THE DETERMINATION OF THE DIRECTION OF TRANSPORT OF FLUVIATILE ARENITES BY ORIENTATION ANALYSES OF THE DETRITAL MICA

A simple method of determining the strike and dip of the AB planes of mica flakes in etched slides of arenaceous sediments by means of a universal stage mounted on a stereomicroscope is described. The polar stereograms constructed from such measurements in fluviatile sandstones show monoclinic symmetry due to up-current imbrication which compares well with the orientation of plant fossils, current bedding structures and the general current pattern. An angular discordance exists between these features and the direction of sole marks. An example is given of how analyses of the orientation and imbrication of mica flakes can assist in the solution of sedimentation problems of coalfield geology.     

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Mathematical Geosciences -     

Regionalization of global electromagnetic induction data: a theoretical model

Systematic differences are noted between those global response functions (Q = I₁/E₁; $\text{W}\text{= Z/H}$) derived from single observatories and those derived from global averages. Using a simple first-order model to simulate global scale lateral heterogeneities, we argue that reasonable differences in the depth to the conductosphere (d (average) = 400 km in one hemisphere, 600 km in the other) result in significant differences in the response functions at single observatories. These differences appear to be easily resolvable within the expected error-bars of actual observations. At this point it is believed that regional differences in global response parameters can indeed be used to infer large-scale differences in the depth to the conductosphere, providing that systematic biases introduced in data processing and interpretation are minimized.     

The analcime-phyric phonolites (blairmorites) and associated analcime kenytes of the Lupata Gorge,Mocambique

The petrology and mineralogy of three lavas from the Lupata Gorge, Mocambique, containing primary euhedral analcime phenocrysts, up to 1.5 cm in diameter, as well as potassium feldspar and nepheline phenocrysts, are described. Electron microprobe analyses of these phases and a whole rock analysis of the blairmorite are given. Reference to published and unpublished experimental work indicates that these rocks must have been generated at water pressures between about 5 and 13 kbars, implying depths of origin of between 20 and 50 km. Very rapid transport to the surface and quenching is implied. It is suggested that these indications of the considerable depth of origin of these rocks, taken together with the absence of associated intermediate and basic rocks, lends credence to the hypothesis of D. K. Bailey that the voluminous associations of salic igneous rocks found in parts of the African continent, unaccompanied by associated basic rocks, are explicable in terms of partial melting, under high water pressures, of the lower part of the crust.