Art. LI.—On an Olivine-andesite of Banks Peninsula.

By R. Speight, M.A., B.Sc.

hard black andesite. These have consolidated at an earlier period, and have been caught up by the liquid lava as it ascended the pipe of the volcano. These blocks look as if they had been rounded by water, and have almost the appearance of concretions. They are in most cases surrounded by concentric layers of decomposed rock. Probably this is due to the interaction of the fluid lava on the inclusions, and this has produced a rock which has weathered in concentric layers, the junction of the two rocks affording a weak place where atmospheric water might readily promote decomposition.

The rock is of a dark-grey colour, but very often shows a tint of red, which disappears on long exposure to the air, as can be seen in the stone in the buildings above mentioned, where the newer parts can easily be distinguished from the older by the peculiar tint of the stone. The rock is full of cavities and steam-holes, but none of these were noticed to be filled with infiltration products. This rendered an accurate determination of the specific gravity almost impossible. On weighing pieces in distilled water the following results were obtained:—

I., 2.621. II., 2.619. Average, 2.620.

On grinding to a fine powder, and using a specific-gravity bottle, a number of different weighings gave results between 2.67 and 2.68.

The rock appears to the eye to be composed of a glassy or finely crystalline ground-mass in which crystals of felspar are porphyritically distributed. Some of these are over half an inch long, but as they crumble readily it is impossible to detach more than mere fragments for examination. They show at times a fine striation and glistening surfaces, but the cleavage is not well marked.

The examination of the rock was conducted in two ways:—

A. A chemical analysis of the rock as a whole, and of the felspar separately.

B. A microscopic examination by means of thin sections.

A. Chemical Analysis.

Portions of the rock were taken and ground up, so that a perfectly general sample might be obtained, and were analysed a number of times. Since the rock was roughly examined first under the microscope a qualitative chemical analysis was unnecessary. I show all the results obtained, with the exception of those in No. III., as they were completely wrong, from known reasons. The last analysis is the most trustworthy, for reasons which I shall point out:—

estimated as pyrophosphate. The variation in the results obtained in this case has been explained.

The alkalies were determined by the Lawrence-Smith method, and were determined separately, by titration with silver-nitrate in the first two cases, and by precipitation with platinic chloride in the last two.

The presence of manganese and phosphorus was also tested for, but with no result. The absence of the last would show that apatite was absent.

For reasons given above, it will be seen that the last analysis is the most trustworthy.

The porphyritic felspar was analysed separately, but I will give the results when treating of it microscopically.

B. Microscopical Examination.

Sections of the rock were made by grinding down thin fragments till they were semi-transparent, care being taken to get specimens from rock which was weathered as little as possible. These were examined under the microscope by means of polarised light. The rock then appeared to consist of a semi-crystalline ground-mass in which crystals of plagioclase, augite, and olivine were porphyritically distributed. All through the rock were traces of weathering, as limonite showed nearly everywhere in the ground-mass.

I. Ground-mass.

The ground-mass was semi-crystalline, but the amount of interstitial glass was comparatively small, nearly the whole of the space between the felspar and olivine being taken up with an interlacing network of felspar microliths. These were probably oligoclase, as in almost every ease the direction of the extinction was nearly in a line with the length of the microlith; but, on attempting to determine what kind of felspar it was, by means of the extinction of twin lamellæ, results were obtained not at all in agreement with this. In some cases the angle of extinction was so high as to make it anorthite. Besides, in many cases, the microliths exhibited undulose extinction, and the determination of the true angle was impossible. Since the occurrence of a more basic felspar than the porphyritic felspar (I shall afterwards show that this is labradorite) is unusual, I conclude that the majority of the felspar is oligoclase, as determined by the first method.

In some cases the amount of glass was so very small that it hardly appeared in the section at all, but in other cases it occupied a, comparatively speaking, large space. It was full of crystallites, which had no effect on the polarised light. The glass, besides, was coloured brown, owing to the presence of

small quantities of limonite, evidently due to the action of atmospheric agencies or percolating water.

II. Porphyritic Minerals.

1. Felspar.—This is by far the most prominent porphyritic mineral, as it can easily be distinguished by the naked eye, while the other minerals can only be distinguished microscopically. It is of all sizes, from almost microscopic dimensions to crystals half an inch in length. The felspar is almost wholly labradorite, but oligoclase is present to a small amount. The evidence for the species of felspar is as follows:—

(i.) Chemical Analysis.

Portions of the rock were taken and broken in order to obtain pieces of the felspar. These were tested chemically,—

• (a.) With the flame, showing well-marked calcium, obscuring the sodium to a certain extent, but a trace of potassium appeared when the flame was observed through blue glass.

• (b.) The presence of CaO was shown by precipitation with ammonium oxalate, when a decided precipitate appeared.

• (c.) By the action of strong hydrochloric acid the mineral was partly dissolved.

• (d.) By quantitative analysis the following results were obtained:—

[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]

The amount of ferric-oxide is probably accounted for by the presence of minute inclusions of limonite.

The large value of the CaO in No. I. is probably due to incorrect weighing, as the calcium was estimated as CaO after ignition. As this analysis was almost qualitative, extreme care was not taken. The second determination is probably exact. All the alkali present was estimated as soda, since the flametest gave such a slight amount of potash. By this analysis the felspar appears to be labradorite, but the amount of CaO is slightly too low for a typical specimen. This may be due to the presence of a small quantity of oligoclase, which subsequent determination shows to be present.

In one of the felspar crystals a remarkable structure was observed. What appeared to be an ordinary narrow band was crossed at small intervals by black marks, so that the whole appearance was like the rungs of a ladder. On examining it with a higher power no difference was observed in the structure. The bars were black throughoutthe revolution of the stage, whether the nicols were crossed or not, so that it was probably some alteration product produced in a rather curious way. (See Pl. XLVII., fig. i.)

2. Olivine.—The olivine occurs in rounded crystals, which are crossed by irregular cracks. Some of these are so much corroded that they show no trace of their original form (see Pl. XLVII., figs. iii., iv., and v.); but in a few cases faces were seen remaining, as in fig. iv., in which the extinction bisected the angle between the straight edges. The alteration which marks nearly all the olivine present is most strongly observed near the periphery and along the cracks. The alteration product is limonite. It is shown in the figures by the dark shading. Sometimes unaltered fragments are seen, and they show the brilliant polarisation colours and rough surface of olivine. Considering the amount of olivine which has been present, the percentage of magnesia (2.75) is rather low, but nearly all the magnesia has been taken away by weathering action, and there is left an aggregate of limonite.

3. Augite.—The amount of augite in this rock is very small compared with the amount of olivine, and the lastmentioned mineral is the most prominent ferro-magnesian mineral. Only a few crystals of augite were observed in the sections made, and these were a pale greenish-brown by ordinary light. They showed no pleochroism, but moderately brilliant colours under polarised light. They were distinguished from the unaltered olivine by their smooth surface and by the absence of irregular cracks. Some showed a banding, evidently due to polysynthetic twinning (fig. ii.).

As this is parallel to ∝ P 1/∝, it gave a means of determination by extinction when an angle of 39° was obtained. In fig. ii. the longer lines represent bandings which also corresponded to cleavage-cracks, while the shorter irregular eracks were parallel to the prism.

4. Magnetite.—The presence of this mineral was shown by examination with reflected light, when numerous black grains appeared, with the blue metallic lustre of magnetite. The presence of this mineral will account for the ferric oxide, which was shown to be present by means of chemical analysis. No other porphyritic minerals were noticed.