Telluric potentials were recorded at nine sites stretching 1000 km from east of Wilcannia (N.S.W.) to west of Coober Pedy (S.A.) in south central Australia. These were combined with data obtained from a simultaneous magnetic variometer study, and analysed to yield magnetotelluric impedance tensors. The estimation of probable errors for the impedance tensors was an important part of the calculations. Numerical calculations for two-dimensional models showed that the observed data could be explained by a conductive-resistive-conductive layering. The top conductive (10 ohm-m) layer is approximately 5 km deep and the ensuing resistive section (1000ohm-m) continues to a depth of about 230km. This is underlain by an indeterminably thick conductive layer. Embedded in this general structure is an extremely good conductor (0.1 ohm-m) which strikes north-south. The detection of this good conductor is the main result of this work. The good conductor lies just east of the Adelaide Geosyncline at a depth of a few kilometres, and is of minimum thickness 10 km. It is thought to be related to the accretion of the eastern portion of the Australian continent on to the stable western Precambrian platform by tectonic processes. The strong anisotropy present in the magnetotelluric data at all sites could not be explained by induction in conductive sediments, and is thought to be caused by localized conductivity inhomogeneities distorting current flow.

SUMSixty-four samples from various sites along the granite batholith of SW. England, from the surrounding country rock, and from Lundy Island have been analyzed for the radiogenic elements potassium, uranium and thorium. An overall picture emerges of a radioactive batholith, uniform along its length, emplaced in a normal and less radioactive country rock. Weathered and kaolinized granites are depleted in U and Th but not K, relative to fresh granite. A proportion of the uranium mineralization found in the region, as well as the many different ages determined for uraninite and coffinite mineralization, are probably caused by the remobilization of the MARY uranium which has been leached from the granites. A traverse across the Dartmoor pluton revealed no marked differences between the megacrystic and poorly megacrystic granites. In contrast, the radiogenic concentrations of the fine grained granites which comprise parts of the Land's End and Carnmenellis plutons are atypical and are possibly rafts of assimilated country rock. Assuming that the radiogenic elements are distributed in exponentially decreasing amounts with depth, then the measured data are consistent with the formation of the batholith near the bast. of an originally normal crust with a subsequent high-level emplacement.THE PURPOSE OF THE STUDY was to utilize the radioactive, heat-producing elements K, U and Th to investigate the uniformity of the various plutons and to study the differences with the country rock. It was hoped that the investigation would assist in the understanding of the geology and petrogenesis of the granite as well as in explaining the uranium mineralization of the region. Radiogeology generally consists of detailed radiometric mapping with scintillation techniques, usually by air or car. Comprehensive radiometric investigations of this type have been made in SW. England, and throughout much of Great Britain, by the Institute of Geological Sciences (Bowie et al. 1973, I.G.S. Annual Reports 195i-7 2) but remain virtually unpublished. The type of survey reported here makes amends for its discrete, less complete areal coverage by yielding absolute, as opposed to relative, elemental concentrations; it is the first of its kind in the British Isles.

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Three heat flow values for south‐west England are presented. Two of the sites, Geevor and South Crofty, are operating tin mines on the northern contacts of the Land's End and Carnmenellis Granites, respectively, while the third, Wilsey Down, is a stratigraphical borehole 5 km north of the Bodmin Moor Granite. After applying topographic corrections values of 128·6 mW m‐2 (3·07 μ cal cm‐2 s‐1) for Geevor, 128·9 mW m‐2 (3·08 (A cal cm‐2 s‐1) for South Crofty and 67·3 mW cm‐2 (1·61 cal cm‐2 s‐1) for Wilsey Down, were determined. The value at Wilsey Down is shown to be consistent with that for an environment in which the Hercynian orogeny was the last significant thermal event. An additional heat source term must clearly be involved at Geevor and South Crofty to account for the unusually high values at these sites. Radiogenic heat production has been determined on granites from these sites and in spite of the fact that it is high it does not fully account for the measured heat flow. A compilation of underground temperature measurements made in the nineteenth century suggests that high heat flow is a general feature of the mineralized belt. At least part of this can be explained in terms of hot spring activity recorded widely throughout the area but the ultimate cause remains to be evaluated.

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