'The origin of _regur_ is a doubtful question.

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The dark coloration was attributed by earlier writers to vegetable matter, and taken to indicate a large amount of humus in the soil; more recent investigations make this doubtful, and in all probability the colour is due to mineral constitution rather than to the very scanty organic constituents of the soil,' It may possibly be formed of 'wind-borne dust', like the loess plains of China (Oldham, in _The Oxford Survey of the British Empire_, vol.ii, Asia, p.