Third Order of the Hypersthene-Augite Andesites.

Remarks.—The same classification is to be employed here as in the case of the two previous orders, but as the rocks in my collection that belong to this order are not numerous (nine sections), it will be sufficient to refer to the general remarks on the order on p. 290.

Remarks.—My sliced specimens (five) are too few for the elaboration of this order to which the classification employed for the other orders is scarcely applicable. This is due to the partial decomposition or imperfect development of the pyroxene of the groundmass. The general characters of the order are given on p. 291.

The Plutonic Rocks.

These rocks are very infrequent and are for the most part hypersthene-gabbros or norites, with a few representatives of diorites^[110] without pyroxene. True plutonic rocks did not come under my observation in the western half of the island (west of Lambasa and Savu-savu), those of Mount Thoka-singa in the Ndrandramea district making the nearest approach (see p. 302). The localities in which they were found are below enumerated:—

The Hypersthene-gabbros.—These rocks also contain monoclinic pyroxene, and are the plutonic equivalents of the hypersthene-augite-andesites which as a rule prevail in the localities where these rocks occur. They are usually dark grey or steel-grey in colour with a specific gravity ranging from 2·7 to 2·84 and have a granitoid aspect. The following characters are common to all the specimens.

They display a mixture of plagioclase and pyroxene, the last filling up the spaces between the felspars and apparently of later formation. The plagioclase crystals, which are 1 to 2 mm. in size, are opaque; and since they are traversed by numerous fine fissures filled with dust-like decomposition products, their appearance is often semi-saussuritic. They are much cross-macled, are at times zoned, and give lamellar extinctions of andesine-labradorite (20°-30°).... The pyroxene includes both the rhombic and monoclinic forms, the last with extinction angles of over 30°. They may be associated or may occur as separate crystals, the rhombic prevailing in the less basic and the monoclinic in the more basic rocks. The rhombic pyroxene is usually more or less converted into bastite which by further change passes into a chloritic material; whilst the augite sometimes undergoes the diallagic change resulting from schillerisation.

Some special features are presented by rocks from different localities. That from Nawi is most basic and looks like a diallage-gabbro. That from the Valanga Range (sp. gr. 2·75) contains some quartz, apparently secondary and filling up the interspaces. The rock from the Na Kula Ridge shows traces of a groundmass; but it comes near the plutonic type.

A Hornblende-gabbro.—This granitoid rock, which is from the Na Kula Ridge, has a specific gravity of 2·72. Hand-specimens display large porphyritic crystals of hornblende (7 mm. long) in a base of opaque felspar and smaller hornblende. In the slide we observe besides the large crystals of plagioclase and hornblende a little pyroxene; but the mass of the rock consists of greenish-brown hornblende, plagioclase, and some secondary quartz, forming a coarse mosaic with a “grain” of about a millimetre. The hornblende is displayed in regular hexagonal sections, markedly pleochroic, and gives extinctions up to 12°. It shows no dark resorption borders; and the larger porphyritic crystals have the same characters. Almost all the plagioclase of the rock is traversed by numerous fine fissures, and often acquires a semi-saussuritic appearance from the presence of dust-like decomposition products. The lamellar extinctions indicate andesine-labradorite. The quartz occurs mostly in nests. The pyroxene is formed of large grains of both the monoclinic and rhombic types.

The Diorites.—The rock forming blocks in the Vunimbua River has a specific gravity of 2·78 to 2·8. It is a pretty rock showing long black blades of hornblende, 10 mm. in length, in an opaque felspar base. In the slide the hornblende, which is dark brown and markedly pleochroic, shows six-sided sections with characteristic prismatic cleavage lines, the longitudinal sections giving extinctions up to 15°. The borders in some cases display traces of resorption. The felspar (plagioclase) is in the form usually of broad regular crystals, 3 to 4 mm. in size, and giving extinctions of andesine-labradorite (28°); they are “clouded” through the presence of fine alteration products associated with numerous fissures. The relation between the hornblende and the plagioclase is not constant. This appears to be partially due to the occurrence of traces of a groundmass.

The diorite forming blocks in the agglomerate of the coast cliffs, west of the Salt Lake Passage, is a remarkable rock showing large blackish hornblende crystals, in the shape of blades 25 mm. long and 3 or 4 mm. broad, set in a base of opaque plagioclase felspar which surrounds the hornblende. The last-named is deep-brown, very pleochroic, yields extinctions up to 22°, and displays but little evidence of resorption. The plagioclase is irregular in shape and exhibits broad lamellæ giving extinctions of acid labradorite (28°-30°). It is traversed by numerous fine fissures filled with decomposition products and contains abundant dust-like materials. (Spec. grav. 2·8).