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High magnification (Fig. 8.2) enables observation of the successive
layers of the coatings. Two things are noted: (1) the base of the layer is
dark (brown), whereas the upper part is lighter (yellow in thin section)
and (ii) the transition of one layer to the next is abrupt, while the
variation in hue within a layer is gradual. Thus one cannot be mistaken
about the orientation of the thin section (Legros 1975, 1976).
Fig. 8.1 In a granitic arène (Pilat massif, France), clay coatings (microlaminar coatings) are
accumulated in successive layers on the lower wall of a pore. The white ellipse highlights the
position of the pore so that it is seen just before its total fi lling up. Height of the photograph: 1
mm. The small black circle is an artefact. Thin section : A. Guyon; photo : author.
Fig. 8.2 Superposed clay coatings observed by the microscope at high magnifi cation ( Thin
section : A. Guyon; photo : author).
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