16. Genus of the Augite-Andesites

Formula.—Aug, matr, flu, gran, non-phen, parv.

Characters.—In the groundmass the felspar-lathes are in flow-arrangement and the augite is granular. Plagioclase phenocrysts are absent, or if present very scanty and not usually over 1 mm. in size. When present the augite phenocrysts are under 2 mm.

Description.—This is a very extensive genus, admitting considerable variation and including most of the aphanitic augite-andesites, where the felspar-lathes are as a rule very small (under ·1 mm. in length), as well as some of the doleritic types where they are very large (·2 to ·4 mm. long). In assigning a rock a place in this genus some regard must be paid to its macroscopic aspect as well as to the presence or absence of plagioclase phenocrysts. In many cases two or three small phenocrysts may be observed in a slide, under a millimetre in size; but they do not give a character to the naked-eye appearance of the rock, and such rocks cannot be distinguished from others that do not display them.

These rocks range in specific gravity from 2·55 to 2·85. This large range is in the main concerned with different degrees of basicity depending on the character of the plagioclase, the relative abundance of the augite granules, &c.; but it is also connected with the amount of interstitial glass. The variety of plagioclase ranges between oligoclase and andesine labradorite. The fluidal structure is nearly always well-marked, and the closely packed felspar-lathes have often the peculiar “felted” appearance of many andesites. A little interstitial glass is present in most rocks.

Many, perhaps nearly all, of the rocks belong either to dykes or to larger intrusive masses. All the four species indicated by the length of the felspar-lathes are represented in my collection, especially the two with smallest felspars. They may again be split up into two sub-species according to the degree of basicity of the rocks.

Species A.—Felspar-lathes between ·02 mm. and ·1 mm. in average length.

(1) Most basic sub-species.... Sp. gr. 2·75-2·85. Dark-brown and dark-grey compact aphanitic rocks showing no plagioclase phenocrysts to the eye. When a few of these phenocrysts are present in a slide they are not usually much over 1 mm. in size, and give extinctions of andesine labradorite (20° to 30°). Augite phenocrysts are often absent, and when present are not over 1 mm. in size and are as a rule scanty, occasionally affording a suspicion of inter-growths with rhombic pyroxene. The felspar-lathes which display marked flow-structure, vary in average length in different rocks from ·05-·08 mm. Lamellar twinning is rare, the extinctions being those of oligoclase andesine (10° to 20°). The usual extinction, as measured from the long axis of the lathe, is 10° to 15° (medium andesine). The augite granules are small (·01-·02 mm.) and abundant. There is generally a little interstitial glass with small magnetite.

(2) Least basic sub-species.... Sp. gr. 2·55-2·75. Dark compact aphanitic rocks especially characteristic of the Ndrawa district. When plagioclase phenocrysts are present, they are very scanty and not generally over a millimetre in size, possessing rectangular clean outlines and showing but few inclusions. They may display carlsbad twinning and zoning, or albite twinning, when they give extinctions of oligoclase andesine (10°-15°). Pyroxene phenocrysts are either absent, or scanty and small, being usually of pale yellow augite with occasional indications of intergrowth with rhombic pyroxene. The felspar-lathes as a rule average ·07 or ·08 mm. and present a dense fluidal arrangement. They rarely display lamellar twinning and give extinctions measured from the long axis of oligoclase and oligoclase andesine (2°-8°). The pyroxene granules are very small, averaging ·01 mm. and less. There is also fine magnetite. A little interstitial glass is usually present. When abundant it is not generally smoky but shows clear fibrous devitrification.... One of the specimens, which is semi-vitreous, exhibits tube-like steam-pores drawn out to a length of 1-1½ centimetres. The felspar microliths are only ·02 mm. in length. The copious glass has the character above described.

Species B.—Felspar-lathes ·1-·2 mm. in average length.

This species may also be sub-divided into two sub-species (more basic and less basic). Since, however, all but one of the fifteen rocks belonging to the species are of the more basic kind my remarks will mainly apply to them. They are dark-brown or dark-grey compact aphanitic rocks, occasionally banded or streaky, in appearance, and ranging in specific gravity from 2·75 to 2·84. They occur in several districts, but are especially characteristic of the Ndrawa district. The plagioclase phenocrysts, if present, are very scanty and small (1 or 1½ mm.). They contain inclusions of the magma and give lamellar extinctions of andesine labradorite (20°-30°). Pyroxene phenocrysts do not generally occur. When present, they are small and of pale yellow augite yielding large extinctions. Occasionally micro-porphyritic augite is well represented. The felspar-lathes, which exhibit a well-marked flow-arrangement, are generally ·13 to ·15 mm. long. Lamellar twinning is uncommon, the extinctions measured from the long axis indicating basic andesine (10°-20°). The augite granules are abundant and small (·01-·02 mm.). Occasional prism-forms yield large extinctions. Magnetite is abundant, its grains corresponding in size to the augite granules. There is as a rule a little residual glass, which shows fibrous devitrification and is not smoky. The banded appearance of some of the rocks arises from the glass collecting in streaks rudely parallel and running in the direction of the “flow” of the felspar-lathes.

The only specimen in my collection of “sliced” rocks belonging to the less basic sub-species is an altered bluish-grey rock (sp. gr. 2·7) from the range between the Mbuthai-sau valley and the Wainikoro plains. Its long parallel untwinned felspar-lathes give the nearly straight extinctions of oligoclase. Fine cracks in the rock are filled with crystalline silica.

The rocks of these species in the collection are for the most part dyke-rocks of the more basic kind. They are blackish or dark-brown, almost doleritic in texture, and range in specific gravity from 2·77 to 2·87. At times they are vesicular or scoriaceous, as in the specimens from an agglomerate at Undu Point and from a flow or dyke at Vunikondi. The most typical of these rocks are those of some of the dykes of the Ndriti basin, which, however, display propylitic alteration in a varying degree. They would be described as semi-doleritic basalts without olivine or as non-porphyritic basaltic andesites. Plagioclase phenocrysts are typically absent, or they are scanty and not over 1 mm. in size. Augite phenocrysts are usually scanty and small. The felspar-lathes, which are more or less in flow-arrangement, are rather stout, and range in average length in different rocks from ·23 to ·35 mm. They often show a few twin-lamellæ which yield extinctions of medium and basic andesine (15-28°). The augite granules are large (·03 mm.) in the Ndriti rocks. Magnetites, usually corresponding in size to the augite granules, are abundant. Interstitial glass occurs often in fair quantity and is dark and semi-opaque.

At times there can be recognised a later generation of minute felspar microliths between the much larger lathes. They display a plexus rather than a flow-arrangement. Whilst the larger parallel lathes of the Vunikondi rock, above referred to, average ·23 mm. long, the felspar microliths of the interspaces average only ·03 mm. The significance of these two crops of felspars in the groundmass is discussed on page [237].

The only rock of the less basic sub-species in my collection is from a dyke near Vatua-karoa. It shows secondary calcite and viridite and other evidences of the propylitic change. The felspar-lathes, which average ·3 mm. in length, give extinctions of oligoclase (0-5°). The specific gravity is 2·72.