An earlier report (Record 1950/041) has described the geophysical survey carried out early in 1950 on the Renison Bell tin field. The present report deals with the work of a second field season which commenced in November 1950 and continued until June 1951. The methods used were again the magnetic and self-potential. The 1950 survey was extended to the south and east by continuing along the Renison Bell Hill base-line a further 1000 feet south and reading stations on both sides of this base-line along traverses extending from the main quartz-porphyry dyke to the headwaters of the Montana Creek. In addition, some of the traverses were extended over the saddle between Dreadnought and Stebbins Hills as far as the Boulder tramway. The report gives an account of the survey work and its results, and provides recommendations for future testing. The geological features of the area covered by the present geophysical survey are briefly described.

Rurnbalara railway siding is 831 miles by rail north of Adelaide, South Australia, and 120 miles by rail south of Alice Springs, Northern Territory. The ochre deposits (Plate 1) are situated in desert country, 15-20 miles northeast of the siding. No permanent water supply has been developed in the vicinity of the mines. Numerous mesas in the district are capped by siliceous laterite (grey billy), and are composed of approximately 150 feet of horizontally-bedded, leached shales and sandstone containing numerous fossils of Lower Cretaceous age. These beds, for which the name Rumbalara Shale is proposed, rest unconformably on the eroded surface of flat-lying, current-bedded sandstones of which the age has not been determined; it is proposed that the sandstone beds be called the De Souza Sandstone. .The ochre occurs in beds 1 to 4 feet thick; the best grade consists of soft, friable, golden-yellow material, free from grit and with good paint-making properties. Throughout the deposits the ochre occurs at the same stratigraphic horizon- at the base of the Lower Cretaceous-and appears to be of sedimentary origin. Excluding 1943 and 1944, annual production during 1940-1948 averaged 500 tons; in 1943 and 1944 production was 1,345 and 1,439 tons respectively. The deposits are very extensive, but prospecting carried out to date indicates that only some of them contain ochre equal in grade to that being mined. However, although there are virtually no proved reserves, the field is probably capable of supplying Australia's requirements of yellow ochre for many years.

The samples from Margaret Bay were collected by H.B. Owen during a recent visit to the northern coast of Australia. The collection was made with the hope that microfossils may be found in the rocks which were collected from a twenty foot section exposed at Margaret Bay. The results of the micropalaeontological examination are discussed herein.

Ore reserves in the Geiger-Main and Whip Lodes at Radium Hill have been calculated from up-to-date plans and assay data provided by the South Australian Mines Department and results obtained agree closely with estimates made by the Mines Department. Tonnage of ore (not including dilution) to 600 feet depth on Geiger-Main and Whip Lodes and to 400 feet in Main Lode South block is estimated at 337,500 tons containing 938 tons U3O8, corresponding to an approximate average of 160 tons U3O8 per 100 feet of vertical depth. The data used, method of calculation, and results, are discussed at length in this report.

The presence of bauxite on one of the Wessel Islands was established in 1949 when specimens of laterite collected from various parts of the Arnhem Land coast were analysed. In October, 1951, the writer spent two days on Marchinbar Island and examined several small laterite residuals and part of one large deposit. The characteristics and distribution of the laterite are discussed in this report.

Gravity observations were made at quarter-mile intervals along the road that joins Bullara and Giralia homesteads. This road crosses the northern end of the Giralia anticline normal to its axis. A seismic reflection profile, which is described in B.M.R. Record 1951 No. 62, has also been run along this road and it is possible to compare the gravity results with the seismic. Gravity station elevations were determined with an accuracy of within 0.1 of a foot and the gravity observations have been reduced to Bouguer anomalies using an elevation correction factor of 0.062 milligals per foot. The results are shown in the form of a profile on the accompanying plate. The surface profile is also plotted and the positions of some of the seismic shot points are shown alongside the gravity station numbers. These results are discussed in this report.

This report provides a summary of volcanic activity in the volcanic centres of New Guinea for the year 1951. Extended descriptions are given of ongoing investigations and the volcanological observations made at Rabaul, Mt. Lamington, and other significant volcanoes.

The Nunyerrie Asbestos Deposits are associated with a remnant of metamorphosed ultrabasic rocks in granite country, which is part of a group of Pre-Cambrian rocks of Archaean age. The ultrabasics form an east northeast trending ridge about 1 mile long and 250 feet average width, which rises to a maximum elevation of 300 feet above the general level of the surrounding country. The belt of ultrabasics is highly sheared and jointed and consists chiefly of serpentines, but talcose rocks, chlorite schists, authophyllite rock and what appear to be hydro-biotite schists also occur. The ultrabasics have an intrusive junction with the surrounding granite rocks, and they are intruded by quartz veins and granitic dykes. The deposits were investigated by the author. This report gives an overview of the deposits. Workings, production figures, reserves, grades, and prices are described.

Four days were spent in the area, observations being severely limited, firstly by lack of knowledge, on the part of the observer, of overall topography such as could be obtained from aerial photos or viewing the area from an aircraft and secondly by the dense fog which covered the area for the greater portion of the time spent there. Captain Fox of Q.E.A. had previously informed me that during the aerial inspection made by Mr. G.A. Taylor, three areas of "burnt" ground had been observed. Mr. Jones during his stay in the area had located two of those and in addition, two more minor areas. During my stay in the area those fumarolic areas were inspected and in addition, the third area located and inspected. The three major areas shall be referred to as the lower, middle and upper fumarolic areas.

A number of fossils were sent in for examination by the Director of Mines, Tasmania. The results of the examination are recorded in this report. Two different horizons are represented.