Shallow friable red soils (euchrozems) and shallow cracking clays (black earths) occur in close proximity on basalt hills of the Darling Downs of Queensland. The euchrozems are mainly restricted to fiat hill crests and are associated with lithosols; the shallow black earths occur on upper pediment slopes, on small convex crests and on depressions on flat crests. The euchrozems are moderately leached and contain kaolin minerals and hematite with minor montmorillonite, while the black earths are dominantly montmorillonite with minor kaolin and hematite. It is proposed that the euchrozems have developed by long continued weathering under stable well-drained conditions while the black earths have formed in sites prone to erosion and in situations with less water available for weathering. The weathering products produced in each situation have ensured continuation of the processes. The presence of two red soils in anomalous positions have been ascribed to (a) soil formation on exposed 'bole' (compacted red clay) layers and (b) local peculiarities of parent material controlling the course of weathering.

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The purpose of classification is to indicate relationships and differences between objects and to establish groups at various levels of generalization; i.e. to produce simplified models based on experience. In soils the object to be classified is often the soil profile, which may be considered to be analogous to the hand specimen in geology and to the individual organism in biology. For purposes of discussion a profile is considered to be a vertical column, up to 20 cm square, and extending to c. 2 m from the surface, to rock or to some distinct soil layer underlying the profile, across which physical, chemical and mineralogical variations are small. There is no conceptual continuum of soil profiles, even though there is a physical continuum across the surface of the earth. Rather there are a number of common forms each of which may be represented by a 'central concept' with intergrades between them. Such a central concept can be recognized at each level of a hierarchy. Soils are natural objects with a complex structure and history, as are organisms, rock specimens and even landforms. They should be considered in the same way as the other natural objects when a classification is being constructed. Their 'history', as conceived at present, should be involved in classification, even though major emphasis is placed on physical, chemical and mineralogical properties. To illustrate these ideas, the Australian Great Soil Groups are re-examined and suggestions are made about possible subgroups.

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