Weathering of a Basalt by Pertusaria Corallina

The weathering phenomena resulting from the encrustation of basalt by Pertusaria corallina have been studied by scanning electron microscopy and a variety of other techniques. The rock consists largely of plagioclase felspar (labradorite) and ferromagnesian minerals, the latter often being replaced by hydrothermally formed ferruginous clay minerals. Lichen weathering results in the extensive etching of the primary rock-forming minerals, particularly labradorite, and in the degradation of the clay minerals to yield a thin ochreous crust of poorly-ordered ferruginous (ferrihydrite) and alumino-silicate materials. These changes are brought about principally by the oxalic acid secreted by the mycobiont, a conclusion supported by observations following experimental mineral alteration.

Evidence of Single-Domain Magnetite in the Michikamau Anorthosite

The Michikamau anorthosite possesses very stable natural remanent magnetization, some of which resists alternating fields up to 1800 Oe. The rock contains two types of opaque grains, fine opaque needles of order 10 × 0.5 μ in the plagioclase felspar, and large equidimensional magnetite particles. Ore microscope studies suggest, but do not establish, that the needles are composed of magnetite. Saturation isothermal remanence and thermal demagnetization studies indicate magnetite as the carrier of remanent magnetization. In order to distinguish the effects of the large grains from those of the needles, mineral separation was used to show that an artificial specimen of essentially pure plagioclase had very similar isothermal remanent magnetization properties to the whole rock. Both indicated magnetite as the magnetic mineral. Thermoremanent properties of the separated mineral fractions indicated magnetite as the dominant magnetic constituent but showed some evidence of laboratory-produced hematite. Theoretical models of grains elongated along [111] and [110] axes are used to show that magnetite needles can exist in stable single-domain configuration in the size and shape ranges of the needles observed in the Michikamau anorthosite. There is thus considerable experimental and theoretical evidence for the conclusion that the stable remanent magnetization of the Michikamau anorthosite is carried by fine single–domain needles of magnetite in the plagioclase felspar.

An investigation of high-temperature optics in some naturally occurring plagioclases

For several years now it. has been generally appreciated that plagioclase felspar formed at high temperature differs optically from that formed at lower temperatures. Much of the evidence for this has been provided by the Austrian workers, notably Köhler (1-3), Scholler (4), and Tertsch (5-7), who have prepared revised curves to supplement the universal-stage stereograms of Reichert (which are for use with lowtemperature material). There are a number of interesting petrological points which have not yet been fully investigated. Much work has been done on synthetic material, and it is not clear to what extent the hightemperature forms occur naturally. There may be two distinct and separate types of plagioclase structure, or the two forms may grade into one another. It may be possible to explain the optical anomalies as an order-disorder phenomenon; a more disordered state has been 'frozenin' in those felspars which ha~e formed a.t a high temperature and cooled rapidly.

Zoning in plagioclase felspar

During the examination of the basalts of Calciferous Sandstone age which are mapped on the one-inch Sheet 23 (Hamilton, Lanarkshire) Scotland, the peculiarities of zoning exhibited by the porphyritic felspar were studied in some detail. As only a summary of the details is possible in the memoir explanatory of the geology of this Sheet, the description of the zoning is offered here, and some conclusions regarding the magmatic history of the zoned felspars are presented.

The Term “Crinanite”

The name crinanite was given in 1911 by Sir John Flett to a suite of N.W. analcite-dolerite dykes occurring in a fairly extensive district in Argyllshire of which Loch Crinan is the centre. A description of the type is accompanied by an analysis and a photomicrograph of a characteristic example from the island of Jura. Writing a few months later of similar dykes in Colonsay, the same author gives a concise definition of the type—“The crinanites, then, are dark coloured, fine-grained, basic rocks consisting mainly of olivine, augite, and plagioclase felspar, with a considerable amount of analoite and zeolites.” A detailed description follows, in which the very perfect ophitic structure of the rocks is emphasized, and another excellent photomicrograph is shown of an example from Colonsay. In his original description Sir John Flett states that the crinanites “present some analogies to the teschenites in mineral composition; from the latter they are distinguished by their finer grain, their perfect ophitic structure, the scarcity of hornblende and biotite, and their occurrence as thin vertical parallel dykes”.

On a new barium plagioclase felspar

In the memoir dealing with the geology of the Broken Hill district, New South Wales, mention is made 1 of a remarkable rock type found in a few places in the vicinity of Broken Hill, which looks not unlike an aplite in the field. A brief description of its appearance in thin section is given. It consists mainly of what appears to be very basic plagioclase. A fair amount of quartz, some epidote and bluishgreen hornblende were noticed, whilst a notable amount of sphene is present.

The Petrology of the Warren House Series, Malvern

THE Warren House Series consists of lavas which belong to a sodic suite, being rhyolites, keratophyres, and spilites. The rhyolites have a fine-grained micrographic groundmass, and some contain phenocrysts of quartz, orthoclase, anorthoclase, and plagioclase felspar. Epidotization of the rocks has occurred, but it is the felspathic and femic constituents of the groundmass which have suffered alteration and not the felspar phenocrysts, the action not being intense enough to affect the latter. The keratophyres consist essentially of small lath-shaped albite crystals which give a trachytic texture to the rock. The spilites are more basic, vesicular, and variolitic or sub-ophitic in texture, while some of them show pillowstructure in the field. The keratophyres and the spilites also exhibit varying degrees of epidotization. Pyroclastic rocks are not very common, but a few interesting varieties have been noted. Intrusions of dolerite occur within the lavas. One type is an ophitic dolerite, which when fresh probably contained olivine. Another type is now an epidiorite. Both these types have been epidotized, the epidiorite to such an extent that it passes into an epidote-uralite rock in places.