The mineral compositions of the clays from a red earth and a black cotton soil from Hyderabad, Deccan State, India, occurring in close proximity in the field are determined. Both soils are derived from the same or from very similar parent rocks, a coarsely crystalline granite or gneiss.For both soils there is practically no variation in the mineralogical composition of the clay throughout the profile, but for any given clay there is some variation with grain size. The main contrast between the two is that the red clay contains predominantly kaolinite or halloysite, whereas the black clay contains mainly beidellite, a member of the montmorillonite group. The topography appears to be the principal factor associated with this difference in minerals, and the processes of weathering believed to have produced the contrasted clays are discussed with reference to experiments on the leaching of felspar in the laboratory and on hydrothermal synthesis.
The Bin and Clashindarroch Forests lie some 40 miles to the north-west of Aberdeen. Clashindarroch Forest, the larger of the two, stretches from the Rhynie-Cabrach road in the south almost to the Huntly–Glass road in the north. The Bogie valley in the east forms another rough boundary for the hill mass that the forest occupies. On the west the area marches with the county boundary. The Bin Forest consists of two main sections—the Bin itself to the north-west of Huntly, and the Balloch which lies between Keith and Grange, being partly in Banffshire. Both of these areas are more or less isolated hills.
Summary
A series of soils from two areas of Tanganyika have been studied mineralogically. They are derived in the main from amphibolite, but only a small proportion of the more resistant ahminosilicate minerals survive in the soil. The clays (<1.4μ) of the red loams are characterized by a predominance of kaolinite, whereas the more poorly drained grey soils have a disordered kaolin. Other components of the clays are iron oxides and small amounts of illite. The pallid soils contain moderate amounts of montmorillonite in the weathering zone, some of which persists into the soil. The mbuga and black valley soils contain montmorillonite or iliite, with subordinate kaolin.
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