequently the core of barkevikite has entirely disappeared. A third type is characterised by a colourless glassy groundmass crowded with acicular green microlites. It carries small and sparse phenocrysts of euhedral barkevikite, segirine-augite, orthoclase, and a little nepheline. In some of the slides are small rounded areas of sodalite. The glassy base is probably highly, alkaline, and the rocks may be termed solvsbergite- and tinguaite-vitrophyres. These rocks undoubtedly represent the hypabyssal or volcanic phases of the syenites described above. The lavaform equivalent of the shonkinite does not appear in the collection, although it probably occurs in the field. 4. Ouachitite. A remarkable rock belonging to the alkaline series occurs as dykes in the sodalite- syenite of Ochilesa (209). In hand specimens it is a dark-green, grey, or brown, compact rock, which effervesces freely with acid, and in which the only identifiable constituent is biotite. It is characterised by an irregular banding due to the alterna- tion of strips differing slightly in texture, colour, or mineral composition. In thin section the bulk of the rock is seen to consist of biotite and a grass-green pyroxene, embedded in a groundmass consisting of allotriomorphic plates of calcite. Anatase derived from ilmenite, and apatite, are important accessories; whilst soda-orthoclase, and a mineral identified as melilite, occasionally appear in the slides. The biotite forms large plates of a bleached yellow tint, with much darker borders. Large cross- sections ranging up to 5 mms. in length are frequent, as are also good basal sections. It is extensively altered to a peculiar yellow mineral, which is usually granular in A CONTRIBUTION TO THE PETROGRAPHY OF BENGUELLA. 557

texture, but occasionally becomes massive and then shows aggregate polarisation. This mineral retains the cleavage of a mica, although it is not so perfect as in the biotite. The double refraction is rather lower than that of biotite, but its pleochroism is usually very feeble. In some sections, however, a pleochroism from orange to a pale-green was observed. The mineral is sensibly uniaxial. These characters agree on the whole with those of the penninite variety of chlorite. In the text-books, however, the double refraction of penninite is stated to be extremely low, whereas the mineral in question has a double refraction not much lower than that of biotite itself. The pyroxene occurs in small euhedral crystals, or as granular aggregates embedded in the plates of calcite and biotite. It is grass-green in colour, with only a slight pleochroism from green to a yellowish-green. The cross-sections are frequently rectangular, with well-marked cleavages parallel to the rectangular sides. There is occasionally a slight truncation of the corners giving rise to octagonal sections. The extinction diagonally bisects the cleavages and the rectangular sections. The crystals are therefore prismatic with only a feeble development of the pinacoid faces. There is great dispersion of the bisectrices, and some of the sections fail to give perfect extinction in white light. Many of the pyroxene crystals are mere shells, the interiors having been entirely replaced by calcite. The biotite and augite are embedded in a groundmass of large allotriomorphic plates of calcite. Within the latter, or enveloping biotite or augite, are found areas of a colourless mineral with very low double refraction and a dark-blue polarisation tint. There is occasionally some approach to a prismatic form, and on the thin edge of sections a cleavage may be observed parallel to the direction of elongation, which is also the negative or " fast-ray " direction. The extinction is straight in regard to the cleavage, the mineral is optically uniaxial, and has a refractive index considerably higher than that of Canada balsam. These characters agree with those of melilite, but the characteristic peg structure of this mineral has not been observed. There are numerous irregular patches of a black mineral, which in very thin section becomes translucent and of a beautiful blue colour. The identification of this mineral as anatase was put beyond doubt by finding it growing, in small rectangular tablets, out of decomposing ilmenite. Apatite is also very abundant in small euhedral crystals. In one of the slides (209d) one of the bands or schlieren consists of biotite and granular pyroxene in a groundmass consisting entirely of grains of apatite. In another section a considerable amount of soda-orthoclase appears in large anhedral plates, with which the biotite forms peculiar poikilitic intergrowths. Notwithstanding the alteration of the rock, and the rather uncertain identification of melilite, the general character of this rock is similar to that of the alnoite group, as defined by ROSENBUSCH,* and it is here provisionally classed with that group. The type alnoite of Alno, Sweden, differs from the rock here described only in containing serpentinised olivine, garnet, and perofskite as the titaniferous * ROSENBUSCH, GesteinUhre, 1910, p. 304. 558 MR G. W. TYRRELL.

mineral in place of ilmenite and anatase. It is very rich in apatite, and contains abundant calcite as the result of the alteration of the augite and melilite. In the absence of olivine and garnet, the Ochilesa rock has closer affinities with the biotite- monchiquite or ouachitite of Arkansas than to the type alnoite. According to ROSENBTJSCH * the ouachitite of Hot Springs, Arkansas, contains a small quantity of melilite, and is free from garnet. It is included by him in the alnoite group.

