Silica diagenesis in the near-surface zones of subdued Tertiary drainage basins in the arid interior of central and southern Australia produced a mosaic of silcretes now exposed as relic duricrust on mesas that are significantly higher than the contemporary base-level of erosion. The silcretes comprise a variety of silica-cemented sediments and possibly soils that exhibit a range of textures and chemical compositions. Acid-weathering conditions in pedogenic environments near the margins of the Tertiary drainage basins are likely to have provided soluble silica for soil waters and groundwater moving slowly through the regolith into depositional zones in the lower reaches of the basins. The silica cement in silcretes ranges from ordered forms of opal to quartz overgrowths, and the existence of incomplete transformations between structural states and between textural forms suggests heterogeneous precipitation and recrystallization in response to local fluctuations in properties such as pH and moisture regime. High concentrations of titanium were precipitated from solution in bedrock fractures as well as in sediments in the piedmont of the ancient drainage basins, but as yet there is no unequivocal explanation for its occurrence. Apart from zircon, enrichment of zirconium and aluminium in areas of high titanium concentration is evident from electron probe microanalyses and X-ray scanning images, but the mineral phase or phases responsible for this distribution are unidentified.

Terminalia Sp.-Dichanthium fecundum Alliance .. (g) E. brevifolia (Mono-association) Alliance (h) Melaleuca Spp. Alliance (i) Bmchychiton Spp.-Terminalia Spp.-E. confertiflora Connnunity '(j) Temdnalia Spp.-'Ficus Spp.-Melaleuca Spp. Community (k) E. camaldulensis Fringing Community (I) Mangrove Communities

The surveys are conducted by a team of scientists (in this case geologist, geomorphologist, pedologist, and botanists) who work together in the field and laboratory. The concepts and techniques have been described by Christian and Stewart (1953). A basic feature is that the areas are described in terms of land systems which are defined as "an area or group of areas throughout which there is a recurring pattern of topography, soils, and vegetation". The technique of surveying large areas in limited time is based on the interpretation of aerial photographs and a basic assumption is that the patterns distinguishable on aerial photographs are a reflection of land characteristics and vice versa. A complete cover of aerial photographs is necessary-in this survey their scale was I : 50,000. II. LoCATION AND SURVEY STATISTICS The Leichhardt-Gilbert area lies wholly in north Queensland and comprises 117,000 sq miles bounded by long. 139°30'E. and 145°30'E. and by lat. Irs. and nos. The team covered 5500 miles of land traverses (Fig. I) in two periods of field work-June to September 1953 and June to August 1954-totalling six months. Six months were spent on interpretation of aerial photographs in the laboratory. lIt. TOWNS AND COMMUNICATIONS The southern part of the area is served by a railway line which crosses from east to west (Fig. 2) and links Mt. Isa (a few miles west of the western limits of the area) with Townsville on the east coast. The largest towns in the area (Cloncurry, Julia Creek, Richmond, and Hughenden) are on this line, as are the smaller settlements of Pentland, Torrens Creek, Prairie, Maxwelton, Nonda, Nelia, Gilliat, Malbon, and Duchess. From this line at Hughenden a branch line goes southwest to Winton beyond the southern boundary of the area and forms an inland link to the other lines of central Queensland. At Cloncurry another branch line runs northwest through the small settlements of Quamby and Kajabbi to Dobbyn and at Duchess a branch runs southwest to Dajarra, which is the main trucking centre for cattle from the northern half of the Northern Territory. The northern half of the area is only poorly served by railways. In the northeast a light line joins Forsayth with Cairns through Einasleigh and Mt. Surprise. The

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A thermoluminescence age of about 20 000 years (� 3 000) B.P. has been obtained for samples of dune sand from near Lake Woods. This gives the time when the sample was last exposed to direct daylight, and it dates a period of dune building associated with an ancestral Lake Woods. The age is compatible with geomorphological evidence.