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Art. LVIII.—Notes on some Rocks from Campbell Island.
[Read before the Philosophical Institute of Canterbury.]
These rocks were collected on Campbell Island by Mr. James Gordon, who forwarded them by Dr. Cockayne to the Canterbury Museum, with the object, I believe, of having them examined for gold. Campbell Island was formerly believed to contain mineral deposits, and whalers stated that tin was found there. Though there is no absolute impossibility that
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such occurs, yet it seems highly unlikely that any payable deposit exists.
During the French expedition to the island in 1874 for the purpose of observing the transit of Venus, a careful study was made by members of the scientific staff, and no traces of any metallic ores were found. Perhaps the presence of iron-pyrites in small amount in one of the specimens has encouraged the hope that gold may occur. As I have not visited Campbell Island I have no personal knowledge of its geological structure; but an excellent account of it is given in Filhol's “Mission de l'Île Campbell,” which I have found of much assistance. I have no knowledge of the field relations of the rocks under consideration.
No. 1.
This is a greyish-white rock, so much weathered that an accurate determination is difficult. The specific gravity is 2.56. To the eye it appears holocrystalline, with crystals of felspar plainly visible; some of these are 5 mm. in length. A brassy yellow mineral is also present, but I could not determine it for certain; it has, however, all the appearance of iron-pyrites. On treating the rock with acid there is a marked effervescence in every part, owing to the presence of calcite. The soft character of the rock makes it difficult to treat microscopically, but thin sections show plainly that it is holocrystalline, and composed principally of much-decomposed felspar. A large proportion of this is so weathered that it could not be determined accurately, but the lamellar twinning shows that some is certainly plagioclase. Whether it is all plagioclase I cannot say, but the occurrence of calcite in large quantity points to the presence of a basic felspar. It is possible that this may be partly due to infiltration from the limestone deposits which occur on the island. The calcite permeates the whole rock, and sometimes occurs in masses 1 cm. in length. There is also an amount of amorphous black matter which is no doubt due to the decomposition of the original ferro-magnesian mineral. I detected none in the slides, but the examination of crushed fragments showed small grains of a brown mineral which is perhaps hornblende.
The foregoing description shows the rock to be either a weathered syenite or diorite; most probably the latter. In the report of the Colonial Laboratory for the year 1903, page 10, there is the analysis of a Campbell Island diorite; but the percentage of CaO is too low for the rock under consideration, and the absence of CO2 is a further proof that the two rocks are different. This may be due to the introduction of a varying
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amount of foreign matter into different portions of the rock-mass, and the two samples may have come from different parts of the same dyke or intrusion.
Campbell Island is formed of volcanic and sedimentary rocks of perhaps early Tertiary age, resting on a platform of metamorphic schists. The existence of granite dykes in these is recorded in Filhol's “Mission de l'Île Campbell,” page 143, but no diorite dykes are mentioned. Such intrusive rocks might be expected to occur in the off islands of New Zealand, as they are not truly oceanic, but part of a continental area which stretched chiefly to the east and south, and which was above sea-level, either wholly or partly, during late Tertiary times.
No. 2.
This is a basic glass or tachylyte—specific gravity about 2.5, hardness 4–5, with a brown streak, fusibility about 3; the water-percentage I found to be 6.3. But the following analysis, taken from the report of the Colonial Laboratory for the year 1903, gives details of its chemical composition:—
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
| Silica | 54.1 |
| Alumina | 18.2 |
| Iron-oxides | 11.2 |
| Lime | 3.8 |
| Magnesia | 0.5 |
| Manganese-oxide | 0.1 |
| Alkalies and undetermined | 4.9 |
| Water | 7.2 |
| — | |
| 100.0 |
This analysis shows the usual characteristic of a basic glass—viz., the high percentage of silica; but it is probable that it may be the glassy equivalent of the andesites found on the island.
Microscopic examination shows that it is undoubtedly a glass, rendered almost opaque by magnetite-dust. This is aggregated at times into cumulous masses, which sometimes show radial arrangement, the intervening spaces being more translucent. Perlitic cracks, due to cooling, are common. Porphyritic crystals of plagioclase and augite of very small dimensions also occur very sparingly.
When placed near the pole of an electro-magnet almost all the powdered rock was strongly attracted, due, no doubt, to the excessive amount of magnetite-dust disseminated throughout it.