VII. BASIC INTRUSIONS. A group of basic rocks including dolerites and basalts is widely spread over the area traversed by Professor GREGORY. Their geological relations suggest that they are the youngest set of igneous rocks in the region. A rock from the railway W. of Lepi (127) is a coarse-grained ophitic olivine- dolerite, consisting of a plexus of broad laths of labradorite (AbiAni) in ophitic relations with irregular plates of brown augite. There is also some anhedral olivine, magnetite-blackened, and partly altered to colourless serpentine ; and titaniferous iron-ore. The rock is mesocratic or mafelsic, i.e. the felspars are roughly equal in amount to the ferro-magnesian minerals. A similar rock forms a dyke W. of Cruz's, Quingenge (236). It is, however, finer in grain, and the iron-ore is more abundant, than in the Lepi rock. The augite has a marked purplish tone. Rocks from near Cubal (300c) and Portella Solo (118) are very basic basalts approaching augitite. The Cubal rock contains numerous large phenocrysts of brown augite in a fine-grained groundmass consisting of augite and plagioclase, of which the former is by far the more abundant. The texture is obscurely ophitic, and the augite phenocrysts enclose very small laths of plagioclase. The only remaining constituents are ilmenite passing over to leucoxene, and small green patches which may represent a little original olivine. The rock from Portella Solo differs from the above only in its non-porphyritic character and the slightly greater proportion of the felspar. It occurs at the margin of a granite mass, and appears to have been recrystallised as a basalt-hornfels, with little or no new mineral formation. Both these rocks are practically devoid of olivine, and in the abundance of augite approach the augitite end-member of the basalt family.

EXPLANATION OF PLATE. Fig. 1. Charnockite, Ochilesa. Ordinary light, x 15. A granulitic aggregate of clear quartz, cloudy felspars, with biotite (centre of field) and pyroxenes (below biotite). The latter is mostly hypersthene. Fig. 2. Hornblende - hyperite, Ochilesa. Ordinary light, x 15. A granulitic aggregate of felspar (plagioclase) with a little quartz, abundant hypersthene, and hornblende. The hypersthene is easily distinguishable by its comparatively high relief. * ROSENBUSCH, Gesteinlehre, 1910, p. 305. A CONTRIBUTION TO THE PETROGRAPHY OF BENGTJELLA. 559

Fig. 3. Hornfelsed porphyry, Bailundo. Ordinary light, x 15. A recrystallised granulitic quartzo- felspathic groundmass, which contains irregular clots of intergrown hornblende, biotite, magnetite, and occasionally cassiterite; and cloudy phenocrysts of felspar (orthoclase and oligoclase). Fig. 4. Shonkinite, Ochilesa. Ordinary light, x 15. Shows pseudo-porphyritic euhedral crystals of barkevikite (centre), and titanaugite (south-eastern quadrant), in a groundmass consisting of orthoclase, and a little sodalite which is not distinguishable in the photograph. The felsic minerals contain numerous small inclusions of titanaugite and barkevikite, and apatite is enclosed within the coloured minerals. Fig. 5. Solvsbergite-vitrophyre, Chieuca. Ordinary light, x 24. Shows small phenocrysts of segirine. augite and magnetite (centre of field), in colourless glassy groundmass crowded with microlites of segirine. Fig. 6. Ouachitite, Ochilesa. Ordinary light, x 15. Shows numerous small euhedral sections of green augite, and large plates of bleached biotite (centre of field), in groundmass of calcite (colourless areas), and a little melilite (?) which cannot be distinguished in the photograph. The irregular black sections are of blue translucent anatase which has been derived by the alteration of ilmenite. RANS. ROY. SOC. EDIN. VOL. LI.

Q. W. TYRRELL: PETROGRAPHY OP BENGUELLA.—PL. I.